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

Many electron transfer centers in biology involve metal complexes, that exh ibit unique spectral features. These reflect highly covalent electronic str uctures, which contribute to the electron transfer function of the protein. The blue copper center has a highly covalent copper-thiolate bond, which p romotes long range electron transfer. The Cu-A center is a mixed valence bi nuclear complex that is completely delocalized even in low symmetry protein environments. The [2Fe-2S] center is valence localized in the mixed valenc e Fe(III)Fe(II) oxidation state, while the mixed valence [2Fe-2S] sub-sites in [4Fe-4S] clusters are completely valence delocalized. Factors which con tribute to electron localization/delocalization in these mixed valence site s are experimentally evaluated using a variety of spectroscopic and electro nic structural methods. These include the very powerful technique of ligand K-edge X-ray absorption spectroscopy for determining the covalency of liga nd-metal bonds. (C) 2000 Elsevier Science S.A. All rights reserved.