Helical 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. (C) 2003 Wiley Periodicals, Inc.