Electron Transfer Across α‑Helical Peptide Monolayers: Importance of Interchain Coupling Jan Pawlowski, Joanna Juhaniewicz, Dagmara Tymecka, and Slawomir Sek* Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland ABSTRACT: Four helical peptides with the general formula (Boc)-Cys-(S-Acm)-(Ala-Leu) n -NH-(CH 2 ) 2 -SH (n = 4-7) were synthesized and further used for the preparation of self-assembled monolayers (SAMs) on gold substrates. The electron-transfer behavior of these systems was probed using current-sensing atomic force microscopy (CS-AFM). It was found that the electron transmission through SAMs of helical peptides trapped between an AFM conductive tip and a gold substrate occurs very efficiently and that the distance dependence obeys the exponential trend with a decay constant of 4.6 nm -1 . This result indicates that the tunneling mechanism is operative in this case. Conductance measurements under mechanical stress show that peptide-mediated electron transmission occurs with the possible contribution of intermolecular electron tunneling between adjacent helices. It was also demonstrated that an external electric field applied between metallic contacts can affect the structure of the peptide SAM by changing its thickness. This explains the asymmetry of the current-voltage response of metal-monolayer-metal junction. ■ INTRODUCTION Peptides are known to be crucial components of proteins that provide different functions in biological systems. These include enzymatic catalysis, control of mass transport, adhesion, regulation of biochemical processes, energy storage, and electron transfer. 1 Such a broad range of functions results from the diversity of the peptides and their ability to adopt numerous structural motifs. Given the above, peptides seem to be excellent components that can be suitably designed to provide specific properties that are useful in nanoscale electronics and biosensing devices. 2,3 In most cases, the successful application of peptides in such nanodevices requires the adsorption of molecules on a conductive substrate in a conformation that enables the efficient mediation of the electron-transfer process. It was demonstrated in numerous papers that peptides assembled into molecular layers immobilized on a metallic surface can act as electron-transfer mediators. 4-10 Moreover, the efficiency of this process can be modulated by the changes in the secondary structure and length of the peptide. 11-13 Among the variety of peptide structural motifs, helical structures seem to be the most efficient electron-transfer mediators. As reported by Kimura’s group, α- helical peptides organized within a self-assembled monolayer (SAM) enable long-range charge transport over an enormous distance of 10 nm. 14,15 The good mediating properties of helices are also confirmed by the relatively weak distance dependence of electron transfer along the peptide bridge. The decay factors reported for helical peptides are in the range of 0.2-5.0 nm -1 . 8,9,12,14 The large spread in the reported decay constants results from the fact that the overall electron transfer through peptides can be affected by two mechanisms: tunneling and hopping. 6 Their contribution varies with the length of the mediating bridge. 16 Tunneling dominates for short bridges, giving rise to a sharp exponential distance dependence. For longer bridges, the hopping contribution prevails, resulting in a much weaker distance dependence. Unfortunately, it is difficult to indicate the sharp transition between these two mechanisms. Usually for helical bridges the increased contribution of hopping, recognized as a weakening of the distance depend- ence, become apparent for bridges exceeding 3.0 nm in length. 9,14,17 Nevertheless, this number cannot be considered to be a stiff limit because the relative changes in the contributions of two mechanisms occur gradually. Another important factor that needs to be considered in a description of electron transfer through the helix is related to the motional freedom of the adsorbed peptide. Molecular dynamics was demonstrated to have a large impact on electron-transfer behavior, and the restriction of some vibrational modes of the molecule results in the suppression of the electron-transfer rate. 9,18 Additionally, the whole picture is complicated by the fact that the structure of the peptide can also be affected by an external electric field. For example, the variation of the helical peptide SAM thickness induced by the potential applied to the underlying substrate was reported by Wain et al. 19 Moreover, Kimura and co- workers have demonstrated that the peptide molecule placed between a metallic substrate and an STM tip changes its conformation from an α-helix to a 3 10 -helix in response to the applied bias voltage. 20 It is known that helical peptides possess large dipole moments along their molecular axes, with a partial positive charge at the N terminus and a partial negative charge at the C terminus. Therefore, the helix can be compressed or stretched depending on whether the external electric field is Received: July 6, 2012 Revised: September 28, 2012 Published: November 26, 2012 Article pubs.acs.org/Langmuir © 2012 American Chemical Society 17287 dx.doi.org/10.1021/la302716n | Langmuir 2012, 28, 17287-17294