X-RAY ABSORPTION-SPECTROSCOPY AND THE STRUCTURES OF TRANSITION-METAL CENTERS IN PROTEINS

This chapter provides a qualitative description of the theoretical basis of the technique and outlines important experimental considerations, before presenting selected examples of the application of X-ray absorption spectroscopy (XAS) to characterize transition metal centers in proteins. X-Ray crystallography is rightly regarded as the most powerful structural technique to provide the architectural details of a protein molecule. However, sometimes the resolution of crystallographic data is insufficient to draw meaningful conclusions concerning the detailed nature of a metal center within a protein. Some proteins refuse to crystallize or yield crystals suitable for a high-resolution structure determination. It is important to establish structural details for proteins, and especially their catalytic centers, when maintained at conditions similar to their working environment-typically, in aqueous media in the presence of substrate, inhibitor, or suitable redox partner. For a metalloprotein, the electron source and detector is the metal atom that is probed, because selective excitation is achieved by scanning a range of X-ray wavelengths particularly appropriate to the element of central interest. © 1991 Academic Press, Inc.