The stability of insulin in the presence of short alkyl chain imidazolium-based ionic liquids

Abstract

Fibril or aggregation formation in insulin (In) has been a subject of severe biomedical and biotechnological complications. The search for a novel solvent/co-solvent that can provide long term stabilization for In monomeric form has not been completed yet. In this quest, for the first time we have successfully explored the stability of the monomeric form of In in the presence of ammonium-based ionic liquids (ILs) [A. Kumar and P. Venkatesu, RSC Adv., 2013, 3, 362–367]. Further, in continuation, in this study, we have established the stability of In in the presence of imidazolium-based ILs with different anions. These anions represent the Hofmeister series of anions of ILs. In this regard, we have carried out UV-vis, fluorescence, circular dichroism spectral analysis and dynamic light scattering (DLS) measurements of In in various concentrations of these ILs. Our experimental findings reveal that Br− and Cl− ILs stabilized the native state while the rest of the ILs with anions such as SCN−, HSO4−, CH3COO− and I− were denaturants for the In. Further, the results show that IL–In interactions are difficult to classify on the basis of the Hofmeister series, as bromide containing ILs show more stabilizing properties on the In structure. The disulfide bonds were almost intact in the presence of Br− IL as compared to Cl− and the rest of the Hofmeister anions. On the other hand, a strongly hydrated kosmotropic anion like HSO4− interacts with the structure of In, leading it towards complete denaturation of the In structure. Additionally, all ILs failed to protect the native state of In with increasing temperature.