Electron transport properties of a helical peptide dithiol were investigated by using scanning tunneling microscopy under ultrahigh vacuum. A helical peptide, which carries two thiol moieties at both terminals but with different protecting groups, was synthesized. One terminal was deprotected and was incorporated into defects of the pre-assembled dodecanethiolate monolayer on gold. Then, the other terminal was deblocked to bind with gold nanoparticles, resulting in formation of metal-molecule-metal junction. Current-distance measurement of gold-peptide dithiol-gold nanoparticle by scanning tunneling microscopy clearly showed a plateau region in the current response, indicating the physical contact of the tip with the gold nanoparticle. Keeping this contact, current-voltage measurement was carried out to show a rectification behavior. The enhancement of electron transport at the negative bias can be explained by taking the dipole effect of the helical peptide into consideration. (c) 2005 Elsevier B.V. All rights reserved.