Low-frequency fourier transform infrared spectroscopy of the @entity26 -evolving and XXXX acceptor complexes in photosystem II.

@entity3276 @entity26 @entity921
The low-frequency (<1000 cm-1) region of the IR spectrum has the potential to provide detailed structural and mechanistic insight into the photosystem II/ @entity26 evolving complex (PSII/OEC). A cluster of four @entity921 ions forms the core of the OEC and diagnostic @entity921 -ligand and @entity921 -substrate modes are expected to occur in the 200-900 cm-1 range. However, water also absorbs IR strongly in this region, which has limited previous Fourier transform infrared (FTIR) spectroscopic studies of the OEC to higher frequencies (>1000 cm-1). We have overcome the technical obstacles that have blocked FTIR access to low-frequency substrate, cofactor, and protein vibrational modes by using partially dehydrated samples, appropriate window materials, a wide-range MCT detector, a novel band-pass filter, and a closely regulated temperature control system. With this design, we studied PSII/OEC samples that were prepared by brief illumination of O2 evolving and Tris-washed preparations at 200 K or by a single saturating laser flash applied to O2 evolving and inhibited samples at 250 K. These protocols allowed us to isolate low-frequency modes that are specific to the QA-/QA and S2/S1 states. The high-frequency FTIR spectra recorded for these samples and parallel EPR experiments confirmed the states accessed by the trapping procedures we used. In the S2/S1 spectrum, we detect positive bands at 631 and 602 cm-1 and negative bands at 850, 679, 664, and 650 cm-1 that are specifically associated with these two S states. The possible origins of these IR bands are discussed. For the low-frequency QA-/QA difference spectrum, several modes can be assigned to ring stretching and bending modes from the neutral and anion radical states of the @entity3276 acceptor. These results provide insight into the PSII/OEC and demonstrate the utility of FTIR techniques in accessing low-frequency modes in proteins.
