Thermal and conformational stability of insulin in the presence of imidazolium-based ionic liquids

Abstract

Stabilization of the monomeric form of insulin (In) under condition of low pH has been a recent challenge. This research aims to reveal the effect of a series of 1-butyl-3-methylimidazolium-based ionic liquids (ILs) on the stability of In dissolved in highly acidic solution (pH 2.0). Differential scanning calorimetry was applied to assess the thermal stability of In in the presence of these ILs. In addition, we monitored the IL-induced changes in the In secondary structure using Fourier transformed infrared spectroscopy. The peak of In thermal denaturation was shifted to higher temperatures in the presence of the tested acetate, trifluoroacetate and dicyanamide salts. At the same time, chloride and thiocyanate ILs had no effect on the thermal stability of the insulin, while the tricyanomethanide salt slightly destabilized the protein. The change in the In conformation affected not only the position but also the sharpness and the shape of the transition peak. As a whole, those ILs which were able to preserve or enhance helical structure of In produced stabilizing effect and those which stimulated the formation of unordered and random-coiled structures deteriorated its thermal stability. No aggregation of In in the presence of the imidazolium-based ILs was observed under the tested acidic media.