Fourier Transform Infrared Analysis of the S-State Cycle of Water Oxidation in the Microcrystals of Photosystem II.

Photosynthetic water oxidation is performed in photosystem II (PSII) through a light-driven cycle of intermediates called S states (S0-S4) at the water oxidizing center. Time-resolved serial femtosecond crystallography (SFX) has recently been applied to the microcrystals of PSII to obtain the structural information on these intermediates. However, it remains unanswered whether the reactions efficiently proceed throughout the S-state cycle retaining the native structures of the intermediates in PSII crystals. We investigated the water oxidation reactions in the PSII microcrystals using flash-induced Fourier transform infrared (FTIR) difference spectroscopy. In comparison with the FTIR spectra in solution, it was shown that all of the metastable intermediates in the microcrystals retained their native structures, and the efficiencies of the S-state transitions remained relatively high, although those of the S2 → S3 and S3 → S0 transitions were slightly lowered possibly due to some restriction of water movement in the crystals.