Self-assembled monolayers of alpha-helical peptide on gold surface were employed as model systems for the investigations of mediated electron transfer. The measurements of electron transfer efficiency through single helical peptide molecules were performed using scanning tunneling spectroscopy (STS). The molecules were trapped between the gold tip and the substrate. The electrical contact between the molecule and the gold probe was achieved by the use of peptides containing thiol groups present at each end of the helix. The electron transfer behavior of the peptides was examined as a function of tip-substrate distance at fixed bias voltage and as a function of bias voltage at a fixed distance between the tip and the substrate. The data obtained from these experiments indicated that there is a directional dependence of electron transmission through peptide. It is suggested that the observed electron transfer behavior is connected with the electric field generated by the molecular dipole of the helix.

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1. Distance dependence in peptide-mediated long range electron transfer