Interrupted Helical Structure of Grafted Polypeptides in Brush-Like Macromolecules

Brush-like polymers are a type of graft polymers with densely and regularly spaced side chains. Their unique topologies and large sizes afforded a number of potential applications in nanotechnology. Incorporating helical polypeptides as grafted chains in brush polymers may allow for the construction of large macromolecules with structurally well-defined, rod-like domains. However, due to a congested local environment in brush polymers, the conformational structure of grafted polypeptides can deviate from the rigid-rod alpha-helical structure observed in homopolypeptides. Because of synthetic challenges, the effect of macromolecular environments on the conformational structures of grafted polypeptides has not been systematically studied in brush polymers. Here we synthesized a small library of polynorbornene-g-poly(gamma-benzyl-L-glutamate) (PN-g-PBLG) with variable PN, PBLG lengths and grafting densities, and we studied solvent induced helix-coil transition of grafted PBLGs in these brush polymers to explore their specific conformational structures. Instead of forming rigid-rod alpha-helices in helicogenic solvents, the grafted PBLGs in PN-g-PBLG, especially those with high grafting density and high molecular weights (MWs), adopt interrupted helical structures that can be represented by a broken rod model, which was also confirmed by the nuclear overhauser enhancement spectroscopy (NOESY) experiments. The behavior also exists in other types of grafted polypeptides in brush polymers, e.g., PN-g-poly(epsilon-benzyloxycarbonyl-L-lysine) (PN-g-PZLL). These findings and the structural analysis methods we developed in this study provide the guidance to the rational design and characterization of polypeptide-containing macromolecules with complex architectures.