LAUSR

Helical molecules have recently attracted interest due to their capability for robust spin polarization of transmitted electrons. By means of mechanically controlled break Au junctions, we analyze the transport properties of single lysine-doped and cysteine-terminated polyalanine (PA) molecules of various lengths (2.4--5.4 nm). The conductance varies exponentially with the (effective) length of the molecules and does not depend on the temperature (90--300 K), thus electron tunneling is the dominant transport mechanism. The decay constant for the PA molecule is found to be $3.5\phantom{\rule{0.28em}{0ex}}{\mathrm{nm}}^{\ensuremath{-}1}$, significantly smaller compared to those of other organic molecules, emphasizing the high conductivity along the helical polypeptides and reflecting a low tunneling barrier, which decreases further for fields exceeding $5\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0.28em}{0ex}}\mathrm{V}/\mathrm{cm}$. The conductance histograms of all PA molecules investigated reveal characteristic satellite peaks, which correlate with the apparent molecule length in multiples of characteristic peptide sequences. We attribute this effect to a racheting of interdigitated molecules adsorbed on each side of the electrodes during the opening/closing cycles.