Self-assembly of a rodlike polypeptide on solid surfaces: Role of solvent, molecular weight, and time of assembly

The interplay between preparative conditions and molecular characteristics during the self-assembly of a rigid, rodlike, alpha -helical polypeptide was investigated. Poly(gamma -benzyl-L-glutamate) was prepared by a novel synthetic route wherein the polymerization of the N-carboxyanhydride was initiated with an amine terminated disulfide, and the SSPBLG was characterized using light scattering, viscometry, and size-exclusion chromatography. Self-assembly of SSPBLG on gold substrates was found to proceed rapidly and high molecular weight (similar to 27 kDa) polypeptides formed self-assembled monolayers (SAMs) on a time scale of minutes. Characterization of the SAMs by polarization modulation infrared absorption spectroscopy (PM-IRRAS) was used to explore the effects of the molecular weight of the polypeptide, the time of assembly, and the solvent. An increase in the length of the polypeptide from 66 to 156 Angstrom caused an increase in the average tilt of the helix axis from the normal to the surface. The use of a nonpolar solvent (benzene), where antiparallel aggregation of helices is known to occur, did not prove to be an effective strategy for improving orientation of the longer polypeptides. The time of assembly was found to play a key role, and long times of assembly led to a decrease in surface coverage and increase in tilt angle. This effect was more pronounced for the short polypeptide than the long polypeptide. Self-assembly of a debenzylated SSPBLG showed that physisorption due to hydrophobic interactions was important in the preparation of SAMs formed from long polypeptides. Reduction of physisorption by use of the debenzylated SSPBLG led to an enhancement in surface coverage and decrease in the tilt angle of the higher molecular weight SSPBLG.