Elucidation of the effect of some cholinium amino acid ionic liquids on the thermal and the conformational stability of insulin

Abstract The major concerns about protein-based drugs are their stability i.e. maintaining the protein in the folded state throughout processing and storage, as well as the preparation of novel formulation. Stabilization of the monomeric form of insulin ( In ) under the condition of low pH has been a recent challenge. In our earlier investigation, we found that 1-butyl-3-methylimidazolium-based ionic liquids (ILs) containing acetate, trifluoroacetate or dicyanamide anions enhance In thermal stability and prevent protein aggregation. In the present study, six non-toxic ILs containing biocompatible cholinium cation [Chol] and an anion charged amino acid (asparginyl (Asp), glutaminyl (Glu), lysinyl (Lys) and arginyl (Arg)) were synthesized using a two-step procedure. Their effect of the ILs on the In secondary structures was evaluated using FTIR spectroscopy. Rearrangement in the protein molecules, an increase in the beta-structures on behalf of the α-helices, partial denaturation but no aggregation was observed in all In solutions containing ILs. Differential scanning calorimetry was applied to elucidate the effect of ILs on thermal stability of In . Interestingly, in presence of [Chol][Glu] and [Chol] 2 [Asp] the denaturation temperature of the In was shifted to higher temperatures with 9.6 and 4.1 °C, respectively.