Preparation and thermoresponsive properties of helical polypeptides bearing pyridinium salts

Water-soluble polypeptides bearing various pyridinium groups (i.e., pyridinium, 2-methylpyridinium, 3-methylpyridinium, 4-methylpyridinium) and Cl− counter-anions have been prepared by nuleophilic substitutions between poly(γ-3-chloropropyl-L-glutamate) (PCPLG) or poly(γ-6-chlorohexyl-L-glutamate) (PCHLG) and pyridine or y-methyl pyridine (y = 2, 3, and 4). Polypeptides bearing pyridinium groups and BF4− counter-anions were prepared by ion-exchange reactions from the polypeptide–pyridinium conjugates with Cl− counter-anions. CD analysis revealed that water-soluble polypeptides adopted α-helical conformation in solutions with a fractional helicity in the range of 24.7–41.4%. Variable-temperature UV-vis spectroscopy revealed that polypeptides bearing 3-methylpyridinium and BF4− counter-anions (i.e., P11 and P15) showed upper critical solution temperature (UCST)-type transitions in aqueous solutions. The UCST-type phase transition temperatures increased as increasing the hydrophobicity of the spacer groups, the polymer concentrations, and the concentrations of NaBF4.