Long-range electron transfer across peptide bridges: The transition from electron superexchange to hopping

Long-range electron transfer rate constants for complexes of the type [(bpy)2RuIIL−Pron−apyRuIII)(NH3)5]5++ proline residues (n) varying 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 kmax values from both techniques coalesce into one data set. The distance dependence of the reactions is consistent with a smooth transition from a superexchange mechanism with attenuation constant β = 1.4 A-1 to a hopping mechanism with attenuation constant β = 0.17 A-1. The transition occurs between n = 3 and 4 prolines, and the virtual hopping rate constant at the shortest distance is about 1 x 106 times slower than that observed for the superexchange value.