Functional Polystyrene Derivatives Influence the Miscibility and Helical Peptide Secondary Structures of Poly(γ-benzyl L-glutamate)

We synthesized polystyrene (PS) and poly(acetoxystyrene) (PAS) homopolymers through atom transfer radical polymerization (of styrene and 4-acetoxystryene monomers, respectively) and then prepared poly(vinylphenol) (PVPh) through acetoxyl hydrazinolysis of PAS with hydrazine monohydrate. To investigate the influences of these three functionalized polystyrene derivatives on the helical peptide secondary structures and miscibility behavior of polypeptide homopolymers, we blended PS, PAS, and PVPh with a low-molecular-weight poly(gamma-benzyl L-glutamate) (PBLG) homopolymer and analyzed these blends using differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, solid state nuclear magnetic resonance (NMR) spectroscopy, and wide-angle X-ray diffraction (WAXD). Variations in the intermolecular interactions (e.g., pi-pi, dipole-dipole, hydrogen bonding) strongly affected the miscibility behavior and secondary structures of PBLG. The weak pi-pi interactions between PS and PBLG resulted in only partial miscibility, with the alpha-helical secondary structure of PBLG remaining almost unchanged upon increasing the content of the PS homopolymer; in contrast, the stronger dipole-dipole interactions between PAS and PBLG and the hydrogen-bonding interactions between PVPh and PBLG led to complete miscibility, with the content of alpha-helical PBLG secondary structures increasing upon increasing the contents of both the PAS and PVPh homopolymers. Because the hydrogen-bonding interactions in the PVPh/PBLG blends were stronger than the dipole-dipole interactions in the PAS/PBLG blends, the fractions of alpha-helical secondary structures in the PVPh/PBLG systems were larger than those in the PAS/PBLG systems. Indeed, the contents of alpha-helical conformations in these three blend systems correlated strongly with the strength of their intermolecular interactions.