Absence of Mn-Centered Oxidation in the S2 → S3 Transition: Implications for the Mechanism of Photosynthetic Water Oxidation

摘要

A key question for the understanding of photosynthetic water oxidation is whether the four oxidizing equivalents necessary to oxidize water to dioxygen are accumulated on the four Mn ions of the oxygen-evolving complex (OEC), or whether some ligand-centered oxidations take place before the formation and release of dioxygen during the S3 → [S4] → S0 transition. Progress in instrumentation and flash sample preparation allowed us to apply Mn Kβ X-ray emission spectroscopy (Kβ XES) to this problem for the first time. The Kβ XES results, in combination with Mn X-ray absorption near-edge structure (XANES) and electron paramagnetic resonance (EPR) data obtained from the same set of samples, show that the S2 → S3 transition, in contrast to the S0 → S1 and S1 → S2 transitions, does not involve a Mn-centered oxidation. On the basis of new structural data from the S3-state, manganese μ-oxo bridge radical formation is proposed for the S2 → S3 transition, and three possible mechanisms for the O–O bond formation are presented.