title: Proton/hydrogen transfer affects the S-state-dependent microsecond phases of P680+ reduction during water splitting.
abstract: 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.
authors.affiliation: Department of Biochemistry and Chemistry, Imperial College, London, United Kingdom.
authors.forename: M J
authors.initials: MJ
authors.lastname: Schilstra
authors.forename: F
authors.initials: F
authors.lastname: Rappaport
authors.forename: J H
authors.initials: JH
authors.lastname: Nugent
authors.forename: C J
authors.initials: CJ
authors.lastname: Barnett
authors.forename: D R
authors.initials: DR
authors.lastname: Klug
mesh_terms: D002734:Chlorophyll / Q000378:metabolism*; D017666:Deuterium Oxide / Q000378:metabolism; D004579:Electron Transport; D006859:Hydrogen / Q000378:metabolism*; D007700:Kinetics; D045342:Light-Harvesting Protein Complexes; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism*; D011522:Protons; D018724:Spinacia oleracea; D014867:Water / Q000378:metabolism*
chemical_list: D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D011522:Protons; D014867:Water; D002734:Chlorophyll; C025167:P-680; D006859:Hydrogen; D017666:Deuterium Oxide