Abstract
The oxidation states of the Mn4CaO5 cluster of the photosynthetic oxygen-evolving complex remain controversial. New quantum chemical models for the dark-stable S1 state suggest that a low-valent Mn(III-IV-III-II) form, opposed to the commonly accepted high-valent Mn(III-IV-IV-III) assignment, agrees with available structural data. We examine the magnetic properties of the models and the consequences for electronic structure and catalytic progression of assuming a neutral second-sphere residue D1-His337. The low-valent model reproduces the experimental diamagnetic ground spin-state. However, the protonation assignment introduces a critical flaw because neutral His337 becomes itself the locus of subsequent oxidation, in contradiction to physiological catalytic progression.
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