Molecular dynamics simulation of liquid water under the influence of an external electric field

Molecular dynamics simulations of liquid water were performed at 258K and a density of 1.0?g/cm³ under various applied external electric field, ranging 0～10¹⁰?V/m. The influence of external field on structural and dynamical properties of water was investigated. The simple point charge (SPC) model is used for water molecules. An enhancement of the water hydrogen bond structure with increasing strength of the electric field has been deduced from the radial distribution functions and the analysis of hydrogen bonds structure. With increasing field strength, water system has a more perfect structure, which is similar to ice structure. However, the electrofreezing phenomenon of liquid water has not been detected since the self-diffusion coefficient was very large. The self-diffusion coefficient decreases remarkably with increasing strength of electric field and the self-diffusion coefficient is anisotropic.     

The partition and transport behavior of cytotoxic ionic liquids (ILs) through the DPPC bilayer: Insights from molecular dynamics simulation

A molecular dynamics (MD) simulation with atomistic details was performed to examine the partitioning and transport behavior of moderately cytotoxic ionic liquids (ILs), namely choline bis(2-ethylhexyl) phosphate (CBEH), choline bis(2,4,4-trimethylpentyl) phosphinate (CTMP) and choline O,O-diethyl dithiophosphate (CDEP) in a fully hydrated dipalmitoylphosphatidylcholine (DPPC) bilayer in the fluid phase at 323?K. The structure of ILs was so selected to understand if the role of dipole and dispersion forces in the ILs distribution in the membrane can be possible. Several analyses including mass density, electrostatic potential, order parameter, diffusion coefficients and hydrogen bond formation, was carried out to determine the precise location of the anionic species inside the membrane. Moreover, the potential of the mean force (PMF) method was used to calculate free energy profile for transferring anionic species from the DPPC membrane into the bulk water. While less cytotoxic DEP is located within the bulk water, more cytotoxic TMP and BEH ILs were found to remain in the membrane and the energy barrier for crossing through the bilayer center of BEH was higher. Various ILs have no significant effect on P–N vector. The thickness of lipid bilayer decreased in all systems comprising ILs, while area per lipid increased.     

Molecular modelling of ionic liquids in the ordered mesoporous carbon CMK-5

We performed classical molecular dynamics simulations of the ionic liquids (ILs) [dmim⁺][Cl^?] and [emim⁺][NTf₂^?], confined in a model CMK-5 material, which consists of amorphous carbon nanopipes (ACNPs) arranged in a hexagonal array. We compare our findings against the behaviour of the same ILs inside an isolated ACNP (i.e. no IL adsorbed on the outer surface of the ACNP) and inside a model CMK-3 material (which is similar to CMK-5, but is formed by amorphous carbon nanorods). Our results indicate that the presence of IL adsorbed in the outer surface of an uncharged ACNP in CMK-5 affects the dynamics and the density of an IL adsorbed inside the ACNP and vice versa. ILs adsorbed outside the nanopipes in CMK-5 (i.e. with IL also adsorbed inside the nanopipes) have faster dynamics and remain closer to the carbon surfaces when compared to the same ILs adsorbed on CMK-3 materials. The trends are IL-specific: [dmim⁺][Cl^?] has slower dynamics when inside an isolated ACNP than when inside the ACNPs in CMK-5, but in contrast, [emim⁺][NTf₂^?] moves faster when it is inside an isolated ACNP than when it is inside the ACNPs in CMK-5 (i.e. with IL adsorbed outside the nanopipes).     

Tyrosinase activity in ionic liquids

Activity of mushroom tyrosinase was studied in three ionic liquids, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm][PF₆]), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF₄]) and 1-butyl-3-methylimidazolium methylsulfate ([BMIm][MeSO₄]), and was compared to that in chloroform. Kinetic parameters of the enzyme were determined and the results indicate that the enzyme in ionic liquids basically follows the same catalytic mechanism as in water, and that the ionic liquids may affect the enzyme activity by direct interacting with the enzyme and thus hindering the E–S binding due to their high hydrophilicity and polarity.     

Structural modifications of nucleosides in ionic liquids

Nucleoside chemistry represents an important research area for drug discovery, as many nucleoside analogs are prominent drugs and have been widely applied for cancer and viral chemotherapy. However, the synthesis of modified nucleosides presents a major challenge, which is further aggravated by poor solubility of these compounds in common organic solvents. Most of the currently available methods for nucleoside modification employ toxic high boiling solvents; require long reaction time and tedious workup methods. As such, there is constant effort to develop process chemistry in alternative medium to limit the use of organic solvents that are hazardous to the environment and can be deleterious to human health. One such approach is to use ionic liquids, which are ‘designer materials’ with unique and tunable physico-chemical properties. Studies have shown that methodologies using ionic liquids are highly efficient and convenient for the synthesis of nucleoside analogs, as demonstrated by the preparation of pharmaceutically important anti-viral drugs. This article summarizes recent efforts on nucleoside modification using ionic liquids.     

Simulation of the effect of an external GHz electric field on the potential energy profile of Ca2+ ions in the selectivity filter of the CaVAb channel

Ca_V channels are transmembrane proteins that mediate and regulate ion fluxes across cell membranes, and they are activated in response to action potentials to allow Ca²⁺ influx. Since ion channels are composed of charge or polar groups, an external alternating electric field may affect the ion‐selective membrane transport and the performance of the channel. In this article, we have investigated the effect of an external GHz electric field on the dynamics of calcium ions in the selectivity filter of the Ca_VAb channel. Molecular dynamics (MD) simulations and the potential of mean force (PMF) calculations were carried out, via the umbrella sampling method, to determine the free energy profile of Ca²⁺ ions in the Ca_VAb channels in presence and absence of an external field. Exposing Ca_VAb channel to 1, 2, 3, 4, and 5 GHz electric fields increases the depth of the potential energy well and this may result in an increase in the affinity and strength of Ca²⁺ ions to binding sites in the selectivity filter the channel. This increase of strength of Ca²⁺ ions binding in the selectivity filter may interrupt the mechanism of Ca²⁺ ion conduction, and leads to a reduction of Ca²⁺ ion permeation through the Ca_VAb channel.     

Green microwave-assisted synthesis of cellulose/calcium silicate nanocomposites in ionic liquids and recycled ionic liquids

Fabrication of biomass materials by a microwave-assisted method in ionic liquids allows the high value-added applications of biomass by combining three major green chemistry principles: using environmentally preferable solvents, using an environmentally friendly method, and making use of renewable biomass materials. Herein, we report a rapid and green microwave-assisted method for the synthesis of the cellulose/calcium silicate nanocomposites in ionic liquids and recycled ionic liquids. These calcium silicate nanoparticles or nanosheets as prepared were homogeneously dispersed in the cellulose matrix. The experimental results confirm that the ionic liquids can be used repeatedly. Of course, the slight differences were also observed using ionic liquids and recycled ionic liquids. Compared with other conventional methods, the rapid, green, and environmentally friendly microwave-assisted method in ionic liquids opens a new window to the high value-added applications of biomass.     

Efficient hydrolysis of chitosan in ionic liquids

Effective hydrolysis of chitosan, the N-deacetylated product of chitin, remains challenging. Here, we report acid-promoted hydrolysis of chitosan in imidazolium based ionic liquids with good total reducing sugars (TRS) yield under mild conditions. TRS yield reached over 60% in the presence of about 6.0 wt% concentrated hydrochloric acid at 100 °C within 7 h. Kinetic modeling of a typical experimental data set suggested that the hydrolysis most likely followed a consecutive first-order reaction sequence, where k₁ and k₂, the rate constants for TRS formation and degradation, were determined to be 0.01372 and 0.00015 min⁻¹, respectively. Our method may be useful to explore new applications of natural chitin resources.     

Self-aggregation of trehalose in the mixed solvents of 1,3-dimethylimidazolium ionic liquid and water

Abstract

Conformational variations of solvated trehaloses in binary mixtures of 1,3-dialkylimidazolium ([dmim]Cl) ionic liquids and trehalose as well as ternary mixtures of trehalose, [dmim]Cl and water have been studied by molecular dynamics (MD) simulations with and without polarisable force fields. The interaction energy between anion Cl^? and water is stronger than that between water itself in the [dmim]Cl-water mixtures. Isolated water clusters were found in the binary [dmim]Cl-water mixtures with 60.0 and 75.0% mole fraction of water, but a continuous water network appears when the concentration of the mixture increases to 99.9%. In the case of binary mixtures of trehalose and [dmim]Cl, both non-polarisable and polarisable models demonstrated that the pyranose rings of trehalose displayed chair conformations. MD simulations with polarisation model could sample larger conformation space than that with non-polarisable model. A self-aggregation behaviour of trehalose was found in the ternary trehalose-[dmim]Cl-water mixtures, which can be rationalised by the stronger non-bonded interaction energy between trehalose molecules and anion Cl^? than that between trehalose molecules and water.     

Penicillin acylase catalysis in the presence of ionic liquids

Several ionic liquids were used as reaction media for penicillin G acylase catalysis. In all the assayed ionic liquids, [bmim]PF6 proved good media for PGA-catalyzed hydrolysis. A novel [bmim]PF6/water two-phase system is provided for 6-aminopenicillanic acid (APA) production, which will be more benefical than aquous batch systems used widely in industrial production of APA.     

Sequence-specific destabilization of azurin by tetramethylguanidinium-dipeptide ionic liquids

The thermal unfolding of the copper redox protein azurin was studied in the presence of four different dipeptide-based ionic liquids (ILs) utilizing tetramethylguanidinium as the cation. The four dipeptides have different sequences including the amino acids Ser and Asp: TMG-AspAsp, TMG-SerSer, TMG-SerAsp, and TMG-AspSer. Thermal unfolding curves generated from temperature-dependent fluorescence spectroscopy experiments showed that TMG-AspAsp and TMG-SerSer have minor destabilizing effects on the protein while TMG-AspSer and TMG-SerAsp strongly destabilize azurin. Red-shifted fluorescence signatures in the 25 °C correlate with the observed protein destabilization in the solutions with TMG-AspSer and TMG-SerAsp. These signals could correspond to interactions between the Asp residue in the dipeptide and the azurin Trp residue in the unfolded state. These results, supported by appropriate control experiments, suggest that dipeptide sequence-specific interactions lead to selective protein destabilization and motivate further studies of TMG-dipeptide ILs.     

H, F-HOESY and PGSE diffusion studies on ionic liquids: The effect of co-solvent on structure

¹H, ¹⁹F-HOESY studies on the ionic liquid based on 1-butyl-3-methylimidazolium tetrafluoroborate, [C₄C₁im]BF₄, [C₄C₁im][N(OTf)₂], and, partially, [C₄C₁im]PF₆, are reported. The addition of methanol separates the ions; however, in dichloromethane solution the anions and cations show strong HOESY contacts even after dilution and taken together with the PGSE diffusion measurements, the data suggest strong ion pairing in this solvent.     

Biodegradability of imidazolium and pyridinium ionic liquids by an activated sludge microbial community

Ionic liquids (ILs) are novel organic salts that have enormous potential for industrial use as green replacements for harmful volatile organic solvents. Varying the cationic components can alter the chemical and physical properties of ILs, including solubility, to suit a variety of industrial processes. However, to complement designer engineering, it is crucial to proactively characterize the biological impacts of new chemicals, in order to fully define them as environmentally friendly. Before introduction of ILs into the environment, we performed an analysis of the biodegradability of six ILs by activated sludge microorganisms collected from the South Bend, Indiana wastewater treatment plant. We examined biodegradability of 1-butyl, 1-hexyl and 1-octyl derivatives of 3-methyl-imidazolium and 3-methyl-pyridinium bromide compounds using the standard Organisation for Economic Cooperation and Development dissolved organic carbon Die-Away Test, changes in total dissolved nitrogen concentrations, and ¹H-nuclear magnetic resonance analysis of initial and final chemical structures. Further, we examined microbial community profiles throughout the incubation period using denaturing gradient gel electrophoresis (DNA-PCR-DGGE). Our results suggest that hexyl and octyl substituted pyridinium-based ILs can be fully mineralized, but that imidazolium-based ILs are only partially mineralized. Butyl substituted ILs with either cation, were not biodegradable. Biodegradation rates also increase with longer alkyl chain length, which may be related to enhanced selection of a microbial community. Finally, DGGE analysis suggests that certain microorganisms are enriched by ILs used as a carbon source. Based on these results, we suggest that further IL design and synthesis include pyridinium cations and longer alkyl substitutions for rapid biodegradability.     

NMR evidence for the participation of triflated ionic liquids in glycosylation reaction mechanisms

A systematic low-temperature NMR study of a glycosylation reaction was performed in the presence of different ionic liquids and acidic catalysts. The influence of the triflate anion derived from [emim][OTf] on the stereochemistry of the glycosylation products was evaluated.     

Phosphonium-ammonium-based di-cationic ionic liquids as antibacterial over the ESKAPE group

Emergence of antibioresistance is currently a major threat of public health worldwide. Hence there is an urge need of finding new antibacterial material. Herein, we report a simple and eco-friendly method to synthesize homo and heterodicationic ionic liquids based on quaternary phosphonium and ammonium salt. In order to investigate the structure activity relationship (SAR) we measured the MICs of a series of 16 derivatives with structural variations (nature of cations and counter-ions, size of linker and alkyl side chains as well as structural symmetry) over a range of Gram-positive and Gram-negative bacterial strains from the ESKAPE group. Some of the tested structures exhibit high antimicrobial activities (MIC = 0.5 mg/L) and are active over a wide range of bacteria from Gram-positive to Gram-negative. Overall, these results reveal the strong potential of di-cationic derivatives as antibacterial agents and the determination of activities from structural features gives decisive information for future synthesis of such di-cationic structures for biocidal purpose.     

Chemoenzymatic dynamic kinetic resolution of rac-1-phenylethanol in ionic liquids and ionic liquids/supercritical carbon dioxide systems

Continuous dynamic kinetic resolution processes in different ionic liquid/supercritical carbon dioxide biphasic systems were carried out by simultaneously using both immobilized Candida antarctica lipase B (Novozym 435) and silica modified with benzenosulfonic acid (SCX) catalysts at 40°C and 10 MPa. SCX was seen to act as an efficient heterogeneous chemical catalyst for the racemization of (S)-1-phenylethanol in different ionic liquid media ([emim][NTf₂], [btma][NTf₂] and [bmim][PF₆]). Coating both chemical and enzymatic catalysts with ILs greatly improved the efficiency of the process, providing a good yield (76%) of (R)-1-phenylethyl propionate product with excellent enantioselectivity (ee = 91–98%) in continuous operation.     

Lipase-catalyzed glycerolysis in ionic liquids directed towards diglyceride synthesis

This work examined the lipase-catalyzed glycerolysis of triglycerides (TG) in a list of commercially available ionic liquids (ILs) with varied cations and anions for the purpose of developing an efficient reaction protocol for diglyceride (DG) production and to understand whether ILs could assist the reaction systems. The reaction performances (reaction rate, TG conversion and DG yield) were found to be greatly dependent on the structure and property of ILs. The reactions in [TOMA·Tf₂N] and Ammoeng 120 produced comparable yield of DG to those most efficient conventional systems but with less by-products. Temperature enhancement generally yields positive effect on the conversion of TG, which was much more significant for the ILs with high viscosity. Unusually, increasing substrate concentration in many types of ILs led to enhanced conversion and yield; whereas the increase of glycerol/TG ratio resulted in a dramatic improvement of the reactions in the ILs with strong acidic anions. This work also sorted out some promising IL candidates, namely the ILs with good DG formation selectivity and the ones being able to achieve high TG conversion, which offered possibility to design binary IL systems. Overall, this study presented the first attempt concerning evaluation and characterization of lipase-catalyzed glycerolysis of TG for DG production in IL-based systems.