Time-Resolved Infrared Detection of the Proton and Protein Dynamics during Photosynthetic Oxygen Evolution

Photosynthetic oxygen evolution by plants and cyanobacteria is performed by water oxidation at the Mn4CaO5 cluster in photosystem II. The reaction is known to proceed via a light-driven cycle of five intermediates called S-i states (i = 0-4). However, the detailed reaction processes during the intermediate transitions remain unresolved. In this study, we have directly detected the proton and protein dynamics during the oxygen-evolving reactions using time-resolved infrared spectroscopy. The time courses of the absorption changes at 1400 and 2500 cm(-1), which represent the reactions and/or interaction changes of carboxylate groups and the changes in proton polarizability of strong hydrogen bonds, respectively, were monitored upon flash illumination. The results provided experimental evidence that during the S-3 -> S-0 transition, drastic proton rearrangement, most likely reflecting the release of a proton from the catalytic site, takes place to form a transient state before the oxidation of the Mn4CaO5 cluster that leads to O-2 formation. Early proton movement was also detected during the S-2 -> S-3 transition. These observations reveal the common mechanism in which proton release facilitates the transfer of an electron from the Mn4CaO5 cluster in the S-2 and S-3 states that already accumulate oxidizing equivalents. In addition, relatively slow rearrangement of carboxylate groups was detected in the S-0 -> S-1 transition, which could contribute to the stabilization of the S-1 state. This study demonstrates that time-resolved infrared detection is a powerful method for elucidating the detailed molecular mechanism of photosynthetic oxygen evolution by pursuing the reactions of substrate and amino acid residues during the S-state transitions.