Water‐Soluble Thermoresponsive α‐Helical Polypeptide with an Upper Critical Solution Temperature: Synthesis,Characterization, and Thermoresponsive Phase Transition Behaviors

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 γ‐4‐chloromethylbenzyl‐_l‐glutamate‐based N‐carboxyanhydride ( 3 ), post‐polymerization of poly(γ‐4‐chloromethylbenzyl‐_l‐glutamate) ( 4 ), and ion‐exchange reaction. Circular dichroism (CD) analysis reveals that the resulting polypeptides adopt an α‐helical conformation in water with a fractional helicity in the range of 30%–56% at 20 °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.