Where Plants Make Oxygen: a Structural Model for the Photosynthetic Oxygen-Evolving Manganese Cluster

TLDR

Photosynthetic oxygen evolution occurs at a catalytic site comprising four manganese atoms with essential calcium and chlorine cofactors, organized as di‑μ‑oxo bridged manganese binuclear clusters linked by a mono‑μ‑oxo bridge, a proximal calcium atom, and a halide. The study presents a structural model of the oxygen‑evolving manganese cluster and determines its manganese oxidation states using X‑ray absorption spectroscopy. X‑ray absorption spectroscopy was employed to model the cluster structure and assign manganese oxidation states. In dark‑adapted samples, manganese exists in oxidation states III and IV, and dichroism in oriented membranes indicates the cluster lacks high symmetry.

Abstract

In the photosynthetic evolution of oxygen, water oxidation occurs at a catalytic site that includes four manganese atoms together with the essential cofactors, the calcium and chlorine ions. A structural model and a determination of the manganese oxidation states based on x-ray absorption spectroscopy are presented. The salient features, in both higher plants and cyanobacteria, are a pair of di-μ-oxo bridged manganese binuclear clusters linked by a mono-μ-oxo bridge, one proximal calcium atom, and one halide. In dark-adapted samples, manganese occurs in oxidation states (III) and (IV). Data from oriented membranes display distinct dichroism, precluding highly symmetrical structures for the manganese complex.