Side chain impacts on pH- and thermo-responsiveness of tertiary amine functionalized polypeptides

The systemic investigation of the structural impacts of side chains on the pH- and thermo-responsiveness of tertiary amine functionalized poly(l-glutamate)s (TA-PGs) was carried out. The TA-PGs polymers were effectively synthesized by Cu(I)-catalyzed azide-alkyne cycloaddition click reaction of azido tertiary amines with poly(γ-propargyl-l-glutamate) (PPLG). Turbimetric measurements were performed to characterize the pH- and temperature-induced phase transition of TA-PGs in aqueous solution, which suggested a structural dependence of the properties on the N-substituted groups and the "linkers" between 1,2,3-triazole ring and the tertiary amine groups in the side chains. In detail, the pH responsive properties of TA-PGs were basically determined by the hydrophobicity of the N-substituted groups in the side chains and the pH transition point (pH_t) decreased as the increasing hydrophobicity of the N-substituted groups, while the temperature-responsiveness of TA-PGs were affected by either the N-substituted groups or the "linkers." TA-PGs with a moderate N-substituted amine group (e.g., DEA, PR, and PD) or a branched "linker" (e.g., iso-propylene and 2-methylpropylene group) were more likely to express the LCST-type phase transition tuned by pH variation. These structure-property relationships revealed in this study would help to develop the applications of TA-PGs in smart drug delivery systems.