Characterization of the Mn Oxidation States in Photosystem II by Kβ X-ray Fluorescence Spectroscopy

Abstract

The nature of the Mn oxidation states involved in photosynthetic oxygen evolution has remained controversial, despite intense study by X-ray absorption and electron paramagnetic resonance spectroscopy. As an alternative approach, high-resolution Kβ X-ray fluorescence spectra have been recorded on the dark-adapted S1 state and the hydroquinone-reduced state of the oxygen-evolving complex in photosystem II. By comparison of the Kβ chemical shifts with those of appropriate model compounds, the S1 state of photosystem II is found to contain equal amounts of Mn(III) and Mn(IV). In the hydroquinone-reduced sample, a significant fraction of the Mn is reduced to Mn(II). The results are compatible with models involving conversion of Mn(III)2Mn(IV)2 to Mn(II)2Mn(IV)2 clusters.