1-butyl-3-methylimidazolium-hexafluorophosphate and 1-butyl-3-methylimidazolium-nitrate

We present in this work the first molecular simulation study of an enzyme, the serine protease cutinase from Fusarium solani pisi, in two ionic liquids (ILs): 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) and 1-butyl-3-methylimidazolium nitrate ([BMIM][NO(3)]). We tested different water contents in these ILs at room temperature (298 K) and high temperature (343 K), and we observe that the enzyme structure is highly dependent on the amount of water present in the IL media. We show that the enzyme is preferentially stabilized in [BMIM][PF6] at 5-10% (w/w) (weight of water over protein) water content at room temperature. [BMIM][PF6] renders a more nativelike enzyme structure at the same water content of 5-10% (w/w) as previously found for hexane, and the system displays a similar bell-shape-like dependence with the water content in the IL media. [BMIM][PF6] is shown to increase significantly the protein thermostability at high temperatures, especially at low hydration. Our analysis indicates that the enzyme is less stabilized in [BMIM][NO(3)] relative to [BMIM][PF6] at both temperatures, most likely due to the strong affinity of the [NO(3)]- anion toward the protein main chain. These findings are in accordance with the experimental knowledge for these two ionic liquids. We also show that these ILs "strip off" most of the water from the enzyme surface in a degree similar to that found for polar organic solvents such as acetonitrile, and that the remaining waters at the enzyme surface are organized in many small clusters.

Topics: Carboxylic Ester Hydrolases; Computer Simulation; Enzyme Stability; Fungal Proteins; Fusarium; Hydrogen Bonding; Imidazoles; Ionic Liquids; Models, Molecular; Protein Conformation; Serine Endopeptidases; Temperature; Water

A united-atom model of 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF(6)]) and 1-butyl-3-methylimidazolium nitrate ([BMIM][NO(3)]) is developed in the framework of the GROMOS96 43A1(1) force field. These two ionic liquids are parametrized, and their equilibrium properties in the 298-363 K temperature range are subjected to validation against known experimental properties, namely, density, self-diffusion, shear viscosity, and isothermal compressibility. The ionic radial/spatial distributions, pi interaction, gauche/trans populations of the butyl tail, and enthalpies of vaporization are also reported. The properties obtained from the molecular dynamics simulations agree with experimental data and have the same temperature dependence. The strengths and weakness of our model are discussed.

Topics: Computer Simulation; Imidazoles; Ionic Liquids; Models, Chemical; Molecular Structure; Quantum Theory; Temperature