Electron transfer through a self-assembled monolayer of a double-helix peptide with linking the terminals by ferrocene.

A unique molecular structure, a double-helix peptide, was self-assembled on gold, and the electron transfer through the monolayer was studied. The double-helix peptide consists of two 9mer 3(10)-helical peptide chains having a disulfide group at each N terminal and being linked by a ferrocene dicarboxylic acid between the C terminals. Each helical peptide chain has three naphthyl groups in a linear arrangement along the helix. The monolayer properties and the electron transfer from the ferrocene unit to gold were studied with reference peptides with a similar double helix but without naphthyl groups, a single helix with a dicarboxylic ferrocene unit, and a single helix with a monocarboxylic ferrocene unit. It was demonstrated that the naphthyl groups on the side chains had no effect on electron transfer, and the electron-transfer rate in the double-helix monolayer was not promoted, despite the two electron pathways in the molecule. We propose that in the double-helix monolayer, molecular motions are suppressed, possibly by its rigid structure tethered by the two linkers on gold to cancel out acceleration effects of the 2-fold electron pathways and the ferrocene substitution number. The factors that affect the electron-transfer reaction across the helical peptide SAMs are discussed in depth.