Chapter 7 - X-ray absorption and emission spectroscopy in biology

Abstract

X-ray spectroscopy identifies electronic and geometric structures of metals and their environments in biological and bioinorganic systems. The X-ray absorption spectrum can be divided into two regions. The X-ray absorption near-edge structure (XANES) allows the oxidation state and coordination symmetry of a metal (more generally a trace element) to be determined. From the extended X-ray absorption fine structure accurate metal–ligand distances can be determined, the type of ligand can be roughly identified, and an estimate of the coordination number is obtained. Using physical and mathematical approximations, we explain the relation between chemical structure and its manifestation in X-ray absorption spectroscopy (XAS), and translate this to relevant biological examples, viz., metal cation (V, Fe, Ni, Cu, Zn, Mo, and W)-containing proteins, and other trace elements: Se, Br, and I. X-ray emission spectroscopy gives information complementary to that from XAS, viz., the spin state of the metal of interest as well as a more accurate definition of the type of ligand, and allows the XANES to be recorded with high-energy resolution.