The effect of alkyl side groups on the secondary structure and crystallization of poly(ethylene glycol)-block-polypeptide copolymers

The solid state and melt nanoscale structures of a series of poly(ethylene glycol)-block-poly(γ-alkyl-l-glutamate)s (PEG-b-PALG) copolymers bearing different alkyl side groups, i.e., n-butyl, n-hexyl, n-octyl, and n-dodecyl, have been investigated in the present work. An interesting effect of alkyl side groups on the secondary conformation of polypeptide block, crystallization of PEG segments and morphology of the copolymers was observed. With increasing the length of alkyl side groups or raising the temperature, the predominant secondary structure gradually changed from α-helix to β-sheet conformation. Differential scanning calorimetry, wide-angle X-ray diffraction analysis and polarized optical microscopy indicated that the crystallization and morphology of PEG segments were markedly restricted by the polypeptide block. Therefore, the alkyl side groups not only exhibited significant influence on the conformation of polypeptide block, but also affected the crystallization of PEG segments. The secondary structure, crystallization and spherulite morphology of these synthetic biomaterials, which were significantly influenced by alkyl side groups, may play an important role in their physicochemical properties, such as self-assembly, as well as in some biomedical applications.