Excitation energy transfer in the LHC-II trimer: from carotenoids to XXXX in space and time.

@entity38 @entity51092 @entity2772
Exciton model for description of experimentally determined excitation energy transfer from carotenoids to @entity38 in the LHC-II trimer of @entity2772 is presented. Such an approach allows connecting the excitonic states to the spatial structure of the complex and hence descriptions of advancements of the initially created excitations in space and time. Carotenoids were excited at 490  nm and at 500  nm and induced absorbance changes probed in the Chl Q(y) region to provide kinetic data that were interpreted by using the results from exciton calculations. Calculations included the 42 @entity38 and the 12 carotenoids of the complex, Soret, Q(x) and Q(y) states of the @entity38 , and the main absorbing S(2) state of the carotenoids. According to the calculations excitation at 500  nm populates mostly a mixed Lut S(2) Chl a Soret state, from where excitation is transferred to the Q(x) and Q(y) states of the Chl a's on the stromal side. Internal conversion of the mixed state to a mixed Lut S(1) and Chl a Q(y) state provides a channel for Lut S(1) to Chl a Q(y) energy transfer. The results from the calculations support a picture where excitation at 490  nm populates primarily a mixed @entity51092 S(2) Chl b Soret state. From this state excitation from @entity51092 is transferred to iso-energetic Chl b Soret states or via internal conversion to S(1) Chl b Q(y) states. From the Soret states excitation proceeds via internal conversion to Q(y) states of Chl b's mostly on the lumenal side. A rapid Chl b to Chl a transfer and subsequent transfer to the stromal side Chl a's and to the final state completes the process after 490  nm excitation. The interpretation is further supported by the fact that excitation energy transfer kinetics after excitation of @entity51092 at 490  nm and the Chl b Q(y) band at 647  nm (Linnanto et al., Photosynth Res 87:267-279, 2006) are very similar.
