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 hydro gen transfer, we studied the effects of H2O/D2O exchange on the P680() reduction kinetics in the nano-and microsecond domains. We concentra ted on studying the period-4 oscillatory (i.e., S-state-related) part of the reduction kinetics, by analyzing the differences between the P6 80(+) reduction curves, rather than the full kinetics. Earlier observa tions that P680(+) reduction kinetics have microsecond components were confirmed: the longest observable lifetime whose amplitude showed per iod-4, oscillations was 30 mu s. We found that solvent isotope exchang e left the nanosecond phases of the P680(+) reduction unaltered. Howev er, a significant effect on the oscillatory microsecond components was observed. We propose that, al least in the S-0/S-1 and S-3/S-0 transi tions, hydrogen (proton) transfer provides an additional decrease in t he free energy of the Y(Z)(+)P680 state with respect to the Y(Z)P680() state. This implies that relaxation of the state Y(Z)(+)P680 is requ ired for complete reduction of P680(+) and for efficient water splitti ng. The kinetics of the P680(+) reduction suggest that it is intraprot ein proton/hydrogen rearrangement/transfer, rather than proton release to the bulk, which is occurring on the 1-30 mu s time scale.