FTIR Detection of Structural Changes in a Histidine Ligand during S-State Cycling of Photosynthetic Oxygen-Evolving Complex

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Abstract
Changes in structural coupling between the Mn cluster and a putative histidine ligand during the S-state cycling of the oxygen-evolving complex (OEC) have been detected directly by Fourier transform infrared (FTIR) spectroscopy in photosystem (PS) II core particles from the cyanobacterium Synechocystis sp. PCC6803, in which histidine residues were selectively labeled with l-[15N3]histidine. The bands sensitive to the histidine-specific isotope labeling appeared at 1120−1090 cm-1 in the spectra induced upon the first-, second-, and fourth-flash illumination, for the S2/S1, S3/S2, and S1/S0 differences, at similar frequencies with different sign and/or intensity depending on the respective S-state transitions. However, no distinctive band was observed in the third-flash induced spectrum for the S0/S3 difference. The results indicate that a single histidine residue coupled with the structural changes of the OEC during the S-state cycling is responsible for the observed histidine bands, in which the histidine modes changed during the S0-to-S1 transition are reversed upon the S1-to-S2 and S2-to-S3 transitions. The 1186(+)/1178(−) cm-1 bands affected by l-[15N3]histidine labeling were observed only for the S2/S1 difference, but those affected by universal 15N labeling appeared prominently showing a clear S-state dependency. Possible origins of these bands and changes in the histidine modes during the S-state cycling are discussed.

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