Manganese K-edge X-ray absorption spectra of the cyclic S-states in the photosynthetic oxygen-evolving system

Abstract

A set of Mn K-edge XANES spectra due to the redox states S0-S3 of the OEC were determined by constructing a highly-sensitive X-ray detection system for use with physiologically native PS II membranes capable of cycling under a series of saturating laser-flashes. The spectra showed almost parallel upshifts with relatively high K-edge half-height energies given by 6550.9±0.2 eV, 6551.7±0.2 eV, 6552.5±0.2 eV and 6553.6±0.2 eV for the S0, S1, S2 and S3 states, respectively. The successive difference spectra between S0 and S1, S1 and S2, and S2 and S3 states were found to exhibit a similar peak around 6552-6553 eV, indicating that one Mn(III) ion or its direct ligand is univalently oxidized upon each individual S-state transition from S0 to S3. The present data, together with other observations of EPR and pre-edge XANES spectroscopy, suggest that the oxidation state of the Mn cluster undergoes a periodic change; S0: Mn(III,III,III,IV) → S1: Mn(III,IV,III,IV) → S2: Mn(III,IV,IV,IV) → S3: Mn(IV,IV,IV,IV) or Mn(III,IV,IV,IV)·L(+) with L being a direct ligand of a Mn(III) ion.