Synthesis and UCST-type phase behavior of α-helical polypeptides with Y-shaped and imidazolium pendants

Abstract

Thermoresponsive polypeptides with Y-shaped and ionic liquid (IL) pendants (PPLG-(ImX)₂, X = Br, I, or BF₄) were synthesized by bromination of poly(γ-allyl-L-glutamate) with bromine, nucleophilic substitution with NaN₃, copper-mediated 1,3-dipolar cycloaddition, and ion-exchange reaction. FTIR and CD analyses revealed α-helical conformations in both the solid-state and aqueous solution. UV-vis spectroscopy results revealed reversible upper critical solution temperature (UCST)-type phase behaviors in methanol and water for PPLG-(ImBF₄)₂, and in ethanol for PPLG-(ImI)₂. The UCST-type solution phase transition temperature (T_pt) of PPLG-(ImX)₂ (X = I or BF₄) increased with increasing of polymer concentration. The T_pt of PPLG-(ImBF₄)₂ increased with increasing of NaBF₄ concentration, whereas it decreased with increasing of NaI or NaCl concentrations. Moreover, PPLG-(ImI)₂ with a Y-shaped pendant showed a higher T_pt in ethanol than PPLG-ImI which was a traditional IL-based polypeptide with a single IL group per repeating unit, whereas PPLG-(ImBF₄)₂ with a Y-shaped pendant showed a lower T_pt in methanol and water than its counterpart, PPLG-ImBF₄, indicating that the T_pt can be collectively adjusted by incorporation of Y-shaped pendants and counter-anions.