MN_4CA CLUSTER OF THE WATER-OXIDATION ENZYME STUDIED BY POLARIZED X-RAY SPECTROSCOPY OF PS II SINGLE CRYSTALS

摘要

Polarized XAS studies of single crystals provide a method of resolving and orientationally selecting specific Mn K-edge and near-edge features and structural details from EXAFS that are frequently unresolved in the spectra of unor rented samples. PS II membranes can be oriented on a substrate such that the membrane planes are roughly parallel to the substrate surface. This imparts a one-dimensional order to these samples; while the z axis for each membrane (collinear with the membrane normal) is roughly parallel to the substrate normal, the x and y axes remain disordered. Exploiting the plane-polarized nature of synchrotron radiation, spectra can be collected at different angles between the substrate normal and the X-ray e-vector. The dichroism of the absorber-back-scatterer pair present in the oriented samples is reflected in, and can be extracted from, the resulting X-ray absorption spectra.