Mid- to Low-Frequency Fourier Transform Infrared Spectra of S-State Cycle for Photosynthetic Water Oxidation inSynechocystissp. PCC 6803

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Abstract
Flash-induced Fourier transform infrared (FTIR) difference spectra for the four-step S-state cycle and the effects of global 15N- and 13C-isotope labeling on the difference spectra were examined for the first time in the mid- to low-frequency (1200−800 cm-1) as well as the mid-frequency (1700−1200 cm-1) regions using photosystem (PS) II core particles from cyanobacterium Synechocystis sp. PCC 6803. The difference spectra clearly exhibited the characteristic vibrational features for each transition during the S-state cycling. It is likely that the bands that change their sign and intensity with the S-state advances reflect the changes of the amino acid residues and protein matrices that have functional and/or structural roles within the oxygen-evolving complex (OEC). Except for some minor differences, the trends of S-state dependence in the 1700−1200 cm-1 frequency spectra of the PS II cores from Synechocystis were comparable to that of spinach, indicating that the structural changes of the polypeptide backbones and amino acid side chains that occur during the oxygen evolution are inherently identical between cyanobacteria and higher plants. Upon 13C-labeling, most of the bands, including amide I and II modes and carboxylate stretching modes, showed downward shifts; in contrast, 15N-labeling induced isotopic shifts that were predominantly observed in the amide II region. In the mid- to low-frequency region, several bands in the 1200−1140 cm-1 region were attributable to the nitrogen- and/or carbon-containing group(s) that are closely related to the oxygen evolution process. Specifically, the putative histidine ligand exhibited a band at 1113 cm-1 which was affected by both 15N- and 13C-labeling and showed distinct S-state dependency. The light-induced bands in the 900−800 cm-1 region were downshifted only by 13C-labeling, whereas the bands in the 1000−900 cm-1 region were affected by both 15N- and 13C-labeling. Several modes in the mid- to low-frequency spectra were induced by the change in protonation state of the buffer molecules accompanied by S-state transitions. Our studies on the light-induced spectrum showed that contributions from the redox changes of QA and the non-heme iron at the acceptor side and YD were minimal. It was, therefore, suggested that the observed bands in the 1000−800 cm-1 region include the modes of the amino acid side chains that are coupled to the oxidation of the Mn cluster. S-state-dependent changes were observed in some of the bands.

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