Structure and Orientation of the Mn4Ca Cluster in Plant Photosystem II Membranes Studied by Polarized Range-Extended X-ray Absorption Spectroscopy

X-ray absorption spectroscopy has provided important insights into the structure and function of the Mn4Ca cluster in the oxygen-evolving complex (OEC) of Photosystem II (PS II). The range of Mn extended X-ray absorption fine structure (EXAFS) data collected from PSII until now has been, however, limited by the presence of Fe in PS II. Using a crystal spectrometer with high energy resolution to detect solely the Mn K alpha fluorescence, we are able to extend the EXAFS range beyond the onset of the Fe absorption edge. This results in improvement in resolution of the Mn-backscatterer distances in PS II from 0.14 Angstrom to 0.09 Angstrom. The high-resolution data obtained from oriented spinach PS II membranes in the S1 state show that there are three di-?-oxo-bridged Mn-Mn distances of ∼2.7 Angstrom and ∼2.8 Angstrom in a 2:1 ratio and that these three Mn-Mn vectors are aligned at an average orientation of ∼60 deg relative to the membrane normal. Furthermore, we are able to observe the separation of the Fourier peaks corresponding to the ∼3.2 Angstrom Mn-Mn and the ∼3.4 Angstrom Mn-Ca interactions in oriented PS II samples and determine their orientation relative to the membrane normal. The average of the Mn-Ca vectors at ∼3.4 Angstrom is aligned along the membrane normal, while the ∼3.2 Angstrom Mn-Mn vector is oriented near the membrane plane. A comparison of this structural information with the proposed Mn4Ca cluster models based on spectroscopic and diffraction data provide input for refining and selecting among these models