Distance dependence of long-range electron transfer through helical peptides.

Helical peptides of 8mer, 16mer, and 24mer carrying a disulfide group at the N-terminal and a ferrocene moiety at the C-terminal were synthesized, and they were self-assembled on gold by a sulfur-gold linkage. Infrared reflection-absorption spectroscopy and ellipsometry confirmed that they formed a monolayer with upright orientation. Cyclic voltammetry showed that the electron transfer from the ferrocene moiety to gold occurred even with the longest 24mer peptide. Chronoamperometry and electrochemical impedance spectroscopy were carried out to determine the standard electron transfer rate constants. It was found that the dependence of the electron-transfer rates on the distance was significantly weak with the extension of the chain from 16mer to 24mer (decay constant beta = 0.02-0.04). This dependence on distance cannot be explained by an electron tunneling mechanism even if increased hydrogen-bonding cooperativity or molecular dynamics is considered. It is thus concluded that this long-range electron transfer is operated by an electron hopping mechanism.