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

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 S<INF>1</INF> 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 S<INF>1</INF> 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)<INF>2</INF>Mn(IV)<INF>2</INF> to Mn(II)<INF>2</INF>Mn(IV)<INF>2</INF> clusters.