Formation of oriented helical peptide layers on a gold surface due to the self-assembling properties of peptides

Several hydrophobic α-helical peptides containing a disulfide group were synthesized, and the formation of oriented self-assembled monolayers (SAMs) on gold surface was investigated. The orientation of helices in the SAMs was determined by Fourier transform infrared reflection-absorption spectroscopy measurements. The tilt angle of the helix axis from the surface normal was sensitively affected by the choice of solvent used in the preparation of SAMs, the nature of component amino acids, the molecular structure (either one-helix or two-helix peptide), and the length of peptide chain. The tilt angle was smaller when ethanol was used in the preparation of SAMs rather than N,N-dimethylformamide. Lipo-(Ala-Aib)₈-OBzl (Lipo and OBzl represent a lipoic acid group and a benzyl ester group, respectively) showed a smaller tilt angle than Lipo-(Lys(Z)-Aib)₈-OCH₃. A one-helix peptide with a lipoic acid group at the N-terminal showed a smaller tilt angle than a two-helix peptide, in which the two helix peptides were connected by a disulfide linkage. The longest peptide, Lipo-(Ala-Aib)₁₂-OBzl, showed the smallest tilt angle of 30° when the SAM was prepared in an ethanol solution. Taken together, helix-helix interaction should play a more important role in regulating the orientation of helical peptides in the SAM than the Au-disulfide interaction. Lipo(Ala-Aib)₁₂-OBzl formed a nearly vertically oriented SAM with a parallel orientation of helices due to the highly self-assembling properties of the peptide.