OF THE OXYGEN-EVOLVING MANGANESE COMPLEX IN A PHOTOSYSTEM-II MEMBRANEPREPARATION - AN X-RAY-ABSORPTION SPECTROSCOPY STUDY

X-ray absorption spectroscopy has been performed on oriented photosyst em II membrane particles isolated from spinach. Structural features of the tetranuclear Mn cluster and the orientation of the cluster with r espect to the lipid bilayer were determined in both the S-1 and S-2 st ates of the Kok cycle. Variation of the sample orientation with respec t to the X-ray e-vector yields highly dichroic K-edge and extended X-r ay absorption fine structure spectra (EXAFS), indicative of an asymmet ric tetranuclear cluster. Mn-Mn vectors at 2.72 and 3.38 Angstrom can be resolved from these measurements using quantitative analysis. The 2 .72-Angstrom vector, consisting of at least two component vectors, is oriented at an average angle of 60 degrees +/- 7 degrees to the membra ne normal, with an average of 1.1 +/- 0.1 interactions per Mn atom. Th e 3.38-Angstrom vector, most probably an average of two vectors, makes an angle of 43 degrees +/- 10 degrees with respect to the membrane no rmal, with an average of 0.45 +/- 0.07 backscatterer per Mn atom. Upon advance to the S-2 state, the orientation of these vectors and the av erage numbers of backscatterers are approximately invariant. Analysis of more subtle features of the EXAFS reveals changes accompanying this S-state advance that are consistent with the oxidation of Mn during t his transition. However, the dominant structural features of the oxyge n-evolving complex remain constant in the S-1 and S-2 states. The stru cture of the Mn complex and the orientation of the complex in the memb rane within the context of dichroism of the X-ray absorption data are discussed.