Bond-Mediated Electron Tunneling in Ruthenium-Modified High-Potential Iron−Sulfur Protein

High-potential iron−sulfur proteins (HiPIPs) are found in photosynthetic purple nonsulfur bacteria. The three-dimensional structure of Chromatium vinosum HiPIP features two short segments of α-helix, three strands of antiparallel β-pleated sheet, and a small helix near the N-terminus. The cubane [Fe_4S_4] cluster is attached covalently to the polypeptide matrix through Fe−S^γ bonds to cysteines 43, 46, 63, and 77. The side chains of Tyr19, Phe48, Trp60, Phe66, Trp76, Trp80, and other nonpolar residues encapsulate the cluster in a hydrophobic cavity that is inaccessible to solvent. Tyr19, which contacts the [Fe_4S_4] core, has been suggested to play a particularly important structural role. In both oxidation states, the cysteinyl and core inorganic sulfur atoms are involved in H-bonding interactions with peptide NH protons