Electric properties of self‐assembled monolayers of helical peptides by scanning tunneling spectroscopy

Abstract

AbstractHelical peptides having a disulfide group at one terminal, Lipo‐(Leu‐Aib)12‐OBzl (Lipo and OBzl represent lipoic acid and benzyl ester, respectively, SSL24B), and Boc‐NHCH2CH2NHCO‐Fc‐CO‐Ala‐(Leu‐Aib)8‐lipoamide (Boc, Fc, and lipoamide represent t‐butyloxycarbonyl, ferrocene, and lipoamine, respectively, FcL16SS), were synthesized, and self‐assembled monolayers (SAMs) of the helical peptides were prepared on gold. When the SSL24B SAM was observed at high‐bias voltage by scanning tunneling microscopy (STM) under ultrahigh vacuum, bright spots with higher conductivity appeared in the STM image. The image was restored again to a homogeneous surface with lower bias voltage, suggesting that Au–S linkage should be reversibly changed to a conductive state. The current/voltage (I‐V) curve by scanning tunneling spectroscopy (STS) at the bright domains demonstrated a rectification behavior where current increases drastically at a positive bias voltage above 3.0 V. However, the I‐V curve of the FcL16SS SAM showed a current increase at negative bias voltage below −1.5 V. The redox group is, thus, a determining factor for the direction of this flow. However, STS at low‐bias voltage and with the tip being inserted in the peptide SAM revealed a zero‐conductance region in the I‐V curve. The peptide layer may act as a molecular capacitor because of a large dielectric constant of the helical peptide. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3493–3500, 2003