Conductance of r-Helical Peptides Trapped within Molecular Junctions Slawomir Sek,* Aleksandra Misicka, Karolina Swiatek, and Elwira Maicka Department of Chemistry, UniVersity of Warsaw, Pasteura 1, 02093 Warsaw, Poland ReceiVed: May 19, 2006; In Final Form: July 14, 2006 Self-assembled monolayers of R-helical peptides on a gold surface were employed as model systems for the investigation of mediated electron transfer. The peptides contained 14, 15, 16, and 17 amino acid residues. The measurements of electron transmission through single molecules of helical peptides were performed using scanning tunneling spectroscopy (STS). The molecules were trapped between the gold tip and the substrate. 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 conductance behavior of the peptides was examined as a function of tip-substrate distance at fixed bias voltage. Measurements performed with peptides containing different numbers of amino acid residues indicate that the distance dependence of electron transmission through an R-helix is weaker than that through simple n-alkyl bridges. Introduction The importance of peptides in mediating electron transfer in biological systems is unquestionable. 1-3 However, the exact mechanism of this process is still controversial. It has been shown in numerous reports that two mechanisms can contribute to electron transfer through peptides: superexchange and hopping. 4-10 Theoretical considerations as well as some experimental studies indicate that the first mechanism is valid for short-chain peptides whereas the second contributes sig- nificantly to electron transfer for longer spacers. 4,6 The dis- tinction between these two mechanisms can be made on the basis of the distance dependence of electron transfer. Consider- ing the change of an arbitrarily chosen parameter related to the rate of electron transport as a function of the length of the molecular bridge, the dependence is expected to be exponential for the superexchange mechanism and linear for hopping. 4,10,11 Thus, the distance dependence in the case of the hopping mechanism is much weaker than that for super- exchange tunneling. Such a model seems to explain efficient electron-transfer mediation observed for long amino acid sequences. 5,9,12 Moreover, a transition between the super- exchange and hopping mechanisms was experimentally ob- served for oligoprolines. 6 Unfortunately, the conclusions from studies on particular bridges cannot be generalized for all systems because a number of factors other than the length of the spacer influence the overall electron transfer through pep- tides. These include the details of the amino acid sequence, the secondary structure of the peptide, and the presence of hydro- gen bonds. For example, upon comparison of the electro- chemical data obtained for R-helical peptides, it is evident that the rate constants of electron transfer vary signifi- cantly although the lengths of the spacers are similar. 5,9 This probably reflects the differences in the details of amino acid sequences. Several groups have also demonstrated the influ- ence of the secondary structure of the peptide on electron- transfer efficiency. 13-17 Shin and co-workers showed in their theoretical study that the parameter that describes the ability of the bridge to mediate the electron transfer, i.e., the distance decay constant (), is sensitive to the secondary structure of the peptide backbone in the case of the superexchange mech- anism. 13 Ghiggino and co-workers observed the different distance dependences for photoinduced electron transfer through R-helical and -sheet structures and found corresponding de- cay constants of 0.66 and 0.73 Å -1 , respectively. 14,15 A very weak distance dependence for 3 10 -helices of aminoisobutyric oligomers was reported by Maran and co-workers, who at- tributed this result to the effect of intramolecular hydrogen bonding on the electron-transfer rate. 16 Kraatz and co-workers also observed a shallow distance dependence of the electron- transfer rate for collagen-like peptide assemblies with interstrand hydrogen bonds. 17 All of these results clearly show that the variability in electron-transfer efficiency for peptide bridges is related to a number of factors that control the structures of the individual peptide backbones as well as the larger peptide assemblies. We believe that the present work will contribute to the understanding of the mechanism of electron transfer through peptide bridges. We report here conductance measurements of single R-helical peptide molecules with different lengths. The efficiency of electron transmission through the peptides was investigated using the scanning-tunneling-microscopy- (STM-) based molecular junction method, which involves the entrapment of single molecules between the STM tip and the substrate. This approach has been successfully utilized for conductance mea- surements of dithioderivatives of alkanes, carotenoid polyenes, DNA, viologen, and short-chain peptides consisting of 2-4 amino acid residues. 18-26 In this article, we describe molecular junction experiments performed with monolayers of R-helical peptides containing 14-17 amino acid residues. Because the peptides are asymmetric, we used a monolayer preparation procedure leading to the uniform orientation of molecules on the surface. Previously, it was shown that molecular junctions formed with R-helical peptides produce asymmetric current- voltage responses; thus, control of the orientation of the molecules is crucial to obtain reproducible results. 27 The efficiency of electron transmission through peptides was com- pared to that through n-alkanedithiols. * To whom correspondence should be addressed. E-mail: slasek@ chem.uw.edu.pl. 19671 J. Phys. Chem. B 2006, 110, 19671-19677 10.1021/jp063073z CCC: $33.50 © 2006 American Chemical Society Published on Web 09/12/2006