19] X-ray absorption spectroscopy of metalloproteins

Abstract

Publisher Summary This chapter describes the x-ray absorption spectroscopy of metalloproteins. X-ray absorption spectroscopy is the measurement of the absorption of X-radiation by a material as a function of the incident photon energy. Two sources of information are contained in such a spectrum: the absorption edge fine structure (AEFS) and the extended X-ray absorption fine structure (EXAFS). When applied to metalloproteins, the AEFS spectrum is often useful in ascertaining the oxidation state and site symmetry of the metal centers as well as the nature of the surrounding ligands. This information is derived from careful measurements of the energies and intensities of the electronic transitions which occur as the incident photon energy is scanned through an absorption edge, typically over 30 eV. From the EXAFS spectrum, the distance between a metal center and its ligand in a metalloprotein may be obtained. The small modulations in the absorption coefficient, which occur from 100-1000 eV above an absorption edge of the metal, are caused by variations in the probability of photoelectron emission. This probability reflects the extent to which the photoelectron wave back-scattered from a nearby atom to the metal center is in- or out-of-phase with respect to the emitted photoelectron wave. Theoretical methods have been developed to Fourier-transform the modulations in absorption amplitude with energy to give the distances between atoms.