Proton and hydrogen currents in photosynthetic water oxidation

The photosynthetic processes that lead to water oxidation involve an evolution in time from photon dynamics to photochemically-driven electron transfer to coupled electron/proton chemistry. The redox-active tyrosine, Y-z, is the component at which the proton currents necessary for water oxidation are switched on. The thermodynamic and kinetic implications of this function for Y-z are discussed. These considerations also provide insight into the related roles of Y-z in preserving the high photochemical quantum efficiency in Photosystem II (PSII) and of conserving the highly oxidizing conditions generated by the photochemistry in the PSII reaction center. The oxidation of Y-z by P-680(+) can be described well by a treatment that invokes proton coupling within the context of non-adiabatic electron transfer. The reduction of Y-z(.), however, appears to proceed by an adiabatic process that may have hydrogen-atom transfer character. (C) 2000 Elsevier Science B.V. All rights reserved.