The influence of dipole moments on the mechanism of electron transfer through helical peptides

The life time of aromatic radical cations is limited by reactions like β-elimination, dimerization, and addition to the solvent. Here we show that the attachment of such a radical cation to the C-terminal end of an α-/3(10)-helical peptide further reduces its life time by two orders of magnitude. For PPII-helical peptides, such an effect is only observed if the peptide contains an adjacent electron donor like tyrosine, which enables electron transfer (ET) through the peptide. In order to explain the special role of α-/3(10)-helical peptides, it is assumed that the aromatic radical cation injects a positive charge into an adjacent amide group. This is in accord with quantum chemical calculations and electrochemical experiments in the literature showing a decrease in the amide redox potentials caused by the dipole moments of long α-/3(10)-helical peptides. Rate measurements are in accord with a mechanism for a multi-step ET through α-/3(10)-helical peptides that uses the amide groups or H-bonds as stepping stones.