The state of manganese in the photosynthetic apparatus. 3. Light-induced changes in X-ray absorption (K-edge) energies of manganese in photosynthetic membranes

Abstract

Photosynthetic water oxidation by higher plants proceeds as though five intermediates, S 0-S 4, operate in a cyclic fashion. In this study of the manganese involvement in the process, a low temperature EPR signal is used as an indicator of S-state composition for manganese X-ray absorption K-edge measurements of a spinach Photosystem II preparation. A dramatic change is observed in the edge properties between samples prepared in states S 1 and either S 2 or S 3, establishing a direct relation between the local environment of Mn and the S-state composition. Samples in S 2 or S 3 exhibit a broadening of the principal absorption peak and a shift to higher energy by as much as 2.5 eV relative to S 1 samples. The magnitude of these changes is directly related to the EPR signal intensity induced by illumination. Models are discussed in which these data may be interpreted in terms of a conformation-induced change in Mn ligation and/or oxidation during the S 1 to S 2 transition.