30th Annual SSRL Users' Meeting — October 9-10, 2003

X-ray Spectroscopy of Photosystem II Single Crystals

J. Yano,1,2 K. Sauer,1,2 V. Yachandra,1 J. Messinger,3 J. Kern,4 and A. Zouni4

1Physical Biosciences, LBNL, Berkeley, CA 94720
2Department of Chemistry, University of California, Berkeley, CA 94720
3Max-Planck-Institut fr Strahlenchemie, D-45470 Mlheim an den Ruhr, Germany
4Max-Volmer Laboratorium der TU-Berlin, D-10623, Berlin, Germany

Photosystem II (PS II) is a membrane-bound protein complex found in green plants and cyanobacteria. During a light-driven process by PS II, single electron photooxidations of chlorophyll are coupled to the four- electron oxidation of water to evolve dioxygen. The catalytic site of this reaction is a protein-bound tetranuclear Mn complex. The crystal structure of PSII was recently reported by Zouni et al. (Nature 409, 739, 2001) and then by Shen et al. (PNAS 100, 98, 2003) with resolution of 3.6 . Our primary goal is to utilize X-ray spectroscopy of single crystals of PS II in conjunction with the low-resolution electron density of the Mn complex obtained from X-ray diffraction studies to derive the structure of the Mn complex at high resolution. We have obtained the Mn K- edge and EXAFS spectra of the single crystals of PS II with the e-vector of the X-rays oriented parallel to the three principal a, b and c axes of the crystal. The diffraction pattern for each crystal was collected in situ to determine the orientation of the crystal with respect to the X-ray electric-field vector. The X-ray spectra and diffraction data are collected at 10 K to minimize radiation damage. The dipole forbidden 1s-3d transition and the allowed main K- edge show distinct dichroism. The EXAFS spectra also show distinct dichroism in the Fourier peaks assigned to Mn-O/N vectors at ~1.8 - 2.0 and the Mn-Mn/Ca vectors at 2.7 and 3.3 . The S/N ratio of the spectra was improved by averaging data from many crystals.