Thermodynamic analysis of destabilization of myoglobin by fatty acid ionic liquids

Ionic liquids (ILs) have been the focus of significant amounts of study regarding their interaction with biomolecules for a variety of applications including stabilization and destabilization of biomolecular structure, enhanced activities for biotechnology applications, and as designer solvents to promote specific interactions related to biomolecular function. Here we evaluated the impact of four different fatty acid ionic liquids (FAILs) on the stability of the model protein myoglobin and how the effects of these ILs compared to their individual components. The ILs were composed of a biocompatible cation, either choline or tetramethylguanidine, and an anionic fatty acid, either octanoic acid or decanoic acid. Using a combination of absorbance, fluorescence, and circular dichroism spectroscopy, we evaluated the impact of varying FAIL concentrations on the thermodynamic stability of myoglobin when denatured by either temperature or the chaotrope guanidinium hydrochloride. Both series of FAILs destabilized the myoglobin compared to controls, however the FAILs generally were equally or less destabilizing than the fatty acid component alone. Destabilization by the FAILs was determined to affect both the ΔH_unfolding and ΔS_unfolding values for myoglobin, with larger impacts on both terms from the octanoate-based series of molecules. Interestingly, the octanoate-containing series of molecules exhibited stronger disruption of the myoglobin compared to the decanoates. The highest concentrations of choline- and TMG-octanoate resulted in a change in ΔH_unfolding between 75 and 100 kJ, while the octanoate alone caused a change of >120 kJ. The ΔS_unfolding followed a similar trend, although to a lesser extent, ranging between 0.08 and 0.35 JK^-1 for the octanoate series.