Flash-Induced Fourier Transform Infrared Detection of the Structural Changes during the S-State Cycle of the Oxygen-Evolving Complex in Photosystem II

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Abstract
Fourier transform infrared (FTIR) difference spectra of all flash-induced S-state transitions of the oxygen-evolving complex were measured using photosystem II (PSII) core complexes of Synechococcus elongatus. The PSII core sample was given eight successive flashes with 1 s intervals at 10 °C, and FTIR difference spectra upon individual flashes were measured. The obtained difference spectra upon the first to fourth flashes showed considerably different spectral features from each other, whereas the fifth, sixth, seventh, and eighth flash spectra were similar to the first, second, third, and fourth flash spectra, respectively. The intensities at the wavenumbers of prominent peaks of the first and second flash spectra showed clear period four oscillation patterns. These oscillation patterns were well fitted with the Kok model with 13% misses. These results indicate that the first, second, third, and fourth flash spectra represent the difference spectra upon the S1 → S2, S2 → S3, S3 → S0, and S0 → S1 transitions, respectively. In these spectra, prominent bands were observed in the symmetric (1300−1450 cm-1) and asymmetric (1500−1600 cm-1) stretching regions of carboxylate groups and in the amide I region (1600−1700 cm-1). Comparison of the band features suggests that the drastic coordination changes of carboxylate groups and the protein conformational changes in the S1 → S2 and S2 → S3 transitions are reversed in the S3 → S0 and S0 → S1 transitions. The flash-induced FTIR measurements during the S-state cycle will be a promising method to investigate the detailed molecular mechanism of photosynthetic oxygen evolution.