Vertical orientation with a narrow distribution of helical peptides immobilized on a quartz substrate by stereocomplex formation

Abstract

Second-harmonic generation (SHG) of a donor–π–acceptor (D–π–A) chromophore attached to helical peptides was used for the evaluation of the self-assembled monolayer (SAM) structure of a stereocomplex of helical peptides. A stereocomplex SAM of a left-handed helical conjugate (D17) and a right-handed helical conjugate (L17) showed an SHG intensity four times larger than a stereocomplex SAM of a left-handed helical D17 and a right-handed helical peptide without the D–π–A chromophore (LA16), which agrees well with dependence of SHG intensities on the surface densities of the D–π–A chromophore. The SHG intensities of enantiopure SAMs of D17 and L17 are, however, 47% and 27% of the stereocomplex SAM of D17 and L17, respectively. These differences can be explained only after taking a larger distribution of the tilt angle of the chromophore in the enantiopure SAMs than in the stereocomplex SAM of D17 and L17. On the basis of these analyses, it is concluded that the stereocomplex SAM of a left-handed helix and a right-handed helix constitutes a well-ordered structure, where the tilt angle of the helical peptide from the surface normal becomes small with a narrow distribution due to stereocomplex formation.