Electron Transfer across Polypeptides. 2. Amino Acids and Flexible Dipeptide Bridging Ligands

Article abstract of DOI:10.1021/ja00318a029

A series of cobalt(III)-L-ruthenium(III) complexes (I-VIII) with bridging amino acid and dipeptides derivatized with an isonicotinoyl (iso) group at the N-terminal has been synthesized, n)-Co(NH3)5>³⁺ (I-VIII; n = 0, 1, 2) where for n = 1, AA = Gly, Phe, and Pro and for n = 2, (AA)2 = GlyGly, GlyPhe, GlyLeu, and PhePhe.The effect of these flexible bridging groups on the rate of intramolecular electron transfer and its temperature dependence have been studied.The intramolecular electron-transfer rates for the Gly, Pro and Phe amino acid complexes were compared with that of the parent isonicotinic acid complex.The unimolecular rate constant (25 deg C, 1 M HTFA), ΔH^*, and ΔS^* for the intramolecular electron transfer from the Ru(II) site to the Co(III) site for the Gly, Pro, and Phe amino acid complexes are 3.8 * 10^-5 s^-1, 19.9 kcal/mol, -12 eu; 9.9 * 10^-5 s^-1, 18.0 kcal/mol, -16 eu; and 3.9 * 10^-5 s^-1, 19.4 kcal/mol, -14 eu, respectively.For the GlyPhe, GlyLeu, GlyGly, and PhePhe dipeptide complexes, the corresponding unimolecular rate constants, ΔH^* and ΔS^*, are 8.6 * 10^-6 s^-1, 20.3 kcal/mol, -13.5 eu; 15 * 10^-6 s^-1, 14.6 kcal/mol, -31.5 eu; 9.9 * 10^-6 s^-1, 13.3 kcal/mol, -37 eu; and 11.6 * 10^-6 s^-1, 11.2 kcal/mol, -44 eu, respectively.For the amino acid cases the rates were insensitive to the amino acid side chain.In the dipeptide cases the rate constants are very similar, but the differences between the four flexible dipeptides studied were reflected in the temperature dependence of the rate constant.These differences in the activation parameters are related to the differences in the peptide conformation and hydration properties.The slowness of electron transfer in this series of complexes is attributed to the high reorganizational energy around the cobalt site and to the unfavorable driving force.The reactions, however, go to completion because of the rapid release (ca. t_1/2 < microseconds) of the ligands from the Co(II) site.