J. Phys. Colloques
Volume 47, Numéro C8, Décembre 1986EXAFS and Near Edge Structure IV
|Page(s)||C8-1121 - C8-1128|
J. Phys. Colloques 47 (1986) C8-1121-C8-1128
THE STATE OF MANGANESE IN THE PHOTOSYNTHETIC APPARATUS DETERMINED BY X-RAY ABSORPTION SPECTROSCOPYV.K. YACHANDRA, R.D. GUILES, A. McDERMOTT, R.D. BRITT, J. COLE, S.L. DEXHEIMER, K. SAUER et M.P. KLEIN
Laboratory of Chemical Biodynamics, Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720, U.S.A.
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.