XAFS OF DINUCLEAR METAL SITES IN PROTEINS AND MODEL COMPOUNDS

Extensive use has been made of X-ray absorption fine structure (XAFS) spectroscopy for investigating the local structural environment of metal ions in metalloproteins. Although it is widely accepted that XAFS provides accurate structural information for the nearest neighbors to the metal (i.e., the ligands), the use of XAFS for determining metal-metal distances in multi-nuclear proteins is more problematic. We review the origin of the information in XAFS spectra and discuss some of the limitations that apply in extracting structural data from XAFS spectra. Recent advances in the theory and application of XAFS for determining metal-metal distances are reviewed, with particular emphasis on dinuclear iron and manganese proteins and models. For distances less than 3 Angstrom, it is straightforward to determine accurate metal-metal separations using XAFS. For distances > 3 Angstrom, the unique assignment of an XAFS feature to a metal-metal interaction continues to be difficult, despite recent advances in XAFS theory and analysis. However, when additional information is available to constrain the possible metal site structures, XAFS can provide very accurate metal-metal distances. Future developments that may improve this situation are discussed.