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 /His 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 N coordinating cavity. Our study provides an unprecedented example of a light-induced electron-transfer process in a Tyr /His model of photosystem II, evidencing the movement of both the phenol and imidazole protons along an isoenergetic pathway.

Proton-controlled Action of an Imidazole as Electron Relay in a Photoredox Triad

“…S13). Therefore, we favor an alternative explanation where the pyridines of the uncoordinated terpyridine in 1 could function as internal base accepting the proton from the oxidized imidazole, probably via trace water molecules, in a similar way to recent findings on a free-base porphyrin-imidazole-phenol system [12]. This function is not available when a metal ion is coordinated (in 3 and 2).…”

“…This work provided supportive ground that implementation of such a mimetic module borrowed from natural photosynthesis can indeed lead to further optimization of the water splitting in a photoelectrochemical cell. Photooxidation processes in biomimetic models of the tyrosine/histidine pair of PSII have been investigated in detail especially with respect to the mechanism of protoncoupled electron transfer involving this hydrogen-bonded couple [7][8][9][10][11][12]. On the other hand, different photoredoxcatalyst molecular assemblies have been reported for the water activation process [13].…”

Proton-coupled multi-electron transfer and its relevance for artificial photosynthesis and photoredox catalysis

“…Related reactions have been observed in so-called proton-relays, 44 and were recently termed E2PT. 45,46 The photogenerated radical pair state in the triad from Fig. 2b has a lifetime of 1.9 ms in de-aerated pyridine-pyridinium buffer at room temperature, and this is very similar to the typical lifetimes of classic charge-separated states in comparable molecular triads under similar conditions.…”

Phthalocyanine as a Bioinspired Model for Chlorophyll f‐Containing Photosystem II Drives Photosynthesis into the Far‐Red Region

“…Recent proposals for the involvement of the Tyr Z /His 190 couple in the proton exit pathway or in electronic and magnetic coupling with the OEC have gathered more interest [7–9] . Numerous synthetic models replicating P 680 /Tyr Z /His 190 functions have helped to gain insights in this first light‐induced electron/proton transfer process [10–18] . For all these models, ruthenium trisbipyridine and porphyrin chromophores have been used as the photosensitizer module whereupon irradiation in the Metal to Ligand Charge Transfer band (MLCT, 450 nm) [10, 12, 14] or Q bands (500–650 nm) [15, 17] respectively, is used to initiate the electron transfer flow resulting in the phenoxyl radical formation.…”