Peptide molecular junctions: distance dependent electron transmission through oligoprolines.

We have investigated the efficiency of electron transmission through thiolated oligoproline derivatives of general formula: Cys-(Pro)(n)-CSA, where CSA is a cystamine linker and n=1-4. The conductance measurements were performed using STM-based molecular junction approach. We have noted that the conductance of the oligoprolines decays exponentially with increasing length of the molecules and the decay constant was 4.3 nm(-1). This indicates that electron transfer is dominated by superexchange mechanism. Based on this observation, we have concluded that the height of the barrier is affected by the specific conformation of the peptide backbone. Such conclusion is supported by the fact that the oligoprolines do not form intramolecular hydrogen bonds, which could provide alternative electron transfer pathways.