Evidence against the Hopping Mechanism as an Important Electron Transfer Pathway for Conformationally Constrained Oligopeptides

An electron transfer (ET) between a donor and an acceptor through a molecular bridge usually occurs by the superexchange mechanism where electron tunneling occurs without transient occupation of the states of the bridge. The ET may also occur by an incoherent hopping mechanism where the bridge is more directly involved in the process. We have studied the electrochemically induced ET process from a p-cyanobenzamide moiety to a peroxide acceptor across alpha-aminoisobutyric acid (Aib) homo-oligomers of different lengths. The peptides based on the Aib unit are constrained and stabilized by intramolecular hydrogen bonds, which lower the energy of the bridge and thus favor the hopping mechanism. Nevertheless, our results indicate that the latter mechanism can be discarded.