Concerted One-Electron Two-Proton Transfer Processes in Models Inspired by the Tyr-His Couple of Photosystem II

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 composed of 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 electrochemical oxidation of the phenol will take place. Also, theory predicts a decrease in the redox 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.

Synthesis, Structure, and Redox Properties of a trans-Diaqua Ru Complex That Reaches Seven-Coordination at High Oxidation States

“…The access to five different reversible oxidation states thanks to PCET mechanisms is exceptional for complexes containing just a single metal center. 17 This extremely rich redox chemistry of Table 1, entries 8 and 9 for selected values and Figure S14 for their Pourbaix diagrams). 37 The redox potential values and the shape of the diagram differ from those of 3 III (OH2)2 + due to the different electronic and geometrical effects exerted by tda 2-compared to bpy.…”

“…13,14,15,16 This allows to reach reactive high oxidation states via proton-coupled electron transfer at a relatively low energy. 17,18 For the particular case of the water oxidation, the best catalysts described so far involve the formation of seven coordinate complexes at high oxidations states, stabilized by sigma donor groups such as carboxylates. 19,20,21,22,23,24 A good example of this family of catalysts is the [Ru 25 Complex 4 II (50 mg, 0.1 mmol) was dissolved in a mixture of water (2.5 mL) and pyridine (7.5 mL).…”

Water Molecules Gating a Photoinduced One‐Electron Two‐Protons Transfer in a Tyrosine/Histidine (Tyr/His) Model of Photosystem II

“…[6] Moore and co-workers demonstrated ac oncerted E2PT in as trongly coupled Ty r Z /His 190 model. [7] However,t he addition of ac ovalently bound pending base lowered the potential of the phenol drastically,l eaving open questions on the functioning of such acrucial cofactor as relay for both electron and proton between the P 680 + and the OEC. We report here the light induced reactions occurring at af used porphyrin-imidazole-phenol compound as model for the biological sequence operating in P 680 + /Tyr/His.K inetic studies from laser flash photolysis clearly evidence that water molecules are gating aphotoinduced E2PT process,allowing the phenoxyl radical to form without af all of its oxidative power.…”

Water Molecules Gating a Photoinduced One‐Electron Two‐Protons Transfer in a Tyrosine/Histidine (Tyr/His) Model of Photosystem II

“…Savéant and co‐workers reported a one‐electron two‐proton coupled transfer (E2PT) in a synthetic model featuring a H‐bonded alcohol as a proton relay from the oxidized phenol to a pending base . Moore and co‐workers demonstrated a concerted E2PT in a strongly coupled Tyr Z /His 190 model . However, the addition of a covalently bound pending base lowered the potential of the phenol drastically, leaving open questions on the functioning of such a crucial cofactor as relay for both electron and proton between the P 680 + and the OEC.…”