Redox potentials along the redox-active low-barrier H-bonds in electron transfer pathways

Abstract

Low-barrier H-bonds form when the pK_a values of the H-bond donor and acceptor moieties are nearly equal. Here, we report redox potential (E_m) values along two redox-active low-barrier H-bonds in the water-oxidizing enzyme photosystem II (PSII), using a quantum mechanical/molecular mechanical approach. The low-barrier H-bond between D1-Tyr161 (TyrZ) and D1-His190 is located in the middle of the electron transfer pathway. When the proton is at D1-His190, E_m(TyrZ) is the lowest and can serve as an electron donor to the oxidized chlorophyll P_D1˙⁺. E_m(TyrZ) and E_m(D1-His190) are equal, and the TyrZ⋯D1-His190 pair serves as an electron acceptor to Mn₄CaO₅ when the proton is at TyrZ. In the low-barrier H-bond between D1-His215 and plastoquinone Q_B, located at the terminus of the electron transfer pathway, the driving force of electron transfer and electronic coupling between Q_A and Q_B are maximized when the proton arrives at Q_B. It seems likely that local proton transfer along redox-active low-barrier H-bonds can alter the driving force or electronic coupling for electron transfer.