THE STATE OF MANGANESE IN THE PHOTOSYNTHETIC APPARATUS DETERMINED BY X-RAY ABSORPTION SPECTROSCOPY

Abstract
We present our results of X-ray edge and EXAFS studies of Mn in intermediate states (S₀-S₄) of the photosynthetic O₂ evolving complex prepared from spinach and the thermophilic cyanobacterium Synechococcus. We observed a shift to higher energy in the X-ray absorption K-edge energy of Mn upon advancement from the dark adapted S₁ state to the light-induced S₂ state in spinach and in Synechococcus. We have recently anaiysed the Mn K-edges of samples poised in the S₀ and S₃ states. The K-edge inflection energy of the S₀ samples is lower than that of samples poised in the S₁ state, indicating a reduction in the effective positive charge on Mn in S₀ relative to S₁. The K-edge inflection energy of the S₃ state is similar to that of the S₂ state, suggesting an invariance in the oxidation state of Mn on advancing from the S₂ to the S₃ state. We have examined the Mn K-edge spectra of preparations depleted of essential peptides to determine the changes produced in the structure of the Mn complex. Mn EXAFS results for spinach S₁ and S₂ samples and Synechococcus S₁ all show a Mn neighbor at ~ 2.7 Å and two N or O shells at ~ 1.8 and 2.0 Å indicating a µ-oxo bridged Mn complex. We conclude from the edge and EXAFS studies that the light-induced S₁ to S₂ transition involves a change in the oxidation state of Mn with no change in the coordination of Mn in the O₂-evolving complex. The similarity of the edges and EXAFS results from spinach and Synechococcus suggest that the basic structure of the Mn center in the O₂-evolving complex is conserved over a period of two billion years. Finally, an analysis of a composite of the S₁ and S₂ spectra has revealed a fourth shell in the Fourier transform of the EXAFS spectra. Simulations have shown that it is compatible with an additional Mn shell at ~ 3.3 Å or possibly second shell contributions from histidine ligands.