Significance of secondary structure in nanostructure formation and thermosensitivity of polypeptide block copolymers

Well-defined nanostructural control from biological motifs is gaining attention among materials scientists. We are reporting that the β-sheet structure of l-polyalanine plays a critical role in developing a fibrous nanostructure as well as the sol-to-gel transition of amphiphilic poly(ethylene glycol)-l/or dl-polyalanine diblock copolymers. l-isomers underwent transitions from random coils to β-sheets, and to nanofibers as the polymer concentration increased, whereas the dl-isomer remained as a random coil structure without developing any specific nanostructure. At high polymer concentrations, the aqueous polymer solutions underwent a sol-to-gel transition as the temperature increased, a so called reverse thermal gelation. The l-isomer with a preassembled β-sheet secondary structure facilitates the sol-to-gel transition rather than the dl-isomer with a random coil structure. Thus, only the l-isomer showed a sol-to-gel transition in the physiologically important range of 20-40 °C. This report provides fundamental information on the relationship between hierarchical structures of polypeptides and the thermosensitive sol-gel transition of the polypeptide aqueous solution.