STRUCTURE AND ORIENTATION OF THE OXYGEN-EVOLVING MANGANESE COMPLEX OFGREEN-ALGAE AND HIGHER-PLANTS INVESTIGATED BY X-RAY-ABSORPTION LINEARDICHROISM SPECTROSCOPY ON ORIENTED PHOTOSYSTEM-II MEMBRANE-PARTICLES

X-ray absorption spectroscopy at the Mn K-edge has been performed on m ultilayers of photosystem II-enriched fragments of the native thylakoi d membrane prepared from a higher plant (spinach) and a unicellular gr een alga (Scenedesmus obliquus). Spectra collected for various angles between the prevailing orientation of the thylakoid membrane normal an d the X-ray electric field vector contain information on the atomic st ructure of the tetranuclear manganese complex of photosystem II (PS II ) and its orientation with respect to the membrane normal. The previou sly used approach for evaluation of the dichroism of extended X-ray ab sorption fine structure (EXAFS) spectra [George, G. N., et al. (1989) Science 243, 789-791] is modified, and the following results are obtai ned for PS II in its dark-stable state (S-1-state): (1) structure and orientation of the PS II manganese complexes of green algae and higher plants are highly similar or fully identical; (2) two 2.7-Angstrom ve ctors, which, most likely, connect the Mn nuclei of a planar Mn-2(mu-O -2) structure, an at an average angle of 80 degrees +/- 10 degrees wit h respect to the thylakoid normal; (3) the plane of the Mn-2(mu-O-2) s tructures is rather in parallel with the thylakoid plane than perpendi cular. Structural models for the oxygen-evolving manganese complex and its orientation in the thylakoid membrane are discussed within the co ntext of the presented results.