Synthesis, conformation transition, liquid crystal phase, and self-assembled morphology of thermosensitive homopolypeptide

Thermosensitive diethylene glycol-derived poly(L-glutamate) homopolypeptides (i.e., poly-L-EG2-Glu) with different molecular weights (MW) (Mn,GPC = 5380-32520) were synthesized via the ring-opening polymerization (ROP) of EG2-L-glutamate N-carboxyanhydride (EG2-Glu-NCA) in N,N-dimethylformamide solution at 50 degrees C. Their molecular structure, conformation transition, liquid crystal (LC) phase behavior, lower critical solution temperature (LCST) transition, and morphology evolution were thoroughly characterized by means of FTIR, 1H NMR, gel permeation chromatography, differential scanning calorimetry, wide angle X-ray diffraction, polarized optical microscope, transmission electron microscope, and dynamic light scattering. In solid state, the homopolypeptide poly-L-EG2-Glu presented a conformation transition from a-helix to beta-sheet with increasing their MW at room temperature, while it mainly assumed an a-helix of 80-86% in aqueous solution. Poly-L-EG2-Glu showed a thermotropic LC phase with a transition temperature of about 100 degrees C in solid state, while it gave a reversible LCST transition of 34-36 degrees C in aqueous solution. The amphiphilic homopolypeptide poly-L-EG2-Glu self-assembled into nanostructures in aqueous solution, and their critical aggregation concentrations decreased with increasing MW. Interestingly, their morphology changed from spherical micelles to worm-like micelles, then to fiber micelles with increasing MW. This work provides a simple method for the generation of different nanostructures from a thermosensitive biodegradable homopolypeptide. (C) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012