In photosystem II, tyrosine Z (Yz) functions as a redox relay between the photo-oxidized primary donor (P680●+) and the oxygen evolving complex (OEC), where water oxidation takes place. The oxidation of Yz by P680●+ likely occurs with transfer of a proton to a hydrogen-bonded histidine (His190). This change in the protonation state of Yz is thought to play an important role in water oxidation by poising the potential of the mediator between that of P680●+ and the OEC. To mimic the behavior of the Yz-His190 pair, benzimidazole-phenol (BIP) derivatives have been synthesized. A concerted or effectively concerted one-electron two-proton transfer (E2PT) process associated with the electrochemical oxidation of the phenol has been shown to take place in amino substituted BIPs, accompanied by a decrease in the redox potential of the phenoxyl radical/phenol couple by ~300 mV. Theoretical calculations predict that substituents with reduced pKas, such as substituted imines attached to BIP, would still undergo an E2PT process while maintaining considerably higher potential for the phenoxyl radical/phenol couple. This feature is necessary for a redox relay involved in the activation of water oxidation catalysts. BIP-imine constructs were synthesized as alternative models of the Yz-His190 pair and preliminary results indicate that the phenol oxidation regains most of the 300 mV potential lost in the E2PT process of the amino-BIPs. Infrared spectroelectrochemistry is being used to identify bonding changes to confirm the assignment of an E2PT process. One of the aims of this study is to determine how many proton transfers can be associated with one oxidation event. To partly answer that question, a new compound in which BIP is substituted at the 4 position by 4-aminobenzimidazole was synthesized. Preliminary results show good agreement between experimental and calculated midpoint potentials for the phenoxyl radical/phenol couple in an E3PT mechanism.

Multiple proton transfers coupled to a single electron transfer in benzimidazole-phenol derivatives

255th National Meeting (2018)

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