Long-Range Electron Transfer across Peptide Bridges: the Mechanistic Transition from Superexchange to Hopping

R. Abdel Malak, Z. Gao, J. F. Wishart, and S. S. Isied

J. Am. Chem. Soc. 126, 13888 - 13889 (2004). [Find paper at ACS Publications]

Abstract:

Long-range electron transfer rate constants for complexes of the type [(bpy)₂Ru^IIL-Pro_n-apyRu^III(NH₃)₅]^{5+ +} proline residues (n) varing from 0 to 9 were determined by complementary electron pulse radiolysis and flash photolysis techniques from the picosecond to the millisecond time scales. The activationless  k_max values from both techniques coalesce into one data set. The distance dependence of the reactions is consistent with a smooth transition from a superexchange with an rate attenuation constant (b = 1.4 Å^-1) to a hopping mechanism with attenuation constant (b = 0.17 Å^-1). The transition occurs between n = 3 and 4 prolines and the virtual hopping rate constant at the shortest distance is about one million times slower than that observed for the superexchange value.