Monitoring the reactions of photosynthetic water oxidation using infrared spectroscopy

Photosynthetic water oxidation in plants and cyanobacteria is performed at the Mn₄CaO₅ cluster in photosystem II through the light-driven cycle of five intermediates called S_i state (i=0–4). This reaction is essential not only as an electron source for CO₂ fixation but also as a source of energy and molecular oxygen for life on earth. In this review, the recent progress of the studies using infrared spectroscopy regarding the mechanism of photosynthetic water oxidation was summarized. Flash-induced Fourier-transform infrared (FTIR) difference spectra between the individual S-state transitions (S₁→S₂, S₂→S₃, S₃→S₀ and S₀→S₁) revealed the structural changes of polypeptide main chains and amino-acid side chains as well as the reactions of water molecules. Proton release processes were also monitored using the infrared bands of buffer molecules, while the miss probabilities of individual transitions were estimated by monitoring an electron flow from the Mn₄CaO₅ cluster to an exogenous electron acceptor, ferricyanide, using the CN stretching bands of ferricyanide and ferrocyanide. Furthermore, proton and protein dynamics during the S-state transitions were monitored by dispersive-type time-resolved infrared spectroscopy. These infrared measurements will be powerful methods for full understanding of the mechanism of photosynthetic water oxidation.