Structural Destabilization of Azurin by Imidazolium Chloride Ionic Liquids in Aqueous Solution

Alkyl imidazolium chloride ionic liquids (ILs) have been used for numerous biochemical applications. Their hydrophobicity can be tuned by changing the alkyl chain length, and longer-chain ILs can form micelles in aqueous solution. We have investigated the effects of imidazolium chloride ILs on the structure and stability of azurin, which is a very stable Cu²⁺ redox protein with both α-helix and β-sheet domains. Temperature-dependent infrared (IR) and vibrational circular dichroism spectroscopy can provide secondary-structure-specific information about how the protein is affected, and temperature-jump transient IR measurements can quantify the IL-influenced unfolding dynamics. Using these techniques, we can quantify how azurin is destabilized by 1.0 M ILs in aqueous solution. The shorter, less hydrophobic ILs, 1-butyl-3-methylimidazolium chloride and 1-hexyl-3-methylimidazolium chloride likely interact with the α-helix domain and decrease protein melting temperature from 82 °C without IL to 55 °C and disturb the overall tertiary structure, resulting in a looser, more open shape. Thermodynamic analysis indicates that protein destabilization is due to increased unfolding entropy. 1-Octyl-3-methylimidazolium chloride [OMIM]Cl, which forms micelles in solution that may partially solvate the protein, has a more significant destabilizing effect, resulting in a melting temperature of 35 °C, larger unfolding entropy, and relaxation kinetics several orders of magnitude faster than with unperturbed azurin. The temperature-independence of the relaxation time constant suggests that in the presence of [OMIM]Cl, the protein folding potential energy surface has become very smooth.