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

Abstract

We investigate a biomimetic model of a Tyr_Z /His₁₉₀ pair, a hydrogen-bonded phenol/imidazole covalently attached to a porphyrin sensitizer. Laser flash photolysis in the presence of an external electron acceptor reveals the need for water molecules to unlock the light-induced oxidation of the phenol through an intramolecular pathway. Kinetics monitoring encompasses two fast phases with distinct spectral properties. The first phase is related to a one-electron transfer from the phenol to the porphyrin radical cation coupled with a domino two-proton transfer leading to the ejection of a proton from the imidazole-phenol pair. The second phase concerns conveying the released proton to the porphyrin N₄ coordinating cavity. Our study provides an unprecedented example of a light-induced electron-transfer process in a Tyr_Z /His₁₉₀ model of photosystem II, evidencing the movement of both the phenol and imidazole protons along an isoenergetic pathway.

Keywords: TyrZ-His190 model; artificial photosynthesis; photophysics; porphyrin; proton-coupled electron transfer.