J. Am. Chem. Soc. 112, 7278-7286 (1990)
Abstract:
A series of binuclear [(NH3)5Os(Pro)nCo (NH3)5](CF3COO)5 (n=0-4) complexes have been synthesized. Long range intramolecular electron transfer reactions in these polypeptides were studied by the formation of the OsII(Pro)nisoRuIII precursor complexes by using reducing radicals (ca. CO2- and eaq) generated by pulse radiolysis techniques. For the n=0 complex, the intramolecular electron-transfer rate was very fast, and only a lower limit of 5x109 s-1 could be estimated at 25 °C. For the n= 1-3 complexes, the rates and activation parameters for electron transfer were determined to be 3.1 x 106 s-1, DHý = 4.2 kcal/mol, DSý = 15 eu/mol; 3.7 x 104 s-1, DHý = 5.9 kcal/mol, DSý = -19 eu/mol; 3.2 x 102 s-1, DHý = 7.4 kcal/mol, and DSý = -23 eu/mol, respectively. For n = 4, only a rate constant of 50 s-1 at 25 °C was observed. By using a rearranged form of the transition-state expression, a plot of ln k + DHý/RT vs. distance can be used to separate the electronic factor from the nuclear reorganization factor for these electron-transfer reactions. This analysis yielded a slope for the electronic factor beta = 0.65 Å-1. The result of the experiments presented here show that rapid rates of electron transfer across polypeptides can be observed for a metal-to-metal separation of >20 Å even for a low driving force for the reaction (DE° = 0.25 eV). These results can be used to predict fast rates of electron transfer (ca. in the millisecond timescale) across metal-to-metal distances of 40 Å if the driving force and reorganization energy are appropriately controlled.