| Polypeptides, which are polymers of amino acids, exhibit the same secondary structures (α-helix and β-sheet) as seen in proteins, and they have been the focus of research for the development of new materials for biosensors, microoptical devices, and chiral separations. All these applications require the presence of polymers at interfaces. In this work, our objectives were to understand the properties and characteristics of covalently bonded and physically adsorbed poly(amino acid) films synthesized by vapor deposition onto solid and porous substrates and to examine the selectivity of the deposited polymers for chiral molecules.; In order to study the relationship between polymer conformation and film thickness, kinetics, surface energy, and morphology, we performed vapor deposition to synthesize surface-grafted polypeptides from a wide range of α-amino acids: γ-benzyl-L-glutamate, γ-methyl-L-glutamate, β-benzyl-L-aspartate, O-benzyl-L-serine, S-benzyl-L-cysteine, O-benzyl-Lrtyrosine, L-tryptophan, L-phenylglycine, and L-phenylalanine, L-alanine and L-valine were also examined, but surface grafting was not achieved due to the thermal instability of the monomers. Polymer film growth was fit using a Langmuir isotherm model, and a large initial growth rate was observed for helical and parallel β-sheet structures, while a relatively slow rate was found for anti-parallel β-sheets. The morphology of the films containing both physisorbed and chemisorbed polypeptides consisted of large domains (1 to 8 μm), while wispy or granular structures were observed for covalently bonded polymers. However, the molecular conformation did not affect the final morphology of the vapor-deposited films.; In addition to studying the assembly of polypeptides, we also developed a membrane-based chiral separation system. We modified poly(vinylidene fluoride) (PVDF) and aluminum oxide (Anodisc) ultrafiltration membranes with vapor-deposited poly(amino acids) and investigated the enantioselectivity of these membranes for chiral amino acids (tryptophan, phenylalanine, and tyrosine) and drugs (propranolol, atenolol, and ibuprofen). The membrane selectivity increased as the helical content of the polypeptide increased and the membrane pore size decreased. Dramatic separation of propranolol (enantioselectivity of 34) was achieved using poly(L-glutamic acid)-modified Anodisc membranes, and this chiral recognition is believed to result from a combination of hydrogen bonding, solvation of the aromatic groups, and repulsive steric effects. |
