Water-soluble polypeptides bearing 1-alkylimidazolium (methyl or n-butyl) and various counter-anions (i.e., Cl-, I- or BF4-) are prepared by ring-opening polymerization of gamma-4-chloromethylbenzyl-L-glutamate-based N-carboxyanhydride (3), post-polymerization of poly(gamma-4-chloromethylbenzyl-L-glutamate) (4), and ion-exchange reaction. Circular dichroism (CD) analysis reveals that the resulting polypeptides adopt an alpha-helical conformation in water with a fractional helicity in the range of 30%-56% at 20 degrees C and exhibit good conformational stability against temperature variations. The polypeptides exhibit lower critical solution temperature (LCST)-type or upper critical solution temperature (UCST)-type transitions in organic solvents or in water. The UCST-type transition temperature (T-pt) in water is independent on the molecular weight, yet it decreases upon addition of NaCl and increases upon addition of NaI or NaBF4, suggesting a mainly electrostatic interaction mechanism.