Helix-coil transition and micellar structure of poly(ethylene glycol)-block-poly[N5-(2-hydroxyethyl) L-glutamine] in cyclohexanol/water mixed solvents

Abstract

Structure analysis of associated micelles formed by polypeptide-based diblock copolymer PEG-b-PHEG, consisting of poly(ethylene glycol) (PEG) and poly[N5-(hydroxyethyl) L-glutamine] (PHEG), in dilute solution has been investigated. Cyclohexanol/water mixed solvent was used as selective solvent for PEG-b-PHEG, in which PHEG forms associated domains. Circular dichrorism measurements revealed that PHEG block changes its conformation from α-helix state to random coil state with increase of water content in the mixed solvent from 2.3 to 12.1 wt%. Apparent molecular weight and radius of gyration of associated micelles were derived by light scattering measurements, and most plausible structure of the micelle was evaluated. When the water content is small and helix content of PHEG is almost 100%, a cylindrical micelle with large association number was suggested, and a vesicle-like hollow cylindrical micelle with large diameter was proposed. With the increase of water content and decrease of helix content, association number of the micelle decreased and a core-corona type cylindrical micelle with densely packed PHEG core was plausible. These structure changes of the micelles were interpreted by possible chain packing of α-helical and randomly-coiled PHEG and curvature of PHEG core domain. The obtained results indicate that the conformation and packing manner of the core-forming chain strongly influence the structure of the diblock copolymer micelles.