Electronic structures of active sites in electron transfer metalloproteins: contributions to reactivity

Many electron transfer centers in biology involve metal complexes, that exhibit unique spectral features. These reflect highly covalent electronic structures, which contribute to the electron transfer function of the protein. The blue copper center has a highly covalent copperthiolate bond, which promotes long range electron transfer. The CuA center is a mixed valence binuclear complex that is completely delocalized even in low symmetry protein environments. The [2Fe2S] center is valence localized in the mixed valence Fe(III)Fe(II) oxidation state, while the mixed valence [2Fe2S] sub-sites in [4Fe4S] clusters are completely valence delocalized. Factors which contribute to electron localization/delocalization in these mixed valence sites are experimentally evaluated using a variety of spectroscopic and electronic structural methods. These include the very powerful technique of ligand K-edge X-ray absorption spectroscopy for determining the covalency of ligandmetal bonds.