Proton/hydrogen transfer affects the S-state-dependent microsecond phases of P680+ reduction during water splitting

To investigate a possible coupling between P680+ reduction and hydrogen transfer, we studied the effects of H2O/D2O exchange on the P680+ reduction kinetics in the nano- and microsecond domains. We concentrated on studying the period-4 oscillatory (i.e., S-state-related) part of the reduction kinetics, by analyzing the differences between the P680+ reduction curves, rather than the full kinetics. Earlier observations that P680+ reduction kinetics have microsecond components were confirmed: the longest observable lifetime whose amplitude showed period-4 oscillations was 30 microseconds. We found that solvent isotope exchange left the nanosecond phases of the P680+ reduction unaltered. However, a significant effect on the oscillatory microsecond components was observed. We propose that, at least in the S0/S1 and S3/S0 transitions, hydrogen (proton) transfer provides an additional decrease in the free energy of the YZ+P680 state with respect to the YZP680+ state. This implies that relaxation of the state YZ+P680 is required for complete reduction of P680+ and for efficient water splitting. The kinetics of the P680+ reduction suggest that it is intraprotein proton/hydrogen rearrangement/transfer, rather than proton release to the bulk, which is occurring on the 1-30 microseconds time scale.