Proton and hydrogen currents in photosynthetic water oxidation

AbstractThe 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, YZ, is the component at which the proton currents necessary for water oxidation are switched on. The thermodynamic and kinetic implications of this function for YZ are discussed. These considerations also provide insight into the related roles of YZ 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 YZ by P680+ can be described well by a treatment that invokes proton coupling within the context of non-adiabatic electron transfer. The reduction of YZ, however, appears to proceed by an adiabatic process that may have hydrogen-atom transfer character