COMPARISON OF THE MANGANESE OXYGEN-EVOLVING COMPLEX IN PHOTOSYSTEM-II OF SPINACH AND SYNECHOCOCCUS SP WITH MULTINUCLEAR MANGANESE MODEL COMPOUNDS BY X-RAY-ABSORPTION SPECTROSCOPY

The evaluation of Mn X-ray absorption fine structure (EXAFS) studies on the oxygen-evolving complex (OEC) from photosystem II is described for preparations from both spinach and the cyanobacterium Synechococcus sp. poised in the S-1 and S-2 states. In addition to reproducing previous results suggesting the presence of bis(mu-oxo)bridged Mn centers in the OEC, a Fourier transform peak due to scatterers at an average distance of >3 Angstrom is detected in both types of preparation. In addition, subtle but reproducible changes are found in the relative amplitudes of the Fourier transform peaks due to mainly O (similar to 1.8 Angstrom) and Mn (similar to 2.7 Angstrom) neighbors upon cryogenic advance from the S-1 to the S-2 state. Analysis of the peak due to scatterers at similar to 3 Angstrom favors assignment to (per 4 Mn in the OEC) 1-2 heavy atom (Mn, Ca) scatterers at an average distance of 3.3-3.4 Angstrom. The EXAFS data of several multinuclear Mn model compounds containing such scattering interactions are analyzed and compared with the data for the OEC. Structural models for the OEC are evaluated on the basis of these results.