The solvation of the enzyme Candida antarctica lipase B (CAL-B) was studied in eight different ionic liquids (ILs). The influence of enzyme-ion interactions on the solvation of CAL-B and the structure of the enzyme-IL interface are analyzed. CAL-B and ILs are described with molecular dynamics (MD) simulations in combination with an atomistic empirical force field. The considered cations are based on imidazolium or guanidinium that are paired with nitrate, tetrafluoroborate or hexafluorophosphate anions. The interactions of CAL-B with ILs are dominated by Coulomb interactions with anions, while the second largest contribution stems from van der Waals interactions with cations. The enzyme-ion interaction strength is determined by the ion size and the magnitude of the ion surface charge. The solvation of CAL-B in ILs is unfavorable compared to water because of large formation energies for the CAL-B solute cages in ILs. The internal energy in the IL and of CAL-B increases linearly with the enzyme-ion interaction strength. The average electrostatic potential on the surface of CAL-B is larger in ILs than in water, due to a weaker screening of charged enzyme residues. Ion densities increased moderately in the vicinity of charged residues and decreased close to non-polar residues. An aggregation of long alkyl chains close to non-polar regions and the active site entrance of CAL-B are observed in one IL that involved long non-polar decyl groups. In ILs that contain 1-butyl-3-methylimidazolium cations, the diffusion of one or two cations into the active site of CAL-B occurs during MD simulations. This suggests a possible obstruction of the active site in these ILs. Overall, the results indicate that small ions lead to a stronger electrostatic screening within the solvent and stronger interactions with the enzyme. Also a large ion surface charge, when more hydrophilic ions are used, increases enzyme-IL interactions. An increase of these interactions destabilizes the enzyme and impedes enzyme solvation due to an increase in solute cage formation energies.
Whether an ionic liquid (IL) is water-miscible or immiscible depends on the particular ions that constitute it. We propose an explanation, based on molecular simulations, how ions determine the miscibility of ILs and suggest a straightforward and computationally inexpensive method to predict the miscibility of arbitrary new ILs. The influence of ions on the solvation of water is analyzed by comparing molecular dynamics simulations of water in 9 different ILs with varying cation and anion constituents. The solvation of water in ILs is found to depend primarily on the electrostatic water-ion interaction strength, which, in turn, is determined mainly by two factors: primarily, by the size of the ions and secondarily by the amount of charge on the ion surface that is coordinated with water. It is demonstrated that large ions lead to weaker interactions with water, due to the involved delocalization of the ion charge. A large charge on the ion surface, which is determined by the chemical structure of the ion, strengthens water-ion interactions. We observe that whenever the interaction strength of water with ions exceeds a certain threshold, an IL becomes water-miscible. On the basis of these findings, a simple equation is derived that estimates the water-ion interaction strength. With this equation it is possible to predict most of the observed water-miscibilities of a sample of 83 ILs correctly. A linear increase of the water saturation concentration with the estimated water-ion interaction strength is observed in water-immiscible ILs, which can be utilized to predict the water concentration in new ILs.
Molecular dynamics (MD) simulations of 10 different pure and CO2-saturated ionic liquids are performed to identify the factors that determine CO2 solubility. Imidazolium-based cations with varying alkyl chain length and functionalization are paired with anions of different hydrophobicity and size. Simulations are carried out with an empirical force field based on liquid-phase charges. The partial molar volume of CO2 in ionic liquids (ILs) varies from 30 to 40 cm(3)/mol. This indicates that slight ion displacements are necessary to enable CO2 insertions. However, the absorption of CO2 does not affect the overall organization of ions in the ILs as demonstrated by almost equal cation-anion radial distribution functions of pure ILs and ILs saturated with CO2. The solubility of CO2 in ILs is not influenced by direct CO2-ion interactions. Instead, a strong correlation between the ratio of unoccupied space in pure ILs and their ability to absorb CO2 is found. This preformed unoccupied space is regularly dispersed throughout the ILs and needs to be expanded by slight ion displacements to accommodate CO2. The amount of preformed unoccupied space is a good indicator for ion cohesion in ILs. Weak electrostatic cation-anion interaction densities in ILs, i.e., weak ion cohesion, leads to larger average distances between ions and hence to more unoccupied space. Weak ion cohesion facilitates ion displacement to enable an expansion of empty space to accommodate CO2. Moreover, it is demonstrated that the strength of ion cohesion is primarily determined by the ion density, which in turn is given by the ion sizes. Ion cohesion is influenced additionally to a smaller extent by local electrostatic interactions among ion moieties between which CO2 is inserted and which do not depend on the ion density. Overall, the factors that determine the solubility of CO2 in ILs are identified consistently across a large variety of constituting ions through MD simulations.
We propose a new all-atom force field for guanidinium-based ionic liquids (GILs) which is based on the charge distribution in the actual liquid. It comprises all cations that can be built by attaching alkyl chains of variable length to an acyclic or cyclic guanidinium compound and that are paired with nitrate or perchlorate anions. We based the parametrization of the force field on liquid-phase charge distributions to improve the prediction of energetic and dynamic properties of GILs. The impact of electron charge transfer and polarization on various properties of GILs is systematically assessed. A significant average electron charge transfer between -0.12 and -0.06 e from anions to the central guanidinium group of the cations and a strong polarization of acyclic cations are observed by applying a combined quantum mechanical/molecular mechanical (QM/MM) approach. Molecular dynamics simulations of GILs are performed, utilizing the proposed force field. Derived structures approach the accuracy of QM/MM structures, and a previously reported crystal structure remains stable throughout the simulations. Mass densities are reproduced with a deviation of only 1.4% from experimental data. The calculated melting point of a GIL crystal deviates only 8% from the measured value. Self-diffusion coefficients of various GILs are reported, and a comparison with a diffusion coefficient derived from experimental data indicates that the values are within a reasonable range. We observe that the melting point of a GIL crystal was lowered up to 60 K and that diffusion coefficients are substantially increased by a factor of up to 3.5 upon consideration of charge transfer and polarization. The results demonstrate that liquid-phase partial charges are capable of improving the quality of ionic liquid force fields substantially and that their utilization led to a model that can be applied to predict structural, energetic, and dynamic properties of GILs.
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