Simulation of XANES spectra for protein-bound metal centers: analysis of linear dichroism data

Abstract

For protein-bound transition metals, the structure of the metal site (geometric arrangement of the nuclei) and the electronic structure (oxidations state and orbital occupancy) can be investigated by x-ray absorption spectroscopy. While the extended x-ray absorption fine-structure (EXAFS) is theoretically well understood and the advantages of EXAFS spectroscopy on protein-bound metal centers have been repeatedly demonstrated, quantitative analysis of the near-edge structure XANES is more demanding. However, by now XANES simulations, for which a number of codes are available, may be useful to understand geometric and electronic structure effects.For a simplified model of the manganese complex in photosystem II we simulate polarization-dependent XANES spectra using the FEFF 8.2 code [Ankudinov et al, Phys. Rev. B 12 (1998)], which includes self-consistent muffin-tin potentials and polarization-dependent full multiple-scattering calculations. We derive a scheme to analyze the polarization dependence of XANES data that facilitates a treatment of partially uni-directionally oriented samples. Reasonably good agreement between simulations and experiments is obtained for the chosen model of the photosystem II manganese complex and its orientation with respect to the protein-carrying membrane.