Insight into impact of choline-based ionic liquids on bovine β-lactoglobulin structural analysis: Unexpected high thermal stability of protein.

In this present work, we report the effects of different concentrations of various cholinium-based ionic liquids (ILs) on the structural and thermal stability of β-lactoglobulin (β-LG). Results indicated that anions in ILs have played an important role in affecting the thermal and structural stability of β-LG. Biomolecular interactions between β-LG and ILs are carried out by differential scanning calorimetry (DSC), different spectroscopic and dynamic light scattering (DLS). Analysis of experimental data revealed that an excellent thermal stability of β-LG is obtained in presence of choline dihydrogen phosphate [Chn][Dhp] and choline bitartrate [Chn][Bit] where Tm value of β-LG increased to 93.58 and 93.22 °C, respectively, as compared to Tm of β-LG in buffer at 72.48 °C. Similarly, in presence of choline acetate [Chn][Ac] and choline chloride [Chn][Cl] thermal stability of β-LG also increased, however, it was not possible to calculate Tm values in case of choline iodide [Chn][I], because of obtaining two endothermic peaks in DSC curves. On the other hand, choline hydroxide [Chn][OH] acts as complete destabilizer for β-LG native structure as no Tm is obtained in its presence. The obtained results are further confirmed by estimating the thermodynamic parameters such as Gibbs free energy of unfolding (ΔuG), enthalpy of unfolding (ΔuH), heat capacity change (ΔCp) and total entropy change during protein unfolding (ΔuS). Molecular docking studies of β-LG and various choline-based ILs are also performed to know the probable binding conformations using AutoDock Vina and AutoDock tools 1.5.6, results obtained are in correlation with spectroscopic and biothermodynamic data. The combined study by DSC, spectroscopy techniques and molecular docking studies suggest that some of these ILs can be used as industrial green solvents for different biocatalytic processes and also can help in resolving the problems involving protein unfolding and thermal stability studies.