Behavior of rigid macromolecules in self-assembly at an interface

Abstract

Self-assembly (SA) on gold substrates of α-helical poly(γ-benzyl-L-glutamate) [(PBLG); molecular weight ∼20 000 kd] with a disulfide moiety attached at its N-terminus (PBLGSS) was investigated. The SA films were compared with control experiments using unlabeled physisorbed (PS) PBLG and Langmuir–Blodgett (LB) deposited PBLG monolayers. Characterization of the films included angle-dependent x-ray photoelectron spectroscopy, reflection–absorption Fourier transform infrared spectroscopy, ellipsometry, and contact angle measurements. For PBLGSS, a SA monolayer was formed wherein the α-helical conformation was retained and the polypeptides were preferentially bound to gold through the disulfide moiety. This terminal bond to the substrate results in a nonplanar distribution of helix axes (i.e., on average tilted up from the gold surface). This is different from the unlabeled PBLG controls, PS film and LB monolayers, wherein, respectively, less polymer was adsorbed and the helices lie in the plane of the gold substrate. Furthermore, polypeptide aggregation in the solution (e.g., in dichloromethane or 1,4-dioxane) appears to have an influence on the self-assembling behavior of these macromolecules relative to known nonaggregating solvents (N,N-dimethylformamide).