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Abstract
This study examines the influence of cation hydrophobicity on protein solvation in aqueous solutions of Ionic Liquids. Ubiquitin solvation structures and thermodynamics in systems with 1-ethyl-3-methylimidazolium ([EMIM]⁺) and 1-butyl-3-methylimidazolium ([BMIM]⁺) are studied using molecular dynamics simulations, Minimum-Distance distribution functions, and the Kirkwood-Buff theory of solvation. At low concentrations, the larger alkyl chain leads to enhanced water exclusion and increased accumulation of [BMIM]⁺ at the protein surface relative to [EMIM]⁺. The preferential solvation, nevertheless, depends on the ionic liquid concentration differently for each cation. As concentrations increase, [BMIM]⁺ relative accumulation decreases relative to [EMIM]+. This causes a reversal of cation-protein affinities relative to water, and [EMIM]+ displays greater preferential solvation of the protein at higher concentrations. This reversal is a consequence of the saturation of the cation-specific protein surface binding sites, and the different molarities of water in the bulk solutions implied by the cation sizes. These effects are mostly independent of the anion composing the IL.
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Number of components for each simulation box, including cations, anions, and water molecules (Table S1). Post-NPT concentrations of all components in the bulk region (Table S2). Data on preferential solvation (Γcp) and preferential hydration (Γwp) parameters across varying IL concentrations (Table S3). Minimum-distance distribution functions (MDDFs) of anions, cations, and water molecules around the protein (Figures S1–S5, S10, S13, S15). Kirkwood-Buff integrals (KBIs) of water for different IL systems (Figures S14, S16). Coordination numbers of water molecules in the bulk, calculated within 5 Å of the reference species (Figures S10–S12). Density difference maps for solvent components around protein residues (Figures S6–S8).
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