XANES, EXAFS and Kbeta spectroscopic studies of the oxygen-evolving complex in Photosystem II

Abstract

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 S{sub 3} {r_arrow} [S{sub 4}] {r_arrow} S{sub 0} transition. Progress in instrumentation and flash sample preparation allowed us to apply Mn K{beta} X-ray emission spectroscopy (Kb XES) to this problem for the first time. The K{beta} 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 S{sub 2} {r_arrow} S{sub 3} transition, in contrast to the S{sub 0} {r_arrow} S{sub 1} and S{sub 1} {r_arrow} S{sub 2} transitions, does not involve a Mn-centered oxidation. This is rationalized by manganese {mu}-oxo bridge radical formation during the S{sub 2} {r_arrow} S{sub 3} transition. Using extended X-ray absorption fine structure (EXAFS) spectroscopy, the local environment of the Mn atoms in the S{sub 0} state has been structurally characterized. These results show that the Mn-Mn distance in one of the di-{mu}-oxo-bridged Mn-Mn moieties increases from 2.7 {angstrom} in the S{sub 1} state to 2.85 {angstrom} in the S{sub 0} state. Furthermore, evidence is presented that shows three di-{mu}-oxo binuclear Mn{sub 2} clusters may be present in the OEC, which is contrary to the widely held theory that two such clusters are present in the OEC. The EPR properties of the S{sub 0} state have been investigated and a characteristic ''multiline'' signal in the S{sub 0} state has been discovered in the presence of methanol. This provides the first direct confirmation that the native S{sub 0} state is paramagnetic. In addition, this signal was simulated using parameters derived from three possible oxidation states of Mn in the S{sub 0} state. The dichroic nature of X-rays from synchrotron radiation and single-crystal Mn complexes have been exploited to selectively probe Mn-ligand bonds using XANES and EXAFS spectroscopy. The results from single-crystal Mn complexes show that dramatic dichroism exists in these complexes, and are suggestive of a promising future for single-crystal studies of PS II.