COMPARISON OF THE MANGANESE OXYGEN-EVOLVING COMPLEX IN PHOTOSYSTEM-IIOF 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 o n the oxygen-evolving complex (OEC) from photosystem II is described f or preparations from both spinach and the cyanobacterium Synechococcus sp. poised in the S-1 and S-2 states. In addition to reproducing prev ious results suggesting the presence of bis(mu-oxo)bridged Mn centers in the OEC, a Fourier transform peak due to scatterers at an average d istance of >3 Angstrom is detected in both types of preparation. In ad dition, subtle but reproducible changes are found in the relative ampl itudes of the Fourier transform peaks due to mainly O (similar to 1.8 Angstrom) and Mn (similar to 2.7 Angstrom) neighbors upon cryogenic ad vance from the S-1 to the S-2 state. Analysis of the peak due to scatt erers at similar to 3 Angstrom favors assignment to (per 4 Mn in the O EC) 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 wit h the data for the OEC. Structural models for the OEC are evaluated on the basis of these results.