Self-assembled monolayer (SAM) of peptides is investigated in terms of the correlation of their molecular orientation and the dipolar interactions. The used helical peptides containing a disulfide group on the terminal, which can form SAMs on a gold substrate, were synthesized. The molecular orientation was estimated from the reflection absorption IR intensity ratio of amide I and amide II bands which have transition moment almost parallel and perpendicular to the molecular axis, respectively. The adsorption behavior and layer thickness were determined by surface plasmon resonance.

The molecular orientation depended on the dipolar interaction between the peptide molecules and between the peptide dipole and its image. The molecular direction against the substrate affects the orientation probably due to the interaction between the peptide dipole and Au-S polarization. This speculation is supported by the fact that the orientation difference by the molecular direction became smaller when the spacer from the helix body to the disulfide group is longer. The results indicate that the careful molecular design and optimal preparation condition of SAMs allow formation of well-organized and predetermined structure of nano-architecture even by use of large and complex molecules.