The algorithm is efficient and robust, and should be broadly applicable in surface studies, where it can replace manual, intuition based model generation.Determination of the atomic structure of solid surfaces typically depends on comparison of measured properties with simulations based on hypothesized structural models. For simple structures, the models may be guessed, but for more complex structures there is a need for reliable theory‐based search algorithms. So far, such methods have been limited by the combinatorial complexity and computational expense of sufficiently accurate energy estimation for surfaces. However, the introduction of machine learning methods has the potential to change this radically. Here, we demonstrate how an evolutionary algorithm, utilizing machine learning for accelerated energy estimation and diverse population generation, can be used to solve an unknown surface structure—the (4×4) surface oxide on Pt 3Sn(111) which had eluded determination by experimental methods.Machine learning techniques can be implemented to accelerate surface structure determination based on density functional theory. The application of such an algorithm is demonstrated here for a surface oxide on Pt The atomic structure of surfaces and interfaces critically underpins our understanding of the performance of various materials, from heterogeneous catalysts and electrocatalysts to semiconductor electronics and photovoltaics. Despite the enormous importance of such fundamental information and the great advances in experimental instrumentation over the past decades, structure determination for complex surface reconstructions and ultrathin films remains a significant challenge, and many structures remain unsolved. Surface crystallography is labor intensive and prone to error, even when relatively simple structures are considered; complexity, disorder and heterogeneity can easily render such problems intractable. This naturally hinders our ability to interpret experiments that aim to establish structure‐performance relationships for these materials, and especially our ability to predict and explain such relationships through quantum chemical modelling.The oxidation of platinum‐tin alloys provides a clear example of these limitations. Though the materials are of importance for a variety of catalytic processes,5, 6 tApplicability of the global optimization methods at the density functional theory level to larger systems is, however, limited by the computational cost encountered. This is most pronounced for systems involving a large number of atoms, as is the case for surfaces, which require inclusion of a certain number of bulk layers.26, 27We demonstrate here the successful application of such a machine learning enhanced algorithm to the problem of an oxidized platinum‐tin surface. Specifically, we employ the recently developed GOFEE algorithmEvaluate a few random structures at the DFT level.Construct a surrogate energy landscape based on all structures evaluated at the DFT level. The landscape is described with a Gaussian Process Regression (GPR) model based on a global descriptor of the structures.Collect a sample of the most stable, yet significantly different, structures calculated at the DFT level, see Figure .Generate multiple new structural candidates via rattle mutations applied to the sample members.ELCB=E−κσ, where E and σ are the GPR model energy and uncertainty, respectively, and where κ is a constant.Relax the new candidates in the lower confidence bound landscape given by ELCB and perform evaluation at the DFT level (no relaxation).Select the relaxed candidate with the lowest Jump to (ii).Details of the descriptor, the model, the sampling method, and the setup for the DFT calculations are given in the methods section of the Supporting Information.ELCB’s, of all relaxed candidates, the method is driven by the uncertainty estimates from the surrogate model. It thereby effectively performs a Bayesian selection with (minus) ELCB being the acquisition function and with configurational space being represented discretely by the relaxed candidates.The GOFEE method accomplishes a massive increase in efficiency while maintaining accuracy at the ab initio level mainly through two measures: Firstly, all local relaxations are done in an inexpensive machine learned landscape rather than in the expensive DFT landscape. Secondly, by selecting the most promising candidate based on comparing the relaxed lower confidence bounds, the population of structures, which is implemented by deciding at every iteration if a new candidate, just evaluated at the DFT level, should replace one member of the population. In the present work, the population is replaced with a sample drawn from the set of all structures evaluated at the DFT level. The sample indeed plays the role of a population, but differs in that it assesses all DFT data in every iteration and in having potential to change faster than an evolving population. The conceptual difference between a population and a sample‐based approach is sketched in Figure In its original formulation, the GOFEE method has an evolving 3O6 cluster. The sample is generated from the full set of structures evaluated by DFT so far, after exclusion of those more than a fixed energy ΔE above the current lowest‐energy structure. The remaining structures are represented in feature space using the same global descriptor as in the GPR model. They are subsequently divided using the k‐means++ clustering algorithm,The scheme for obtaining the sample is illustrated in Figure Aside from one previous study,3Sn, the structure of which is shown in Figure p3.The system under consideration here is the (111) surface of PtAttempts at structure determination for this phase have been made using several techniques, including scanning tunneling microscopy (STM), low energy electron diffraction (LEED), X‐ray photoelectron spectroscopy and diffraction (XPS/XPD) and low energy ion scattering (LEIS),10, 113Sn(111). The approximate numbers of Sn and O atoms to be placed on this substrate were chosen by assuming that the (4×4) phase consists of a Sn2+ oxide monolayer of some sort. Here inspiration was taken from bulk SnO and SnS, which are composed of van der Waals sheets, with Sn2+ in 4‐fold and 3‐fold coordination, respectively. Supported on Pt3Sn(111), such sheets would exhibit tin densities between 8 and 13 atoms per (4×4) unit cell. With this density range in mind and supposing somewhat higher coverages of oxygen compared to tin due to presumed oxygen affinity of tin in the substrate, we selected 16 combinations, with 9–12 Sn and 11–14 O per cell. The Sn : O composition of the (4×4) phase is hence not deduced from experiment, but rather the structure search algorithm is being applied to a range of Sn : O combinations, and afterward subjected to thermodynamic analysis and comparison with experimental data to identify a likely candidate.For the GOFEE based structure search, we assumed a (4×4) periodic unit cell and a substrate consisting of bulklike Pt11O12 structure represented by the darkest blue color in Figure Figure 9O11, Sn9O13 and Sn9O14), the same minimum‐energy structures were found in all four runs of the search. Most of the structures found exhibit a common feature: a minority of Sn atoms protrude from the surface and show a lateral spacing of ≈5–6 Å, consistent with SPM images of the (4×4) and related phases. The remaining Sn atoms are at the metal interface, with O atoms forming a layer in between. One of the 16 structures found exhibits the p3 symmetry expected from experiment: that with a composition Sn11O12, with three protruding Sn atoms, 8 Sn atoms at the interface, and 12 nearly coplanar O atoms in between. This structure is the very same that was found to be the most stable according to the free energy diagram, Figure For each composition considered the search was repeated four times from independent starting configurations to check for consistency. For all but three compositions  confirms this structure and enables further refinement. Figure l Figure b. Duringl Figure c also sh2+ surface oxide—a contiguous two‐dimensional network related to SnO, but with a structure strongly influenced by bonding with the metal substrate. The basic structural element is Sn in 3‐fold oxygen coordination, with a strongly asymmetrical pyramidal geometry. This geometry differs somewhat from that found in bulk SnO, a layered material composed of buckled SnO sheets where Sn adopts a 4‐fold pyramidal geometry. The trigonal pyramidal geometry found here is nevertheless typical of Sn2+, as seen in halides and in tin(II) sulfide, and attributed to the occupation of a stereoactive lone pair orbital derived from the Sn 5 s state. This geometry has also been observed or predicted in oxidic phases, including layered, mixed‐valence Sn3O4,2(101)2(110).The (4×4) phase can be described as a Sn2+. The strength of these bonds is reflected in outward buckling of surface Pt (opposite to the clean Pt3Sn(111) surface, where outward buckling of Sn is observed2+ bonding is a characteristic feature of Pt/SnO interfaces. The particular structure found here results from optimization of these bonds at the (111) surface, constrained by preferred Sn−O bond geometry.Bonding between Sn in the oxide layer and Pt in the substrate is also typical of SnAlthough these structural features can be rationalized rather easily in hindsight, the structure itself could not be directly deduced from experiment or guessed based on chemical intuition, primarily due to its multi‐layered arrangement and relatively low symmetry. An automated search method based on theory can, however, be used to generate physically plausible structural candidates for comparison with experiment, leading to the correct structure model. This methodology depends on a search algorithm that is both efficient and reliable. Our strategy utilizes the full history of the evolutionary search both to accelerate structure evaluation and to maintain a sufficiently diverse population so that premature convergence is avoided. With this we take a step towards optimal search algorithms in which all components leverage the available data to maximum benefit.3Sn(111) is relatively complex, but it nevertheless represents a rather ideal case, where quantitative diffraction measurements can be used to verify the result of the structure search. In general, surfaces can exhibit considerable degrees of disorder, with various coexisting phases and minority structures that cannot be characterized in detail by averaging techniques. The combination of scanning probe microscopy with theoretical simulations is often the only viable methodology in these cases, resulting in considerable uncertainty and occasionally gross misinterpretation. A reliable global optimization method like GOFEE can enable surface studies of this type to be conducted with much greater confidence.The (4×4) surface oxide on PtBroad application of this approach, however, will also require the ability to characterize larger‐scale features like defects and superstructures. For this, the search algorithms must be able to scale toward hundreds of atoms, resulting in drastically more demanding searches due to the combinatorial scaling of the configuration space and the increased cost of DFT evaluations. This will require even more effective data utilization in search strategies, where the efforts presented in this work, along with recent advances in transferable machine learning potentials47, 483Sn(111). For this application, we further introduced in the evolutionary search method a shift from an evolving population to a k‐means based sample. The latter leverages the full history of structures visited in the search and appears highly robust and efficient for the present system.Surface structure determination is a notoriously difficult problem due to the limitations of experimental techniques and the high cost of accurate theoretical modeling. In this work we have demonstrated how a machine learning driven search algorithm can be used to overcome such limitations, through the characterization of an experimentally observed (4×4) surface oxide on PtMethods, Figures S1 to S4, Table S1.Authors declare that they have no competing interests.1As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.Supporting InformationClick here for additional data file.


To assess whether persistent idiopathic dentoalveolar pain (PIDAP), a diagnosis of exclusion, exhibits common features that can facilitate its diagnosis. PIDAP is defined by the International Classification of Orofacial Pain (ICOP 6.3.) as ‘Persistent unilateral intraoral dentoalveolar pain, rarely occurring in multiple sites, with variable features but recurring daily for more than 2 h per day for more than 3 months, in the absence of any preceding causative event’.In this observational study, participants fulfilling the new ICOP diagnostic criteria of PIDAP were included, covering 16 years of consecutive data. A systematic, retrospective assessment of patients utilizing the new PIDAP criteria on complaints of chronic tooth pain in a referral‐based endodontic practice was undertaken. Non‐idiopathic cases were excluded on the basis of clinical and radiologic findings. A modified neuropathic pain questionnaire was used to describe the painful sensations. Furthermore, allodynia in the periodontal space and sensory changes in the oral mucosa were assessed using a periodontal probe.Amongst the 160 patients assessed, 78 (63 women) fulfilled the strict PIDAP criteria. Pain history of PIDAP included no nocturnal awakening (85%) and a ‘pulling/dragging’ pain quality (59%). In 69% of the patients with PIDAP, pain was associated with a root filled tooth at the same site. In 14% of the cases, no endodontic treatment was performed in the affected quadrant. Mechanical allodynia in the gingival sulcus was observed in 91% of patients with painful teeth or implants.In this observational study, PIDAP mainly affected females and was associated with undisturbed sleep and periodontal allodynia. Dental pain is a relatively common condition with a significant socioeconomic impact .The new International Classification of Orofacial Pain (ICOP) classification emphasizes the difference between neuropathic and idiopathic pain that can result in persistent dentoalveolar pain. The latter was the subject of this observational study. In this context, ‘Idiopathic’ is defined as ‘arising spontaneously or from an obscure or unknown cause’ , a trained endodontist and a physician with a focussed interest in pain, between January 2003 and June 2019. The use of this censored data for publication was approved by the local ethics commission . The majority of patients were referred from other endodontists within Germany because of ongoing pain after treatment , mucogingival lesions, internal or external resorption or possible lesions of endodontic origin resulted in the exclusion of the patient from the study group Table . PatientAll pain interviews and clinical examinations were conducted by the principal investigator. After their dental examinations reported here, the patients diagnosed with PIDAP were referred to headache and facial pain clinics or specialized pain therapists.The medical history was obtained using a health questionnaire that included questions pertaining to current health status Appendix  and the All patients completed a pain questionnaire for atypical or persistent tooth pain. This questionnaire Appendix  consisteThe area or tooth producing the most pain, as indicated by the patients, was recorded. All patients underwent the same intra‐ and extraoral examinations. Basic clinical examinations were performed as described in standard endodontic texts  using a periodontal probe  and applying light pressure. Patients were not asked before probing whether they had experienced any pain. Patients who had a reaction indicating that they might have felt pain were asked to state explicitly whether or not the touch was painful. The reaction was then compared with the corresponding reaction to the same procedure on the contralateral side. The original painful procedure was then repeated, and the patient was asked to comment on any differences compared with the non‐painful contralateral side. Probing of the gingival sulcus was performed in all quadrants.The periodontium was tested for bleeding on probing or pathological pocket depths (>3.5 mm) circumferentially. Patients presenting with one or more of these symptoms in the painful region were excluded from this study Figure . All teehttps://www.iasp‐pain.org/resources/terminology/) or as an unpleasant sensation (dysesthesia). If at least one of these symptoms was present, it was noted as a sensory change in the gingiva in the painful quadrant.The attached gingiva and mucosa in the painful quadrant were tested by touching them lightly with a periodontal probe and asking the patient for comparison to the contralateral side. The patient's reactions were also noted. No sensation, less sensation, painful sensations, or an unpleasant sensation in the painful quadrant compared with the contralateral side were classified as numbness, hypoesthesia or allodynia according to the IASP terminology (https://rstudio.com/products/rstudio/download/) were used to perform a descriptive statistical analysis. Normally distributed data sets (Shapiro–Wilk test) are presented as means and standard deviations, whilst non‐parametric data are presented as medians and ranges.The RStudio software tools  met the inclusion criteria for this study and were diagnosed with PIDAP. The site of pain was the teeth of 75 patients, implants in one patient or edentulous post‐extraction sites in two patients Table . The paiIn 25 cases in the PIDAP group, the persistent pain was preceded for more than 6 months by root canal treatments or other dental procedures that are listed to be relevant to a diagnosis of PTTNP in the ICOP classification, such as extractions, oral surgery, dental implants, orthognathic surgery and other invasive procedures. In 12 cases, persistent pain preceded the root canal treatment, and in 24 cases, no root canal treatment or other invasive dental procedures were performed on the tooth. In 12 cases, the root canal treatment preceded the beginning of persistent pain for less than 6 months, but since sensory changes were found in other quadrants as well, these cases met the criteria for PIDAP, not PTTNP Appendix . Cases iOther chronic pain conditions reported by patients with PIDAP included chronic back pain , irritable bowel syndrome (3), migraine (4), fibromyalgia (2) and tension headache (1). A history of depression was reported in nine cases and psychosis in one. The most frequently reported medical conditions were thyroid disorders (21), including 12 cases of Hashimoto's thyroiditis. Eleven patients had cardiovascular and respiratory disorders. Other disorders or diseases were present in very small numbers of patients. Twenty‐one patients reported no other health problems besides the existing persistent intraoral pain.Drugs taken on a regular basis included l‐thyroxin (18), contraceptive medication (4), drugs used for chronic pain treatment (8), psychotropic drugs (3) and cardiovascular drugs (10).The pain had been present for an average period of 2.7 years before the examination . A group of 17 women had pain persisting for more than or equal to five years. The maximum pain intensity during the 2 weeks prior to assessment ranged from 2 to 10 on the NRS‐11 scale . The mean average pain score in the same period was 3.8 (median: 3), and the mean NRS‐11 scores for female and male patients were 3.8 and 4.1, respectively.Sixty‐six of the 78 (85%) patients stated that the pain did not wake them at night. Furthermore, 64 (82%) patients experienced pain‐free intervals on waking in the morning before the pain set in. Pain was present during the daytime for more than 2 h in all patients.Patients generally used more than one term to describe the nature of their pain. The most frequently used adjectives were as follows: ‘pulling/dragging’, reported by 46 of 78 patients (59%); ‘pressing’ by 41 (52%); ‘burning’ by 33 (42%); ‘tingling’ by seven (9%); ‘electric’ by three (4%); and ‘stabbing’ by 14 (18%). Forty‐three (55%) patients reported experiencing pulsating or throbbing pain at the affected site from time to time. When describing the quality of their pain in more words, patients frequently used non‐standard, figurative expressions that differed from more common ways of describing toothaches, such as ‘like being stabbed with a knife’, ‘like a wound’, ‘like dead tissue that is nevertheless painful’, ‘as if the tooth were to explode’ and ‘as if the bones were moving’.Pressure to the periapical bone exerted by digital palpation was never painful, nor could pain be elicited by vertical tapping on a tooth with an instrument in any case. Discomfort was elicited by horizontal tapping in six patients. Mechanical allodynia of the gingival sulcus was found in 68 of 75 teeth and in one implant. Thus, allodynia in the periodontium/peri‐implant mucosa was observed in 69 of 76 patients with painful teeth or implants (91%). In two of the cases in which the pain affected the edentulous region, this sign could not be tested. The painful section invariably consisted of only part of the gingival sulcus and never included its entire circumference. The same symptoms could frequently be elicited in the sulcus of the adjacent teeth or in the antagonistic or contralateral quadrants. An allodynic area in the gingival sulcus of the neighbouring teeth was present in 53 cases. More than one quadrant was affected in 39 cases: two quadrants in 22 cases, three quadrants in 11 cases and all four quadrants in six cases.The gingiva and mucosa of all patients were checked for sensitivity changes to touch by using the tip of a periodontal probe to slightly touch the gingiva and mucosa in the painful quadrant, first on the non‐painful contralateral side and then on the affected painful side. In 9 cases, patients reported numbness near the site of the pain after repeated comparisons with the non‐painful contralateral side. None of the patients had been aware of this numbness before the examination. Twenty‐eight patients showed sensitivity changes other than numbness in the painful quadrant beyond the gingival sulcus; mechanical allodynia was present in five patients. Seven patients reported reduced sensitivity to touch (hypoesthesia) and 15 reported discomfort when touched with a periodontal probe compared to the non‐painful side. In seven patients, more than one sensitivity change in the fixed gingiva was present concurrently.The painful sensation sometimes increased as a result of the examination, especially on contact with the gingival sulcus. A total of 32 patients reported an increase in pain levels after completion of the examination.In this observational study, strict PIDAP criteria were applied to illuminate the condition from the dentist's perspective. This allowed the identification of PIDAP features that have not been described previously, including the absence of nocturnal pain and the presence of allodynia in otherwise healthy periodontal sites.Until recently, the nomenclature for the conditions under investigation was not standardized , chronic pain conditions (23%), and depression (10%) were reported by the patients. Because this information was questionnaire‐based, its completeness and accuracy were not assured. It was only included to describe the group of patients under investigation in more general terms. Nevertheless, the relatively high frequency of thyroid disorders in this cohort warrants further research. An examination of a large sample of self‐reported chronic pain   has also been described in neuropathic pain. In this regard, mechanisms similar to those of ‘wind‐up’, a progressive, frequency‐dependent increase in excitability, may play a role   had this sensory abnormality, often not only in the painful tooth but also in other teeth and quadrants. Neurophysiological research on this topic has shown that trauma to one dental nerve may alter the processing of pain in neighbouring teeth and other quadrants  and to identify characteristics of specific groups that may be due to changes in the pain processing system typical of PIDAP.Nevertheless, this observational study was limited by the fact that it was not a cross‐sectional study, and the pain and treatment history depended on the patient's memory. As a result, no conclusions can be drawn regarding the prevalence of the conditions under investigation. Furthermore, there was a selection bias because patients were mostly seen in endodontic clinics Figure , which rThe focus of the current work was on how PIDAP patients presented in endodontic practice, and which were the key features of their condition from a dental perspective. In a future study, an attempt will be made to follow the PIDAP patients identified here to assess how their pain was managed.PIDAP is normally a diagnosis of exclusion, but common features may exist;No nocturnal awakening due to pain was reported by most patients with PIDAP;The sulcus around the affected tooth/implant was painful upon light touch of the periodontal probe in a significant proportion of PIDAP patients;PIDAP mainly affected females;The five most frequent pain descriptors were ‘pulling/dragging’ (59%), ‘pulsating’ or ‘throbbing’ (55%), ‘pressing’ (52%) and ‘burning’ (42%).Persistent idiopathic dentoalveolar pain is mainly diagnosed by exclusion. For clinicians, the typical features of PIDAP can help to better differentiate this type of pain from odontogenic and other possible pain conditions. The identification of allodynia in the gingival sulcus of affected teeth/implants appears to be a common PIDAP feature easily detected by the dentist, as is the information that the pain does not wake the patient at night. In view of the rarity of PIDAP, special networks or centres are needed to generate a sufficient number of cases whose evaluation can provide the basis for an improvement in the diagnosis and treatment of patients with persistent toothaches.The use of data pertaining to this observational study was approved by the local ethics committee .The authors have stated explicitly that there are no conflicts of interest in connection with this article.Appendix S1Click here for additional data file.Appendix S2Click here for additional data file.Appendix S3Click here for additional data file.


Turdus) represent a nearly worldwide clade of potential hosts of brood parasitism by Cuculus cuckoos in Eurasia and Africa and Molothrus cowbirds in the Americas. The Eurasian blackbird (Turdus merula) builds an open‐cup nest and is common within much of the common cuckoo's (C. canorus) breeding range. While this thrush is known to be parasitized at most only at low rates by this cuckoo, the species is also a strong rejector of nonmimetic foreign eggs in the nest. Given their open‐cup nesting habits, we predict that Eurasian blackbirds primarily use visual cues in making a distinction between own and parasitically or experimentally inserted foreign eggs in the nest. We then provide a comprehensive and quantitative review of the literature on blackbird egg rejection studies. This review corroborates that vision is the primary sensory modality used by blackbirds in assessing eggs, but also brings attention to some other, less commonly studied cues which appear to influence rejection, including predator exposure, individual experience, stage of clutch completion, and maternal hormonal state. Blackbirds are also able to recognize and eject even highly mimetic eggs (including those of conspecifics) at a moderate rate, apparently relying on many of the same sensory cues. Although the cues involved in foreign egg recognition by Eurasian blackbirds do not appear specialized to nonmimetic cuckoo parasitism, we cannot differentiate between the possibility of egg rejection being selected by mostly conspecific parasitism or by the evolutionary ghost of a now‐extinct, mimetic cuckoo host‐race.Avian brood parasitism is reproductively costly for hosts and selects for cognitive features enabling anti‐parasitic resistance at multiple stages of the host's breeding cycle. The true thrushes (genus  Turdus merula) to distinguish between own eggs and eggs experimentally or parasitically added to the nest. We find that visual cues are most commonly used, but also that indirect cues such as predator exposure, individual experience, and clutch completion stage are also used.We provide a comprehensive, quantitative review of the literature on cues used by Eurasian blackbirds ( We included work from 1989 to 2021. These data, including our calculations of the effect sizes (odds ratios), are summarized in Table Following the effect‐sized‐based quantitative methodology used by Turner & Hauber  in reviesed N = 2 featuredNest desertion cannot be equated with egg rejection in blackbirds  in papers was available. We provide these as a mechanism for future studies to compare or meta‐analyze the results of the papers reviewed.3.1.1In a recent and large‐scale meta‐analysis conducted across all hosts and brood parasites  were the most common experimental paradigm , a set of blackbird‐sized eggs, and a set smaller than blackbird eggs (house sparrow Passer domesticus eggs). Small nonmimetic eggs were ejected at a higher rate than any other egg type, and medium nonmimetic eggs were ejected more frequently than medium mimetic eggs. Large nonmimetic eggs were rejected at a low rate (and large mimetic eggs were never ejected). The authors suggest that this is in part informed by the difficulty blackbirds, as grasp‐ejectors, would have in grasping the large eggs with their beaks; desertion, instead, appears to be associated with large foreign eggs, as these eggs are deserted  significantly more often than controls    but was not affected by nest specific threat (a model magpie Pica pica). The effect of an adult‐specific predator threat decreased as the breeding season reached its height. Ruiz‐Raya et al. (Streptopelia turtur). Rejection was much more frequent in response to heterospecific eggs under all conditions, with the dummy cuckoo eliciting highest rejection rate, followed by the blackbird, followed by the turtledove . This trend (there was not a significant effect of dummy type for either type of egg model) was not the same with conspecific eggs, where exposure to the cuckoo and turtledove elicited the same level of rejection behavior; the blackbird model elicited the lowest level  have higher levels of prolactin than non‐care‐provisioning parents/members of the breeding group  and lowered prolactin is also associated with egg abandonment  dummies such as of feral pigeons (Columba livia)  rejection of cuckoo egg models, though the presence of males at the nest was not as strong a predictor of rejection as the presence of females. Male rufous horneros (Furnarius rufus)  ; Formal analysis (lead); Investigation (lead). Mark E. Hauber: Conceptualization (lead); Funding acquisition (lead); Methodology (lead); Project administration (lead).


Homelessness and mental illness are associated with poor service engagement, health and health service use outcomes. Existing literature suggests that financial incentives may effectively support service engagement of this population, but studies investigating key stakeholder perspectives are lacking. This study aimed to elicit, using qualitative methods, nuanced service user and provider experiences by using financial incentives to support service engagement among adults experiencing homelessness and mental illness.This qualitative study is part of a larger mixed‐methods pragmatic trial of financial incentives  within a community‐based brief case management programme (CATCH) in Toronto, Ontario. Twenty‐two CATCH‐FI participants were purposefully recruited to participate in in‐depth, semi‐structured interviews; five CATCH service providers participated in a focus group and seven key informants in individual interviews. Data collection occurred between April 2019 and December 2020. All interviews and the focus group were audio‐recorded and transcribed. Topic guides prompted participant perspectives on and experiences of using financial incentives to support engagement, health and well‐being. Grounded theory and inductive thematic analysis guided coding and interpretation of transcripts. Triangulation and member‐checking enhanced the analytical rigour and validity of findings.CATCH service providers, key informants and subgroup of CATCH‐FI participants perceived financial incentives to directly facilitate service engagement. The majority of CATCH‐FI participants however highlighted that intrinsic motivation and service quality may be relatively more important facilitators of engagement. Most study participants across stakeholder groups perceived that financial incentives have direct positive influences on health and well‐being in enabling access to basic needs and simple pleasures.Our data suggest that for some adults experiencing homelessness and mental illness, financial incentives can directly support service engagement. In addition, financial incentives may positively impact health and well‐being by easing financial stress and enabling deeper attention to individual health needs. Further research on the effectiveness and acceptability of financial incentives is needed to improve understanding and uptake of a promising intervention to support health and health service use outcomes in an underserved population.Study participants provided input into the study research questions, study design, interview guides and interpretation of findings. Moreover, significant ethical concerns have been raised, particularly regarding perceived coercion and its potential impact on autonomous decision‐making, and the potential for unintended harms; service providers, researchers and planners have raised concerns that money might enable increased substance use, for example.This study aimed to elicit, using qualitative methods, nuanced service user and provider experiences with financial incentives to support service engagement among adults experiencing homelessness and mental illness posthospital discharge in Toronto, Canada.22.1This study describes the qualitative component of a larger mixed‐methods pragmatic randomized controlled trial (RCT), Coordinated Access to Care for the Homeless—Financial Incentives (CATCH‐FI), described in‐depth elsewhere.The CATCH programme itself has also been extensively described and evaluated.2.2N = 176) and met both programme and RCT eligibility criteria. Programme eligibility included: (i) current homelessness or precarious housingCATCH‐FI participants in this qualitative study were recruited from the broader RCT sample  had at least 10 contacts with their CATCH service providers . CATCH‐FI participants were predominantly male , Caucasian  and middle‐aged, with 75% aged 35–64 years. CATCH‐FI participants varied in level of education but the majority had completed high school . Compared to the overall RCT sample, this qualitative sample was comparable in age but included proportionally more female (32% vs. 25%), Caucasian (68% vs. 56%) and high‐school educated (73% vs. 64%) participants.At baseline, most CATCH‐FI participants (n = 5) were CATCH programme staff in frontline  and managerial roles. Key informants (n = 7) were healthcare providers in senior clinical and/or administrative leadership roles at leading local mental health and social service institutions that are partnered with the CATCH programme and serve the broader homeless population in community settings.CATCH service providers and key informants ranged in age, gender, experience and healthcare role. Focus group participants  were conducted with CATCH‐FI participants between April 2019 and December 2020. Key informant interviews averaging 26 min in length (range: 15–51 min) took place between August 2020 and October 2020, while a focus group with CATCH service providers was held in August 2020, lasting 94 min. All interviews including the focus group were audio‐recorded and transcribed verbatim. CATCH‐FI participants received an honorarium of $30 and public transportation fare for completing a qualitative interview and focus group participants received a $10 gift card.The individual interviews and focus group specifically aimed to explore stakeholder perspectives and experiences related to the impact of FI on service engagement and health and well‐being. Topic guides included open questions and specific prompts related to overall perceived facilitators of and barriers to engagement and factors influencing healthcare decisions, as well as specific questions and prompts related to the perceived impact of FIs on service engagement, health and well‐being. More specifically, CATCH‐FI participants enroled in the intervention arm were asked whether FI impacted their decision to contact their case manager or their overall health and well‐being and how. Those enroled in the usual care arm were asked whether they believed FI would have, hypothetically, impacted their engagement, health and well‐being. Topic guides were iteratively developed and refined by the study team to capture rich and diverse perspectives.The research team is led by a primary investigator (PI) with extensive experience conducting qualitative research among people experiencing homelessness and mental illness; and the study is sponsored by the MAP Centre for Urban Health Solutions at Unity Health Toronto, who similarly have a history of successfully engaging this population locally. Together, this team has successfully conducted several previous studies with the study population. Data collection was conducted by trained and experienced interviewers who were known to participants by virtue of having conducted prior quantitative interviews as part of the broader RCT. Rigorous interviewer training, ongoing transcript review by the study PI and study staff and investigator triangulation were also employed to strengthen the quality, rigour and trustworthiness of results. Lastly, a member‐checking process was conducted in which results were reviewed, refined and confirmed with CATCH service providers in August 2021.2.4This study was approved by the Research Ethics Boards at St. Michael's Hospital, Unity Health Toronto  and the Centre for Addiction and Mental Health  in Toronto, Canada. All participants provided either written or verbal informed consent to participate; access to an interpreter was available to facilitate understanding and a capacity‐to‐consent questionnaire was available for use by study staff as needed.2.5κ statistic,κ = 0.79) and service provider data (κ = 0.77); percent agreement on all codes was 99%. All interviews coded for the purpose of the codebook development were later recoded using the developed codebook. As coding progressed, codes were grouped into higher‐order themes. The PI and study staff met regularly to iteratively review and refine the coding framework and emerging themes. Data saturation was achieved in which no new codes or themes emerged from later interviews.Grounded theory and inductive thematic analysis guided the interpretation of transcripts.QSR International NVivo 9 qualitative analysis software was used to support data management, coding and analysis.3In investigating the perceived impact of FI on service engagement, health and well‐being, three primary themes emerged from participant narratives:Cash is king: The first theme describes how FI directly facilitated engagement in a subgroup of CATCH‐FI participants.Not in it for the money: The second theme describes how, for the majority of CATCH‐FI participants, FI was believed to be relatively less important for service engagement compared to other factors, such as an individual's intrinsic motivation and the quality of care offered.Money talks: Finally, the third theme speaks to the universal agreement among study participants across stakeholder groups that FI directly support health and wellbeing by enabling access to basic needs and enjoyment of simple pleasures.‘Cash is king’: Financial incentives are a carrot and some are hungryI found that the incentive kept me in touch with someone that was helping me. It made me feel better because I knew someone was there. It's like, if the $20 thing wasn't there, I wouldn't have made that phone call, I know I would've felt more distanced from [case manager] and the program and like life itself… But I found the frequent contact kept my head more positive. (P4010)Over one‐quarter of CATCH‐FI participants described that FI had or would have a direct impact on their decision to engage in care. A minority of participants FI enroled in the intervention arm described instances of engaging with their case managers with the primary intention of obtaining the incentive: ‘I thought I should make sure that I stay in touch… I knew that I would get an e‐transfer, so it worked’ (P4337). Often, these participants described FI as an effective externally motivating reinforcement of their overall efforts to engage in an ongoing manner: ‘the finances helped keep me going and doing it, so I guess it would be the consistency of [engaging] that was impacted’ (P4081). As one CATCH‐FI participant enroled in the usual care arm who was asked about the hypothetical influence of FI summarized, ‘it gives motivation… something to gain that would motivate people to show up more often than not’ (P4071). Those who decided to engage because of the FI went on to describe an improved overall care experience as a result:n = 7/12). This was primarily due to the perceived universal utility of a financial gain and specific lack of financial capital among the study population. As one individual described,I think clients who are on the street… they have very few rewards and perks in their life, and so it makes sense to me to figure out what is it that they need and want. And one of the things that everybody needs and wants is…some level of finances… so it makes sense to me that we consider this as an option. (SP2)‘Not in it for the money’: The relative value of financial incentivesThe majority of CATCH service providers and key informants similarly believed that FI did or would likely directly influence service engagement (n = 16/22). These participants described feeling appreciative of the incentives, highlighting that an FI was useful but nonessential; they frequently referred to the incentives as a ‘bonus’ or ‘reward’. As one participant summarized, receiving an FI ‘didn't influence’ their decision to engage ‘because I would have done it regardless. But when I found out there was one, I was very happy’ (P4061).The majority of CATCH‐FI participants indicated that FI was not or would not be directly associated with their decision to engage with the intervention (CATCH‐FI participants described two relatively more meaningful facilitators of service engagement: (i) intrinsic motivation, at the service user level; and (ii) quality of care, at the service provider level.Intrinsic motivation matters more: Many CATCH‐FI participants in the intervention arm highlighted the importance of intrinsic motivation in their decision to follow up with their case managers and remain engaged in their care plan. These participants explained that their motivations were internal and primarily focused on improving their immediate circumstances of homelessness and poor health—‘get[ing] out of the shelter system, find myself an apartment, try to get back to work, get off [financial assistance]’ (P4025)—in addition to the overarching desire for personal improvement. As one individual described, their decision to engage ‘wasn't for the money. It was for the better of me’ (P4106). Others echoed that FI was relatively less meaningful compared to this internal drive: [Financial incentives] are not like the pinnacle of why I'm doing it. I'm trying to better myself… to get out of this… I don't want to be stuck’ (P4068). For this subgroup of CATCH‐FI participants, intrinsic motivation was paramount in their decision to engage: the ‘desire to get better, to get out of where I am. That is core’ (P4092). CATCH‐FI participants enroled in the usual care arm echoed this perspective, with one individual suggesting ‘any person who wants help is probably going to seek the help whether there's a financial incentive or not’ (P4126).Quality of care is a stronger hook: For other CATCH‐FI participants in the intervention arm, it was the quality of care provided by the programme as a whole and by individual staff members that primarily influenced their decision to engage in care. As one individual described the programme, ‘this is a lifesaver… my life was saved… that's why I would have done it regardless’ (P4061). This subgroup suggested they were particularly engaged by CATCH service providers who actively listened and addressed their needs. One participant explained how they ‘already enjoyed meeting with [case manager] and having someone I could talk to… who actually understood and didn't try to cut me off… someone who actually looked at me’ (P4004); and another expanded, ‘mainly I went to see him because he provided results and helped me with my healthcare… because of him, I would have gone to see him anyway. Every week, regardless’ (P4023). CATCH‐FI participants enroled in the usual care arm similarly echoed their counterparts' perspective in articulating that an FI ‘would be nice, but that wouldn't have been more of an incentive – I mean, at least not for me because I was getting so much out of the program’ (P4057).The financial incentive I think is very important for certain kinds of clients who that is their focus, but I think a lot of people are just really hungry for that engagement in someone that's looking out for them and they trust. (FG3)In contrast to the majority of CATCH‐FI participants speaking to relatively more important facilitators of engagement, only one frontline service provider or key informant similarly noted that FI might be a less impactful motivator:‘Money talks’: The utility of financial incentives in supporting health and well‐beingThis group of stakeholders did, however, articulate a scenario in which FI could be used to support initial engagement while service providers work toward building trust through the sustained high quality of care. For example, CATCH service providers and key informants described how the prospect of an FI can support initial engagement by ‘tip[ping] the balance’ (SP5) and ‘provid[ing] a moment in time to help with building that connection’ (SP1) to ‘get them through the door for the first part. Then the second part would be following through with what they've identified they want help with’ (SP3).basic needs and simple pleasures.Participants across stakeholder groups agreed that FI directly supports health and well‐being; all 22 CATCH‐FI participants described examples of how receiving FI did or would have improved their health status with five of 12 CATCH‐FI service providers and key informants providing similar perspectives. Overall, study participants described two primary mechanisms by which FI was used to support their health and well‐being: the fulfilment of Addressing basic needs: Study participants, including over two‐thirds of CATCH‐FI participants, described how FI afford more opportunities to meet basic needs that directly support health and well‐being, such as satisfying hunger, taking prescribed medications and reducing stress. As one CATCH‐FI participant in the intervention arm described, ‘It had a mental health [impact], yes. And yeah physical health “cause I was able to eat… I was going like entire days without food”’ (P4010). Other CATCH‐FI participants enroled in the intervention arm described using the FI they received for food, medications, supplies, public transportation fare and to pay bills: basic needs that ‘reduces stress because I'm able to get the little things I need… that money allowed me to pay my cell phone bill and it left me a small amount to get the groceries I needed for the week’ (P4249). The experience was impactful for CATCH‐FI participants, suggesting that the intervention ‘really helps financially when you're on a very low fixed income… I use that money to be able to help me survive’ (P4061). CATCH‐FI participants assigned to the usual care arm similarly described that receiving FI would be useful in directly enabling individuals to meet basic needs that are essential to health and well‐being:I do think [financial incentives] are really, really… useful because there comes a point when you're too broke to do anything. Like it's all well and good that you've made all these appointments and you have all these dreams but when it really comes down to the nitty gritty, if you don't have money, you can't afford to do it. You can't afford to go see your doctor because you can't afford bus fare. You can't afford to get a job because you can't afford interview clothes. (P4031)CATCH service providers and key informants similarly highlighted the perceived utility of FI in meeting basic needs to support health and well‐being: ‘I think that realistically, incentives do support people being able to maintain their wellness and engage in care’ (HP5). For example, CATCH service providers described how incentives in this study were used practically by CATCH‐FI participants ‘to get from one appointment to the next and to accomplish some of these very important tasks that they need to do: to get their ID, or meet up with a doctor… get their medication’ (FG5). Overall, this stakeholder group generally agreed that the use of FI can positively support the health and well‐being of this population insofar as ‘anything that may be able to help people stabilize those basic needs is critical to supporting their ability to engage in slightly less pressing health and social care’ (HP6).Allowing for simple pleasures: Similar to meeting basic needs, FI allowed CATCH‐FI participants to experience simple pleasures, small gains or rewards that directly improved their mental health and well‐being by relieving stress or giving them an enjoyable experience to anticipate. CATCH‐FI participants enroled in the intervention arm described the positive emotions experienced after spending their incentives on small items, such as coffee, restaurant food and cigarettes; one individual reported purchasing illegal drugs with the incentive. Overall, participants in this study consistently described perceived positive impacts on mental health and well‐being as a direct result of receiving an FI. For example, one individual described their experience as follows:It makes you feel better when you have money to spend on yourself… With the financial incentive, I found it gave me something to look forward to. It's like, ‘Yeah, I can have McDonald's and give myself a little treat’… It helped better my life. It helped pull me ahead, basically. (P4010)CATCH‐FI participants in the usual care arm similarly perceived that a financial incentive would improve their mental health and well‐being, suggesting that ‘if you don't feel so destitute and broke all the time, it's gonna lift up your spirits, for sure’ (P4057). While many CATCH‐FI participants spoke of the importance of simple pleasures and their impact on well‐being, CATCH service providers and key informants did not describe this potential utility of FI for the study population.4This qualitative study explored stakeholder perspectives on the use of FIs to support service engagement and improve health and well‐being among adults experiencing homelessness and mental illness in a large urban centre in Canada. Consistent with prior research, our findings suggest that FI may successfully facilitate service engagement for some service users. A subset of service users in this study described the prospect of immediate financial gain as an externally motivating factor that encouraged continued contact with service providers. These findings are consistent with behavioural health economics principles, which suggest extrinsically motivated individuals and those particularly biased toward present and immediate rewards may be especially likely to respond to FI.Our findings are unique, however, in identifying that the majority of service users perceived FI to be less impactful than other key facilitators of service engagement. In particular, service users highlighted the relatively greater value of intrinsic motivation, which renders FI extraneous. This finding aligns with self‐determination theoryOur study findings also speak to the importance of quality of care, which emerged as another key facilitator of service engagement across all stakeholder groups. Many service users in this study described how perceived quality of programming and quality of relationships with case managers independently motivated their decision to engage above and beyond the influence of FI. This finding reiterates the central role of high‐quality, client‐centred and relationship‐based models of care in sustaining engagement in this population. And while our findings indicate quality in and of itself motivates engagement, literature also suggests high‐quality services can support engagement by facilitating intrinsic motivation.Taken together, particularly among the subset of service users who might want or need an extrinsic motivator to support initial engagement, our findings suggest that FI may offer an effective opportunity to initially engage or ‘hook’ some service users; and that a complementary focus on the quality of care by service providers may further help sustain engagement. The need to enhance sustainability is consistent with findings from a recent qualitative study in which FI were associated with initial motivation to engage in low‐barrier human immunodeficiency virus care but were less effective in facilitating sustained engagementBeyond the direct influence on service engagement, participant narratives consistently described clear, positive impacts of FI on health and well‐being. Primarily, participants described that FI enabled them to afford basic needs  and to enjoy simple pleasures  that were in turn perceived to directly support their physical and mental health and overall sense of well‐being. That only one service user reported using their FI to purchase illegal drugs is consistent with previous researchA final notable finding in this study is the divergence in stakeholder perspectives. Overall, service providers consistently expressed the view that FI would likely support service engagement of adults experiencing homelessness and mental health needs. This perspective reflected the assumption that the perceived utility of FI, specifically among this population who lack basic health, social and financial capital, was or would be high and sufficiently motivating to independently drive service engagement. This stakeholder perspective stands in significant contrast to the majority perspective among service user participants, two‐thirds of whom articulated that FI alone were not or would not be a significant influence on their decision to engage in care. This divergence suggests stakeholders may differentially perceive utility and ascribe value to FI. Further research and a better understanding of service user perspectives on the extent to which, how and why FI are useful will be essential to designing and implementing acceptable client‐centred and context‐specific interventions to promote engagement.5This study is part of a larger mixed‐methods RCT and our findings are strengthened by a methodologically rigorous design. Our resulting rich, multistakeholder narrative data add high‐quality evidence to an underdeveloped literature base and significantly improve our understanding of the perceived impact of using FI to support engagement, health and well‐being in an underserved population. Our findings from a large urban centre are context‐specific and reflect the experiences and potential biases of study participants. Nonetheless, our findings may be helpful to other programmes or jurisdictions considering alternative strategies to improve engagement among people experiencing homelessness and mental illness and other underserved populations.Study limitations include the cross‐sectional design and the narrow demographics of the qualitative sample, which was primarily male, Caucasian and at least high school‐educated. Differences between this qualitative sample and the overall trial sample may reflect, in part, the purposive sampling strategy. The study team made several attempts to ensure a representative sample, including through the expansion of eligibility criteria, but recruitment remained challenged by reasons such as poor health status, nonresponse to invitations to participate and refusal to participate. Although sample characteristics were generally representative of the larger trial population, participants with different sociodemographic, health status and service engagement profiles may perceive and experience FI differently.5.1Given the paucity of research in this area, and the need for interventions to improve service engagement of underserved and disadvantaged populations, further research is warranted to compare and contrast various engagement strategies using rigorous methods and large study samples. In particular, additional research should investigate how FI may be timed and dosed to enhance other facilitators of service engagement, such as intrinsic motivation and quality of care. Further, in addition to high‐quality evidence on effectiveness, more research is needed on the ethicality, acceptability and feasibility of this approach within and across diverse subpopulations and service settings. Ethical concerns, including perceived coercion and unintended consequences, remain key barriers that have resulted in underrepresentation in research and practice of a promising intervention.5.2Key stakeholders described that FIs may improve service engagement among some adults experiencing homelessness and mental illness. Stakeholders also described that FIs positively impact health and well‐being, easing financial stress and enabling deeper attention to individual health needs. Findings from this study add to an underdeveloped literature base on stakeholder perspectives and experiences of using FIs to improve engagement, health and well‐being of homeless adults with mental health needs.All authors contributed significantly to this manuscript. Nadine Reid led the interpretation of results and drafted the manuscript. Rebecca Brown and Cheryl Pedersen participated in study coordination, data collection and analysis and editing of the manuscript. Nicole Kozloff and Alexandra Sosnowski contributed meaningful edits to the manuscript. Vicky Stergiopoulos is the primary investigator who overall conceived of and supervised this study and drafting of the manuscript.


In contrast, for the thermodynamically more stable germylenes and stannylenes an alternative mechanism involving the active participation of the ylide ligand in the E−H bond (E=H or PhO) activation process by addition across the element carbon linkage was found to be energetically favored. Furthermore, the boryl substituted tetrylenes allowed for a further activation pathway involving the active participation of the boron element bond. These cooperative mechanisms are especially attractive for the heavier cyclic ylidyltetrylenes in which the loss of the protonated ylide group is prevented due to the cyclic framework. Overall, the present studies suggest that cyclic ylide‐substituted germylenes and stannylenes bear huge potential for cooperative bond activations at mild conditions which should be experimentally addressed in the future.Due to their transition metal‐like behavior divalent group 14 compounds bear huge potential for their application in bond activation reactions and catalysis. Here we report on detailed computational studies on the use of ylide‐substituted tetrylenes in the activation of dihydrogen and phenol. A series of acyclic and cyclic ylidyltetrylenes featuring various α‐substituents with different  Computational studies on the use of ylide‐functionalized heavier carbenes  in bond activation reactions revealed that these species can undergo both, single‐site as well as cooperative activation processes. The latter involve the active participation of the ylide moiety and are particularly attractive for the more stable germylenes and stannylenes, thus opening new perspectives for future catalyst design. Thus, besides alkyl moieties, also silyl or more lately boryl groups have been used in this chemistry.The singlet‐triplet gap and the HOMO‐LUMO separation were found to be a valuable measures for the ability of carbenes and related species to engange in bond activation reactions. In general, small HOMO‐LUMO gaps lead to a higher activity of tetrylenes towards small molecules as was already demonstrated by Bertrand and coworkers by means of the different behavior of N‐heterocyclic carbenes (NHCs) compared to cyclic alkyl(amino)carbenes (CAACs) towards dihydrogen.12α‐substituent (cooperative) have been described in literature.3 via arene elimination .4, 5 Ts‐block metal yldiides we were able to isolate the diylide‐substituted germylene and stannylene 5 and 6 which exhibited unusual electronic properties.[π‐donation from the amino group into the empty p‐orbital at the group 14 element, 5 and 6 exhibited an alignment of all three lone pairs in the C−E−C linkage thus leading to remarkably increased HOMO and LUMO levels and donor strengths. These properties led us to investigate the potential of ylide‐substituted tetrylenes in bond activation reactions. Since attempts to use 5 and 6 in the activation of dihydrogen remained unsuccessful so far, we set out to evaluate the properties of differently substituted ylidyltetrylenes of type YER . Recent calculations by Phukan and coworkers showed that cyclic amino(ylide) substituted tetrylenes exhibit reasonable low barriers for the activation of small molecules, but usually higher LUMO energies than diaminotetrylenes.[Recently, our group has focused on the use of ylides as substituents in main group chemistry.14, 15push‐pull tetrylenes are promising candidates for bond activations. Most importantly, not only single‐site activations at the group 14 element but also bifunctional bond activation pathways were found to be viable processes.Here, we show that particularly cyclic and acyclic 5 was also performed with the PBE0All calculations were performed without symmetry restrictions using the Gaussian16 Revision B.01TosY was chosen due to the facile synthesis of these compounds via the corresponding metalated ylide, whereas the aryl substituted ylides PhY and FY were selected due to their simple design. As second substituents R, a selection of commonly applied electron‐donating and electron‐withdrawing groups were used to study the impact of different electronic properties on the activity of the compounds towards E−H bond activation. Besides a second ylide group, a simple chloro substituent as well as the “classical“ bis(trimethylsilyl)amide group (HMDS) already employed by Lappert in the 1970s1B and 2B were included in this study, since a 1B‐like substituent was previously described to significantly lower the activation barrier for H2‐splitting with germylenes.6F5), pyrimidine (Pyr) and pyridine (Py) were investigated to further examine push‐pull‐effects in ylidyltetrylenes.To study the propensity of ylide‐substituted tetrylenes to undergo bond activations we at first chose a set of tetrylenes with three different ylide groups Figure . The sulTosY‐substituted tetrylenes, the tosyl group coordinates to the group 14 element thus thermodynamically stabilizing the tetrylene. This conformation results – in accordance with previous experimental observationsPhY and FY substituted systems feature the expected co‐planar arrangement. This allows for a stabilizing π‐interaction between the ylide and the group 14 element and hence in shorter C−E distances compared to the corresponding TosYER systems. However, the aryl groups in the ylide backbone of these tetrylenes are not in plane but rotated out of the E−P−C plane, thus preventing further delocalization of electron density into the aromatic substituent. This arrangement has also been observed in other ylide‐substituted main group compounds such as phosphenium cations3 group. Instead, the aryl groups in the ylide backbone arrange parallel to aromatic groups of the R substituents , thus resulting in stabilizing π‐interactions  were found for es Table . Lastly,ST was shown to correlate with the propensity of tetrylenes to undergo bond activations.ST for all compounds, which are given in Table Alike the HOMO and LUMO energies the energy of the singlet‐triplet gap EST increases from the silylenes over the germylenes to the stannylenes. This is the expected trend according to hybridization defects within the groups of the main group elements. However, it must be noted that the formal excitation of an electron must not necessarily take place at the tetrylene centre .ST should be independent of the central atom. This divergent behaviour is predominant for tetrylenes with a high electron density at the group 14 element and for those stabilized by a tosyl group due to the coordination of the oxygen atoms and the elevation of the LUMO energy.The EST decreases from the silylene over the germylene to the stannylene. This presumably is the result of a formal excitation of an electron from the lone pair at the ylidic carbon atom either into the vacant orbital at the group 14 element or in the ylide backbone. Since the lone pair at the ylidic carbon atom is less stabilized with decreasing electronegativity of the central element, the EST decreases when going down the group. This is also in line with the calculated NBO charges  the “classical” splitting at the tetrylene centre5, 34 Ph2YSnB) and 312 kJ/mol (for TosYSnHMDS) for the single‐site activation (Pathway A), the silylenes the lowest (77.6–174.5 kJ/mol). This corroborates with previous studies on other tetrylenes.2 activation, whereas the use of electron withdrawing groups, especially boryl substituents, results in lower barriers. As a result, the lowest barrier of only 78 kJ/mol is found for Ph2YSiB. This barrier is sufficiently low for dihydrogen activation at room temperature. The influence of the nature of the ylide on the activation energies is somewhat smaller than the impact of the R substituent, but still significant. As such, PhY in general provides lower reaction barriers than the electron poorer FY and TosY, especially for heavier tetrylenes. Regarding the thermodynamics of the activation processes it is noteworthy that in contrast to the silylenes and germylenes, most of the reaction processes of the stannylenes are thermodynamically uphill due to the high stability of the +2 oxidation state of tin.In line with the calculated singlet‐triplet and HOMO‐LUMO gaps, the stannylenes showed the highest barriers between 190 . The most distinct difference of the cooperative bond activation pathways (pathway B and C) compared to the single‐site bond activations is the independence of their activation barriers and reaction energies of the group 14 element. While for the activation of H2 at the element center the reaction barriers clearly decrease from Sn to Si and the energy gain increases in the same direction, only small energetic differences can be observed for the cooperative pathways. Thus, the reaction energies for pathway B and C only vary by 30 kJ/mol with the group 14 element, whereas for the single‐site processes differences of even more than 100 kJ/mol can be observed.Next, we addressed the cooperative dihydrogen activation via pathway B and C for a selection of tetrylenes. Table PhYER, the activation energies for pathway B lie between 115 and 150 kJ/mol and are thus slightly too high to be overcome at room temperature but should be accessible at elevated temperatures. This is especially interesting for the thermodynamically more stable germylenes and stannylenes, which exhibit much higher barriers via pathway A.The transition state energies of pathway B and C are in general in the range of those for the single‐site activation for the silylenes. Thus, these pathways are particularly kinetically favored over the single‐site mechanism for the germylenes and stannylenes. In case of the silylenes the barriers of all processes A−C are in the same range. For the acyclic tetrylenes PhYER depend only little on the nature of substituent R, while the differences are greater in the tosyl compound. Similar to the single‐site process the lowest activation barriers are found for the systems with a boryl group. Comparison of the values of the two different acyclic tetrylenes PhYER and TosYER  locate the transition states for all three different pathways A–C. Here, the cooperative bond activation via addition of H2 across the Si−B linkage was found to be the most favourable process. In general, pathway C exhibited very low activation barriers of up to only 41 k/mol for SiV, thus making this process particularly facile and also thermodynamically favourable. In some cases, also the barrier for the reverse reactions (H2 elimination) was found to require less than 130 kJ/mol thus suggesting that also reversible activation processes should be possible. It is also noteworthy that the reaction barriers for the sulfonyl substituted derivatives IV are significantly higher than those for the Ph‐substituted ylide V. This can be explained by the additional O→B‐interaction in IV which stabilizes the tetrylene and thus increases ΔGTS.The cyclic tetrylenes Table  showed spush‐pull systems such as the boryl substituted compounds exhibit low barriers. These compounds even seem to be attractive target systems for reversible hydrogenation processes and thus for catalytic applications. However, to this end, cyclic tetrylenes need to be constructed to prevent ligand dissociation as was already experimentally observed for acyclic ylide‐substituted tetrylenes.Overall, the comparison of the different pathways A−C with the cyclic and acyclic ylide‐substituted tetrylenes clearly shows that the cooperative bond activation reactions are clearly favoured for the stannylenes and germylenes. In particular, I–V and investigated the three different mechanisms discussed above for the activation of phenol  compounds . These compounds are not only capable of activating H−H and O−H bond via single site processes at the divalent group 14 element centre, but also via cooperative mechanisms with the ylidic carbon atom acting as basic site. Accordingly, the reactivity of these species is also influenced by the substituent in the ylide backbone, which thus offers a further handle for reactivity control. Overall, the cooperative activation processes are particularly beneficial for germylenes and stannylenes which usually are more stable than their silicon counterparts and hence feature higher barriers in the “classical” single‐site activation processes. Since the bifunctional bond activation via protonation of the ylide would result in the cleavage of the element‐carbon bond, these processes are especially interesting for cyclic tetrylenes to prevent destructive elimination of the ylide substituent. For the boryl‐functionalised compounds also a further cooperative activation pathway was found to be a viable alternative in which the tetrylene centre acts as base and boron as acid.Overall, the presented calculations suggest that ylide‐substituted heavier carbenes are promising main group systems for reversible bond activations via element ligand cooperation. Experimental endeavours should particularly focus on the construction of cyclic compounds due to their higher stability towards elimination of the ylide.The authors declare no conflict of interest.As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.Supporting InformationClick here for additional data file.


DELLA proteins are master growth regulators that repress responses to a group of plant growth hormones called gibberellins (GAs). Manipulation of DELLA function and signaling was instrumental in the development of high‐yielding crop varieties that saved millions from starvation during the “Green Revolution.” Despite decades of extensive research, it is still unclear how DELLA function and signaling mechanisms evolved within the land plant lineage. Here, we review current knowledge on DELLA protein function with reference to structure, posttranslational modifications, downstream transcriptional targets, and protein–protein interactions. Furthermore, we discuss older and recent findings regarding the evolution of DELLA signaling within the land plant lineage, with an emphasis on bryophytes, and identify future avenues of research that would enable us to shed more light on the evolution of DELLA signaling. Unraveling how DELLA function and signaling mechanisms have evolved could enable us to engineer better crops in an attempt to contribute to mitigating the effects of global warming and achieving global food security. DELLA protein evolution in land plants. DELLA protein‐encoding genes first appeared in the ancestor of land plants. There have been subsequent gene duplications in the vascular plant ancestor and in eudicot flowering plants, with gene losses in ferns and some flowering plant families. The transactivation ability of DELLA proteins originated in the ancestor of land plants, while canonical gibberellin (GA) signaling arose in the ancestor of vascular plants. DELLA protein regulation is well‐researched in flowering plants but more research is required to determine whether regulatory mechanisms are conserved in early‐diverging lineages. DELLA genes first appeared in the common ancestor of land plants and underwent two major duplications during land plant evolution.DELLAs repress gibberellin responses in vascular plants but their function in nonvascular plants remains elusive. This domain is important for their regulatory function and is absent in other GRAS family proteins such as 3DELLA protein‐encoding genes: the first one in the ancestor of vascular plants and the second one in eudicot flowering plants  are low  in Arabidopsis, which form part of a SKIP1‐CUL1‐F‐box (SCF) E3 ligase complex, to bind DELLAs and polyubiquitinate them , which can bind and stabilize DELLA during daytime under short‐day conditions, thus regulating the diurnal rhythmic accumulation pattern of DELLA proteins  dominant gain‐of‐function mutants, which render the DELLA protein unable to be degraded and give rise to GA‐insensitive dwarf plants, and (ii) loss‐of‐function mutants lacking DELLA activity, which give rise to slender plants with constitutively activated GA responses , a site for posttranslational modifications (PTMs) that affect the stability and activity of DELLA proteins Figure . Plant dTarht‐B1b and Tarht‐D1b revealed that they have nucleotide substitutions that generate a stop codon in the N‐terminal domain  via ubiquitination in a GA‐independent manner upon exposure to warm temperatures or shade  to promote flowering under long‐day conditions  mutant background (Atgai‐t6 Atrga‐t2 Atrgl1‐1 Atrgl2‐1 Atga1‐3) and compared gene expression with an Atga1‐3 mutant line, to identify DELLA‐induced transcriptional changes occurring in imbibed seeds and unopened flower buds. As germination in Arabidopsis is regulated by AtRGL2 primarily, as well as AtGAI and AtRGA1, and flowering by AtRGL1, AtRGL2, and AtRGA1, the choice of the quadruple knockout should have been sufficient to enable identification of gene targets regulated by DELLAs during these developmental stages , genes involved in protein phosphorylation, genes encoding transcription factors belonging to the MYB, bHLH, WRKY, and MADS‐box families, and genes regulating responses to disease, stress, and hormones ‐inducible system. This experiment stimulated rapid and high induction of stable AtRGA1, aiming to identify early transcriptional changes that are more likely to be directly induced by AtRGA1 , as well as genes encoding E2 conjugating enzymes and E3 ligases, such as AtXERICO, which is activated by DELLA to induce abscisic acid (ABA) biosynthesis  domain under the control of the AtGAI1 promoter. Activation of expression by either heat‐shock induction or treatment with DEX and cycloheximide (CHX), led to the identification of early transcriptional targets involved in processes such as GA homeostasis, stress responses, and hormone signaling and biosynthesis , regulating seed dormancy  regulating root meristem identity ; however, DELLAs have not shown any direct DNA binding  as bait to screen a library of 1956 Arabidopsis transcriptional regulators for DELLA interaction using yeast‐two hybrid  were the first transcription factors identified to be repressed via interaction with DELLA proteins, establishing a mechanism by which flowering plants are able to integrate light and GA signaling to regulate hypocotyl elongation , which promote GA‐induced etiolation in darkness  to prevent the latter from binding to transcription factors, such as AtPIF3 and BRASSINAZOLE‐RESISTANT 1 (AtBZR1), and promoting histone H3 lysine‐27 trimethylation (H3K27me3) on promoters of target genes regulating GA‐ and brassinosteroid‐induced hypocotyl elongation.In addition to transcription factors, DELLA proteins can also sequester chromatin remodeling factors to inhibit GA responses. Zhang et al.  demonstrOsSLR1 interacts with NO APICAL MERISTEM 29 (OsNAC29) and OsNAC31 to prevent binding to OsNAC targets such as OsMYB61, which induces CELLULOSE SYNTHASE (OsCESA) genes that promote secondary wall cellulose synthesis required for internode development  and AtABI5 and coactivate transcription of SOMNUS (AtSOM), which induces ABA biosynthesis and represses GA biosynthesis, forming a positive feedback loop to inhibit seed germination at high temperatures  and MtNF‐YA1 to coactivate the expression of ETHYLENE RESPONSIVE FACTOR REQUIRED FOR NODULATION1 (MtERN1), which induces downstream gene expression required for the progression of rhizobial infections , a negative regulator of jasmonate (JA) responses , to inhibit GA responses such as germination, flowering, and juvenile‐to‐adulthood phase transition via binding to promoters of GA‐inducible genes such as EXPANSIN 8 (AtEXPA8), PACLOBUTRAZOL RESISTANCE 1 (AtPRE1), and AtPRE5  promoter  in P. patens; however, these proteins lack the defining features of true flowering plant GID1s, such as the catalytic triad forming the GA pocket or the N‐terminal lid required for interaction with DELLA , which has lost the DELLA domain, but is still able to induce dwarfism and remain stable in the presence of GA , P. patens (bryophyte), and Chlamydomonas reinhardtii , suggested that the function of PpDELLAs was most likely to regulate stress responses, and that coordination between the functions regulated by DELLAs increased during the course of evolution  and ent‐kaurene synthase (KS)—although bryophytes possess bifunctional enzymes (CPS/KS)—that catalyze the conversion of trans‐geranylgeranyl diphosphate (GGPP) into ent‐kaurene, as well as ent‐kaurene oxidase (KO), which oxidizes ent‐kaurene into ent‐kaurenoic acid , which is not encoded by the P. patens or S. fallax genomes  could not interact with AtGID1s in yeast cells in a GA‐dependent manner, DELLA from the hornwort N. vincentianus was able to interact with AtGID1s in a GA‐dependent manner, suggesting that DELLA affinity for GID1 homologs may have evolved as early as the hornwort divergence. As hornworts appear to have diverged earliest in the bryophyte lineages  in a very specific tissue type , where selection would favor tight specificity of LjGID1 for GA4 to ensure proper sex organ development.In addition, yeast two‐hybrid assays by Yasumura et al.  showed t6.3SmGID1s were able to complement the function of OsGID1 in the Osgid1‐3 mutant and SmDELLAs were able to repress growth in wild‐type rice, whereas P. patens homologs could not , which also inhibits the biosynthesis of ent‐kaurenoic acid, induces a growth phenotype that cannot be rescued by exogenous application of GA3, suggesting that a diterpene signaling pathway regulating growth exists in P. patens  of the steroid hormone aldosterone, regulating electrolyte homeostasis, evolved an affinity for the hormone before aldosterone had actually emerged (Bridgham et al., PpDELLAs, and by extension early land plant DELLAs, was most likely to regulate stress responses (Briones‐Moreno et al., PpdellaAB mutant performs under various forms of stress. Experiments by Yasumura et al. (PpdellaAB is sensitive to salt stress. Whether this is the case for other bryophytes or other forms of stresses is unknown. Future studies should concentrate on using bryophyte species with sequenced genomes as well as other emerging model species, to carry out more in vivo genetic and biochemical studies to shed more light on the evolution of DELLA signaling in land plants. Analysis of additional bryophyte and charophyte genomes or transcriptomes may pinpoint the emergence of DELLAs more accurately.The question of how DELLA proteins arose in the very first land plants, and what the selection pressures were that retained them during early land plant evolution, still remains unanswered. In silico comparative gene co‐expression network analysis has suggested the hypothesis that the function of a et al.  have shoDELLA alleles or by manipulating some of the DELLA interaction partners could be a potential avenue for enabling production or breeding of a new generation of resilient land plants.Understanding how DELLA signaling mechanisms have evolved and how DELLAs respond to stress and other environmental signals could enable us to engineer better crops, to contribute to mitigating the effects of global warming, and achieving global food security. As DELLA proteins function via protein–protein interactions, targeting their interaction capacity by either identifying novel The authors declare that there are no conflict of interests.


The goal of CASP experiments is to monitor the progress in the protein structure prediction field. During the 14th CASP edition we aimed to test our capabilities of predicting structures of protein complexes. Our protocol for modeling protein assemblies included both template‐based modeling and free docking. Structural templates were identified using sensitive sequence‐based searches. If sequence‐based searches failed, we performed structure‐based template searches using selected CASP server models. In the absence of reliable templates we applied free docking starting from monomers generated by CASP servers. We evaluated and ranked models of protein complexes using an improved version of our protein structure quality assessment method, VoroMQA, taking into account both interaction interface and global structure scores. If reliable templates could be identified, generally accurate models of protein assemblies were generated with the exception of an antibody‐antigen interaction. The success of free docking mainly depended on the accuracy of initial subunit models and on the scoring of docking solutions. To put our overall results in perspective, we analyzed our performance in the context of other CASP groups. Although the subunits in our assembly models often were not of the top quality, these models had, overall, the best‐predicted intersubunit interfaces according to several accuracy measures. We attribute our relative success primarily to the emphasis on the interaction interface when modeling and scoring. Template‐based modeling is based on the observation that homologous proteins often interact in the same way.The field of protein structure prediction is monitored in the Critical Assessment of Structure Prediction (CASP) experiments that explore every aspect of protein structure modeling.We participated in recent CASP and CAPRI experiments, aiming to test our abilities to predict structures of protein complexes using template‐based modeling and free docking.To explore these questions in detail, we participated in the CASP14 experiment, where our group (“Venclovas”) performed relatively well, particularly in the interface prediction. In this article we describe our modeling methods and analyze our results in detail aiming to understand what went right, what went wrong and why.22.1The outline of our modeling workflow in CASP14 is shown in Figure 2.2The outline of multimeric comparative modeling pipeline is provided in 2.3For some targets neither sequence‐based nor structure‐based searches produced alignments with multimeric homologs of sufficient reliability and/or coverage such that these alignments could be used for multimeric comparative modeling. However, in some cases we observed that the proteins identified as potential multimeric templates had similar functional annotations as the CASP targets. If the alignment of CASP server models to these multimeric structures using TM‐align2.4Several targets were predicted to be coiled coils using MultiCoil22.5When no templates could be found for protein complexes, free docking of top five selected monomeric CASP server models was done by Hex2.6For some large target protein complexes structural templates were available only for some of the subunits or domains. In these cases, a hybrid modeling strategy was used, that is, part of the complex was modeled using comparative modeling, whereas remaining subunits were docked to it either by template‐based docking using TM‐align2.7For model selection we used both global structure scores and interface scores. This approach was described previously2.8https://kliment-olechnovic.github.io/voronota/expansion_js/.VoroMQA‐dark is a new model quality assessment method based on the previously published VoroMQA33.1To analyze our performance in CASP14 we used several accuracy measures designed to evaluate various features of multimeric models. For the overall model evaluation we used QS‐score, a distance‐based measure of interface accuracy.The summary of our modeling results based on the QS‐score is presented in Table 3.2z scores of two interface accuracy measures (ICS and IPS) and two global structure accuracy measures (lDDT and TM‐score).The results, presented in Table The comparison, shown in Figure z scores. z Score values were accumulated progressively for targets ordered by the maximum ICS value of all the models produced by any group for a given target. Such an ordering may be interpreted as an estimate of the target difficulty. Figure To look at different features in more detail, we examined per‐target z score differences resulted from small structural improvements  having two different interfaces (T1099v1 and T1099v2). Our models contained high‐accuracy interface 2, yet the interface 1 was incorrect . The rea3.4When modeling a large protein complex templates might be available just for some parts of it. During CASP14 this was the case for heteromeric targets H1060 and H1097. For both of these targets more accurate models were generated for those parts that had templates.H1060 was a viral protein complex containing 27 subunits  using a higher number of diverse input monomers, for example, generated by modeling domain motions and by remodeling flexible loops and tails; (2) ensuring that the docking software always performs a sufficiently exhaustive sampling of conformations; (3) producing structural variations of each oligomeric docking solution using molecular dynamics or other sampling techniques. These enhancements would allow to explore the conformational space more thoroughly, possibly leading to better results.Despite the limitations of our CASP14 modeling protocol, our strong performance suggests that the prediction of interchain contacts using coevolution and deep learning methods still has little impact on modeling of protein‐protein interactions. Why is that? Apparently, there are multiple reasons why interchain contact prediction is harder than intrachain. For example, contact prediction for heteromeric protein complexes requires generating joined multiple sequence alignments. The interacting proteins in the alignment are inferred by genomic distances or by phylogeny,In addition to the issues related to obtaining and analyzing the multiple sequence alignments, training of supervised learning‐based methods for contact prediction using the structures of protein complexes may be limited by the availability of experimental structural data. The number of possible protein complexes is believed to outnumber the number of possible protein folds,Modeling of structures for individual proteins is highly automated, and multiple structure prediction servers are available in CASPTo conclude, the progress in monomeric protein structure prediction has not yet translated into similar breakthrough in structural modeling of protein complexes. A number of issues of both technical and fundamental nature have to be solved to make a leap in producing reliable structural models of protein interactions, and it will be exciting to see what developments will occur in this research area in the nearest future.https://publons.com/publon/10.1002/prot.26167.The peer review history for this article is available at Appendix S1: Supporting informationClick here for additional data file.