Unexpected effects of the alteration of structure and stability of myoglobin and hemoglobin in ammonium-based ionic liquids

Abstract

The nature of solvent–biomolecule interactions is generally weak and non-specific. The addition of ionic liquids (ILs), which have emerged as a new class of solvents, strengthen the stability of some proteins whereas the same ILs weaken the stability of some other proteins. Although ILs are commonly used for the stabilization of biomolecules, the bimolecular interactions of their stabilization–destabilization is still an active subject of considerable interest and studies on this topic have been limited. To reveal the impact of ILs on the stability of proteins, a series of protic ILs possessing a tetra-alkyl ammonium cation [R₄N]⁺ with a hydroxide [OH]⁻ anion were synthesized. In this study, we report the structural stability of heme proteins such as myoglobin (Mb) and hemoglobin (Hb) in a series of ammonium-based ILs such as tetramethyl ammonium hydroxide [(CH₃)₄N]⁺[OH]⁻ (TMAH), tetraethyl ammonium hydroxide [(C₂H₅)₄N]⁺[OH]⁻ (TEAH), tetrapropyl ammonium hydroxide [(C₃H₇)₄N]⁺[OH]⁻ (TPAH) and tetrabutyl ammonium hydroxide [(C₄H₉)₄N]⁺[OH]⁻ (TBAH) by fluorescence and circular dichroism (CD) spectroscopic studies. Our experimental results reveal that less viscous ILs carrying smaller alkyl chain such as TMAH are strong destabilizers of the heme proteins as compared to the ILs carrying bulkier alkyl chains which are more viscous ILs, such as TBAH. Therefore, our results demonstrate that the addition of these ILs to the heme proteins decreases their thermal stability allowing the protein to be in an unfolded state at lower temperatures. Further, we describe the molecular–structural interaction of the heme proteins with the ILs (molecule like a ligand) by the PatchDocking method.