Charge-Ordered α-Helical Polypeptide Monolayers on Au(111)

The growth and electronic levels of molecular monolayer structures of helical polyalanine-based peptides (PA) on Au(111) surfaces were investigated by scanning tunneling microscopy (STM), spectroscopy (STS), and optical ellipsometry under ambient conditions. The self-assembled monolayer (SAM) films revealed a high degree of lateral and rotational order. Moreover, because of the formation of Au-S bonds, resulting from the termination of the helix by cysteine, the PA molecules are oriented, and their intrinsic dipole moment is tilted by around 50° with respect to the surface normal. This charge ordering within the SAM facilitates internal electric fields of up to 1 V/nm, which may renormalize the molecular orbital energies along the helix, thus enabling a high conductance through these peptides. The characteristic bonding scheme and ordering, found in this study for chiral and polar PA molecules, will be important to understand the accompanied spin polarization of propagating electrons as well as the spin exchange mechanism at the hybrid interface.