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

X-ray absorption spectroscopy has been performed on oriented photosystem II membrane particles isolated from spinach. Structural features of the tetranuclear Mn cluster and the orientation of the cluster with respect to the lipid bilayer were determined in both the S-1 and S-2 states of the Kok cycle. Variation of the sample orientation with respect to the X-ray e-vector yields highly dichroic K-edge and extended X-ray absorption fine structure spectra (EXAFS), indicative of an asymmetric 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 membrane normal, with an average of 1.1 +/- 0.1 interactions per Mn atom. The 3.38-Angstrom vector, most probably an average of two vectors, makes an angle of 43 degrees +/- 10 degrees with respect to the membrane normal, 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 average 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 this transition. However, the dominant structural features of the oxygen-evolving complex remain constant in the S-1 and S-2 states. The structure of the Mn complex and the orientation of the complex in the membrane within the context of dichroism of the X-ray absorption data are discussed.