Electron Transfer Across α-Helical Peptide Monolayers: Importance of Interchain Coupling

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.

Structure dependent spin selectivity in electron transport through oligopeptides

“…The same concept was applied in the past both with CP-AFM and STM for investigating length dependent [28][29][30][31][32] and force dependent conduction. 4,[33][34][35][36] The present study demonstrates the use of CP-AFM to study the spin dependent electron transfer through oligopeptide monolayers. A magnetic tip is used in the present case, unlike conventional CP-AFM, to study the spin specific electron conduction by tuning the magnetic polarity of the tip.…”

“…38 Based on the tilt angles as defined in Fig. 2, the thicknesses of the SAMs (d t ) are estimated to be about 1.5 and 1.8 nm, assuming molecular lengths (d) of 2.2 and 2.8 nm 36 for Al5 and Al7, respectively. To verify these values, the thicknesses of the oligopeptide monolayers were measured using ellipsometry and the values obtained were 1.6 ± 0.2 and 1.9 ± 0.1 nm for Al5 and Al7, respectively, in good agreement with the values obtained based on the tilt angles.…”

“…33,36 The current densities can be estimated by computing the contact area from the contact radius using the Johnson Kendall Roberts (JKR) model. 36 For these thin films, this estimate is semi-quantitative-details are provided in the supplementary material. Figure 4(B) shows the variation of current density as a function of the tip-loading force.…”

“…They attributed it to the inter-molecular electron transfer at high loading forces. 36 It is well-documented that for alkyl thiol monolayers, most of the deformation under tip-loading leads to additional tilting of the molecules. 42,43 Additional tilt enhances intermolecular electron transfer by shortening the tunneling distance between the contacts.…”

“…42 Similar observations were reported earlier for oligopeptide monolayers for which it was suggested that the electrons hop through adjacent amide bonds in the helix. 36 However, it is important to realize that oligopeptide helices (elastic modulus 1.2 GPa) 36 are an order of magnitude more flexible than alkyl thiols (elastic modulus ∼13 GPa). 42 Thus, it is reasonable to assume that the tip loading force has relatively greater influence on the compression that affects the ratio of radius (a) to pitch (b) [ Figure 2(B)] as compared to the tilt angle of the entire molecule.…”