FTIR analysis of water reactions during photosynthetic water oxidation: detection of the DOD bending vibrations of functional water molecules

Abstract

Photosynthetic water oxidation is performed in the water oxidizing complex (WOC) in photosystem II (PSII) through a light-driven cycle of S states (S₀-S₄). To clarify the water oxidation mechanism, monitoring water reactions in WOC is essential. In this study, we have studied the water reactions in WOC by detecting DOD bending vibrations using Fourier transform infrared (FTIR) spectroscopy. FTIR difference spectra of the S-state cycle were measured using PSII complexes of T.elongates moderately deuterated with D₂¹⁶O and D₂¹⁸O. By calculating D₂¹⁶O-D₂¹⁸O double difference spectra, DOD bending bands of D₂O molecules in WOC were identified at 1250-1150 cm^-1. The presence of 6-8 peaks in each spectrum indicated that at least two water molecules participate in the reaction in every S-state transitions. The bands at 1240 cm^-1 in the S₂-->S₃ and S₃-->S₀ spectra reflected substrate insertion to WOC from a water cluster in the protein.