Electron transfer in ruthenium-modified proteins

Abstract

The electron-transfer (ET) reactions of metalloproteins are potentially far more complex than those of small molecules. The structural intricacies of proteins are largely responsible for the added complexity. Reactive centers are surrounded by polypeptide matrices that shield these sites from solvent and separate them from their redox partners. The nonuniform charge distributions on the protein surfaces create anisotropic interactions between redox partners, confounding the interpretation of bimolecular reactions. Covalent or electrostatic coupling of redox partners can be exploited to simplify the problem but does not eliminate all of the complicating factors in metalloprotein ET reactions. The energetics and dynamics of nuclear reorientations accompanying protein ET reactions continue to be a source of inquiry. Multiple conformational states in the polymer surrounding the redox sites create the potential for "gating" and directional electron transfer. The peptide matrix also separates the redox sites, leading to questions of how protein ET can be efficient over such long (> 10 Å) distances. One line of research aimed at addressing many of the fundamental issues in protein ET involves the use of proteins modified by the coordination of Ru complexes to surface amino acid residues.