SPECTROSCOPIC STUDIES OF THE MANGANESE COMPLEX OF PHOTOSYSTEM-II

Our recent EPR and EXAFS experiments investigating the structure of th e oxygen-evolving complex of PS II are discussed. PS II treatments whi ch affect the cofactors calcium and chloride have been used to poise s amples in modified forms of the S-states, S-1, S-2 and S-3. X-ray abso rption studies indicate a similar overall structure for the manganese complex between treated and native samples although the influence of t he treatments and cofactors is observed. Manganese oxidation (or oxida tion of a ligand to the manganese cluster) is indicated to occur on ea ch of the transitions S-1 --> S-2 and S-2 --> S-3 in these modified sa mples. The cluster appears to contain at least two inequivalent Mn-Mn pairs. In the native samples the Mn-Mn distance is 2.7 Angstrom, but i n samples where the calcium site is affected, one of the pairs has a 3 .0 Angstrom Mn-Mn distance. The intensity of the 3.3/3.6 Angstrom inte raction is reduced on sodium chloride treatment (calcium depletion) pe rhaps indicating calcium binding close to the manganese cluster. From EPR data we also propose that treatments which affect calcium and chlo ride binding cause a modification of the native S-2 state, slow the re duction of Y-2. and allow an S3 EPR signal to be observed following il lumination. The origin of the S3 EPR signal, a modified S-3 or S-2 X. where X. is an organic radical of unknown charge, is discussed in rela tion to the results from the EXAFS studies.