FTIR detection of the molecular reactions during the oxygen-evolving S-state cycle

Abstract

Oxygen evolution of plants and cyanobacteria takes place as water oxidation in the oxygen-evolving complex (OEC) of photosystem II. Although the reaction is known to proceed through the light-driven cycle of five intermediates, Si states (i = 0-4), its molecular mechanism is not yet understood. In this study, we have used Fourier transform infrared (FTIR) spectroscopy to detect the reactions of proteins and water during the S-state cycle. FTIR difference spectra upon the flash-induced S-state transitions were measured using the core complexes from Synechococcus elongatus at 10 °C. The intensities of prominent peaks showed clear period-four oscillation patterns, indicating that the 1st, 2nd, 3rd, and 4th-flash spectra represent the difference spectra upon the S1®S2, S2®S3, S3®S0, and S0®S1 transitions, respectively. In all the spectra, prominent bands were observed in the regions of carboxylate stretches (1350-1600 cm-1) and amide I modes (1600-1700 cm-1). The behaviors of these bands showed that the reactions of carboxylate groups and the conformational changes of proteins in the S1®S2®S3 transitions are reversed in the S3®S0®S1 transitions. In the weakly H-bonding O-H region (3500-3800 cm-1), bands were observed at positions specific to each S-state transition. These bands exhibited downshifts in H218O, and thus were assigned to the O-H vibrations of either substrate water or inner water coupled to the reaction in OEC. These results show that FTIR analysis is fruitful to unravel the molecular mechanism of the photosynthetic oxygen evolution.