Destabilization of Human Serum Albumin by Ionic Liquids Studied Using Enhanced Molecular Dynamics Simulations

Ionic liquid (IL) containing solvents can change the structure, dynamics, function, and stability of proteins. In order to investigate the mechanisms by which ILs induce structural changes in a large multidomain protein, we study the interactions of human serum albumin (HSA) with two different ILs, 1-butyl-3-methylimidazolium tetrafluoroborate and choline dihydrogen phosphate. Root mean square deviation and fluctuation calculations indicate that high concentrations of ILs in mixtures with water lead to protein structures that remain close to their crystallographic structures on time scales of hundreds of nanoseconds. To overcome potential time scale limitations due to the high viscosity of the solvent, we employed enhanced sampling techniques to estimate the free energy of an experimentally determined important transition within the protein structure. Metadynamics simulations show that the free energy landscape of the unfolding of loop 1 of domain I is different in the presence of ILs than it is in water, consistent with previously published experimental evidence. We then apply essential dynamics coarse graining to systematically predict differences in the dynamics of proteins solvated in IL-water mixtures versus pure water systems. We also demonstrate that the presence of ionic liquids changes the distribution of intermolecular distances among several ligands, indicating that the protein structure swells in the presence of certain ILs, consistent with experimental evidence.