PNNL

Abstract

Nature employs a TyrZ-His pair as a redox relay that couples proton transfer to the redox process between P680 and the water oxidizing catalyst in photosystem II. Artificial redox relays comprising different benzimidazole-phenol dyads (benzimidazole models His and phenol models Tyr) with substituents designed to simulate the hydrogen bond network surrounding the TyrZ-His pair have been prepared. When the benzimidazole substituents are strong proton acceptors such as primary or tertiary amines, theory predicts that a concerted two proton transfer process associated with the electro-chemical oxidation of the phenol will take place. Also, theory predicts a reduction in the oxidation potential of the phenol by ~300 mV and a small kinetic isotope effect (KIE). Indeed, electrochemical, spectroelectrochemical, and KIE experimental data are consistent with these predictions. Notably, these results were obtained by using theory to guide the rational design of artificial systems and have implications for managing proton activity to optimize efficiency at energy conversion sites involving water oxidation and reduction.