The open-cubane oxo–oxyl coupling mechanism dominates photosynthetic oxygen evolution: a comprehensive DFT investigation on O–O bond formation in the S4 state

Abstract

The dioxygen formation mechanism of biological water oxidation in nature has long been the focus of argument; many diverse mechanistic hypotheses have been proposed. Based on a recent breakthrough in the resolution of the electronic and structural properties of the oxygen-evolving complex in the S₃ state, our density functional theory (DFT) calculations reveal that the open-cubane oxo–oxyl coupling mechanism, whose substrates preferably originate from W2 and O5 in the S₂ state, emerges as the best candidate for O–O bond formation in the S₄ state. This is justified by the overwhelming energetic superiority of this mechanism over alternative mechanisms in both the isomeric open and closed-cubane forms of the Mn₄CaO₅ cluster; spin-dependent reactivity rooted in variable magnetic couplings was found to play an essential role. Importantly, this oxygen evolution mechanism is supported by the recent discovery of femtosecond X-ray free electron lasers (XFEL), and the origin of the observed structural changes from the S₁ to S₃ state has been analyzed. In this view, we corroborate the proposed water binding mechanism during S₂–S₃ transition and correlate the theoretical models with experimental findings from aspects of substrate selectivity according to water exchange kinetics. This theoretical consequence for native metalloenzymes may serve as a significant guide for improving the design and synthesis of biomimetic materials in the field of photocatalytic water splitting.