Toward advanced ionic liquids. Polar,enzyme-friendly solvents for biocatalysis

Ionic liquids, also called molten salts, are mixtures of cations and anions that melt below 100°C. Typical ionic liquids are dialkylimidazolium cations with weakly coordinating anions such as (MeOSO₃) or (PF₆). Advanced ionic liquids such as choline citrate have biodegradable, less expensive, and less toxic anions and cations. Deep eutectic solvents are also included in the advanced ionic liquids. Deep eutectic solvents are mixtures of salts such as choline chloride and uncharged hydrogen bond donors such as urea, oxalic acid, or glycerol. For example, a mixture of choline chloride and urea in 1:2 molar ratio liquefies to form a deep eutectic solvent. Their properties are similar to those of ionic liquids. Water-miscible ionic liquids as cosolvents with water enhance the solubility of substrates or products. Although traditional water-miscible organic solvents also enhance solubility, they often inactivate enzymes, while ionic liquids do not. The enhanced solubility of substrates can increase the rate of reaction and often increases the regioor enantioselectivity. Ionic liquids can also be solvents for non-aqueous reactions. In these cases, they are especially suited to dissolve polar substrates. Polar organic solvent alternatives inactivate enzymes, but ionic liquids do not even when they have similar polarities. Besides their solubility properties, ionic liquids and deep eutectic solvents may be greener than organic solvents because ionic liquids are nonvolatile, and can be made from nontoxic components. This review covers selected examples of enzyme catalyzed reaction in ionic liquids that demonstrate their advantages and unique properties, and point out opportunities for new applications. Most examples involve hydrolases, but oxidoreductases and even whole cell reactions have been reported in ionic liquids.     

Effect of protonation and hydrogen bonding on 2, 4, 6-substituted pyrimidine and its salt complex-experimental and theoretical evidence

The short-range properties of alkylpiperazine ionic liquids paired with propionate and lactate anions were analyzed and their affinity for CO₂ molecules studied using density functional theory. Anion–cation interactions led to the development of strong intermolecular hydrogen bonding through the cation amine position, as confirmed through variations in structural and vibrational properties upon pair formation. Topological analysis via the atoms-in-molecules approach revealed the development of intense bond and ring critical points in the intermolecular regions, which is in agreement with charge transfer from lone pairs in anion oxygen atoms of carboxylate groups through antibonding orbitals in cation amine groups. Such anion–cation interactions are weakly dependent on cation alkyl chain length but are remarkably affected by the presence of an anion hydroxyl group. Interactions with CO₂ molecules are stronger for anions than for cations, especially for propionate anions, and are also affected strongly by the anion hydroxyl group.     

Modeling interactions between a β‐O‐4 type lignin model compound and 1‐allyl‐3‐methylimidazolium chloride ionic liquid

Aiming at understanding the molecular mechanism of the lignin dissolution in imidazolium‐based ionic liquids (ILs), this work presents a combined quantum chemistry (QC) calculation and molecular dynamics (MD) simulation study on the interaction of the lignin model compound, veratrylglycerol‐β‐guaiacyl ether (VG) with 1‐allyl‐3‐methylimidazolium chloride ([Amim]Cl). The monomer of VG is shown to feature a strong intramolecular hydrogen bond, and its dimer is indicated to present important π‐π stacking and intermolecular hydrogen bonding interactions. The interactions of both the cation and anion of [Amim]Cl with VG are shown to be stronger than that between the two monomers, indicating that [Amim]Cl is capable of dissolving lignin. While Cl^– anion forms a hydrogen‐bonded complex with VG, the imidazolium cation interacts with VG via both the π‐π stacking and intermolecular hydrogen bonding. The calculated interaction energies between VG and the IL or its components (the cation, anion, and ion pair) indicate the anion plays a more important role than the cation for the dissolution of lignin in the IL. Theoretical results provide help for understanding the molecular mechanism of lignin dissolution in imidazolium‐based IL. The theoretical calculations on the interaction between the lignin model compound and [Amim]Cl ionic liquid indicate that the anion of [Amim]Cl plays a more important role for lignin dissolution although the cation also makes a substantial contribution.     

Exploring the differences and similarities between urea and thermally driven denaturation of bovine serum albumin: intermolecular forces and solvation preferences

The interactions of bovine serum albumin (BSA) with urea/water were investigated by computer simulation. It was revealed that the BSA-hydrophobic residues in urea solutions favored contact with urea more than with water. Energy decomposition analysis showed that van der Waals energy was the dominant driving force behind urea affinity for hydrophobic residues, whereas coulombic attraction was largely responsible for water affinity for these residues. Meanwhile, urea–BSA hydrogen bond energies were found to be weaker than water–BSA hydrogen bond energies. The greater strength of water–BSA hydrogen bonds than urea–BSA hydrogen bonds, and the opposing preferential interaction between the BSA and urea suggest that disruption of hydrophobic interaction predominates urea–protein denaturation. In pure water, hydrophobic residues showed aggregation tendencies at 323 K, suggesting an increase in hydrophobicity, while at 353 K the residues were partly denatured due to loss of hydrogen bonds; thus, disruption of hydrophobic interactions appeared to contribute less to thermal denaturation.     

Molecular dynamics simulations of cellulase homologs in aqueous 1-ethyl-3-methylimidazolium chloride

Pretreating biomass using ionic liquids (ILs) can decrease cellulose crystallinity and lead to improved hydrolysis. However, cellulase activity is often reduced in even low concentrations of ILs, necessitating complete washing between pretreatment and hydrolysis steps. To better understand how ILs interact with enzymes at the molecular scale, endoglucanase E1 from Acidothermus cellulolyticus was simulated in aqueous 1-ethyl-3-methylimidazolium chloride ([Emim]Cl). Homologs with differing surface charge were also simulated to assess the role of electrostatic interactions between the enzyme and the surrounding solvent. Chloride anions interacted with the enzyme surface via Coulomb or hydrogen bond interactions, while [Emim] cations primarily formed hydrophobic or ring stacking interactions. Cations strongly associated with the binding pocket of E1, potentially inhibiting the binding of substrate molecules. At elevated temperatures, cations also disrupted native hydrophobic contacts and caused some loss of secondary structure. These observations suggested that both cations and anions could influence enzyme behavior and that denaturing and inhibitory interactions might both be important in aqueous IL systems.     

Ion solvation and water structure in potassium halide aqueous solutions

The structure of water and the nature of ionic hydration is explored in aqueous solutions of potassium fluoride, chloride, bromide and iodide over a range of concentrations up to 4.8 ion pairs per 100 water molecules, using the combined techniques of neutron diffraction with hydrogen isotope substitution. The diffraction data are interpreted using the method of empirical potential structure refinement, which attempts to build a three-dimensional model of the scattering system consistent with the diffraction data. The water structure is strongly perturbed in the first hydration shells of both anion and cation, but is found to be only mildly perturbed outside of this region, with the largest effects occurring with the smallest anion and highest concentrations. For the potassium ion there are strong orientational correlations in the first hydration shell, with the water molecules lying with their dipole moments pointing almost directly away from the cation on average, but with an angular spread of approximately +/-60 degrees which is mildly dependent on the anion type present. For all the anions the water molecules in the first shell are strongly oriented with one O-H vector pointing directly towards the anion on average, with an angular spread of approximately +/-10 degrees for F(-), increasing to approximately +/-22 degrees for I(-). For both anions and cations the second hydration shell is much more disordered than the first, but there is a weak pattern of orientational correlation which becomes more pronounced with the larger anions. There is some evidence that the fluoride ion structures water significantly in its first hydration shell, but not beyond. The findings throw further light on recent findings that the orientational relaxation time for water outside the first shell of dissolved ions is the same as in the bulk liquid.     

MgATP-Stimulated Release of Luteinizing Hormone Releasing Hormone from Isolated Hypothalamic Granules: Evidence for a Selective Requirement for Monovalent Cations

Abstract: The purpose of this study was to characterize the kinetics and monovalent ion requirements of the MgATP-stimulated release of Iuteinizing hormone releasing hormone (LHRH) from isolated hypothalamic granules. LHRH granules were obtained from homogenates of hypothalami of adult male rats by differential centrifugation and then incubated in buffered media containing KCl. Under these conditions, the pH optimum for the MgATP-stimulated release of LHRH was between 7.5 and 8.0; the rate of release was linear for the first 5 min of incubation; and the magnitude of the release was a saturable function of KCl concentration. When the potassium in the incubation mixture was replaced with various monovalent cations (Li⁺, Na⁺, Cs⁺, choline, or tetraethylammonium), the magnitude of the MgATP-stimulated release of LHRH was inversely related to the size of the cation; release in the presence of Li⁺ was 12%, whereas release in the presence of tetraethylammonium was 0% of the total LHRH. When the chloride in the incubation mixture was replaced with various monovalent anions (Br⁻, I⁻, or isethionate), the magnitude of the MgATP-stimulated release of LHRH was not related to the size of the anion. Moreover, inclusion of anion transport inhibitors in the incubation mixture did not affect LHRH release, suggesting that anion transport is not required for the MgATP-stimulated release of LHRH. In summary, we suggest that the MgATP-stimulated release of LHRH from isolated hypothalamic granules involves an enzymatic event and that this release process does not have a selective requirement for monovalent anions, but has a selective requirement for monovalent cations.     

Cation-pi and amino-acceptor interactions between hydrated metal cations and DNA bases. A quantum-chemical view

Cation-pi interactions between cytosine and hexahydrated cations have been characterized using ab initio method with inclusion of electron correlation effects, assuming idealized and crystal geometries of the interacting species. Hydrated metal cations can interact with nucleobases in a cation-pi manner. The stabilization energy of such complexes would be large and comparable to the one for cation-pi complex with benzene. Further, polarized water molecules belonging to the hydration shell of the cation are capable to form a strong hydrogen bond interaction with the nitrogen lone electron pair of the amino groups of bases and enforce a pronounced sp3 pyramidalization of the nucleobase amino groups. However, in contrast to the benzene-cation complexes, the cation-pi configurations are highly unstable for a nucleobase since the conventional in plane binding of hydrated cations to the acceptor sites on the nucleobase is strongly preferred. Thus, a cation-pi interaction with a nucleobase can occur only if the position of the cation is locked above the nucleobase plane by another strong interaction. This indeed can occur in biopolymers and may have an effect on the local DNA architecture. Nevertheless, nucleobases have no intrinsic propensity to form cation-pi interactions.     

Cation—π and Amino-Acceptor Interactions Between Hydrated Metal Cations and DNA Bases. A Quantum-Chemical View

Abstract

Cation—π interactions between cytosine and hexahydrated cations have been characterized using ab initio method with inclusion of electron correlation effects, assuming idealized and crystal geometries of the interacting species. Hydrated metal cations can interact with nucleobases in a cation—π manner. The stabilization energy of such complexes would be large and comparable to the one for cation—π complex with benzene. Further, polarized water molecules belonging to the hydration shell of the cation are capable to form a strong hydrogen bond interaction with the nitrogen lone electron pair of the amino groups of bases and enforce a pronounced sp³ pyramidalization of the nucleobase amino groups. However, in contrast to the benzene—cation complexes, the cation—π configurations are highly unstable for a nucleobase since the conventional in plane binding of hydrated cations to the acceptor sites on the nucleobase is strongly preferred. Thus, a cation—π interaction with a nucle-obase can occur only if the position of the cation is locked above the nucleobase plane by another strong interaction. This indeed can occur in biopolymers and may have an effect on the local DNA architecture. Nevertheless, nucleobases have no intrinsic propensity to form cation—π interactions.     

Crystal structures of 1 : 1 complexes of meclofenamic acid with choline and ethanolamine

The hydrated 1:1 complex of meclofenamic acid with choline crystallizes in the orthorhombic space group Pna2(1) with a = 9.637(1), b = 12.962(5), c = 33.099(4) A and Z = 8. Crystals of the corresponding anhydrous complex with ethanolamine are triclinic, space group P1, with a = 9.232(3), b = 12.287(5), c = 17.033(3) A, alpha = 70.21(2), beta = 76.72(2), gamma = 68.21(3) degrees and Z = 4. The structures have been solved by direct methods and refined to R values of 0.062 and 0.079, respectively for 1942 and 2852 observed reflections. The four crystallographically independent meclofenamate anions in the complexes have nearly the same molecular geometry which in turn is very similar to that found in the crystal structure of free meclofenamic acid. The choline and ethanolamine molecules assume a gauche conformation with respect to the central C-C bond. The invariant structural features observed in the crystals of the free fenamates are retained by the meclofenamate ions in the complexes. These features are the rigid coplanar geometry of the six-membered ring carrying the carboxyl group, the carboxyl group and the imino nitrogen atom, and the internal hydrogen bond connecting the imino and the carboxyl groups. The crystal structures are stabilised by ionic interactions between the carboxylate groups of meclofenamate ions and choline or ethanolamine cations, and hydrogen bonds. The choline complex exhibits pseudosymmetry and the distribution of molecules in it is nearly centrosymmetric although the space group is noncentrosymmetric. The packing of molecules in the crystals is such that the polar columns are surrounded by non-polar regions. The core of each column in the choline complex is made up of water molecules connected by hydrogen bonds involving disordered protons. The results of the X-ray structure analysis of fenamates and their crystalline complexes provide some insights into structure-function relationships in this family of drugs.     

Mechanism of anion-cation selectivity of amphotericin B channels

Summary Zero current potential and conductance of ionic channels formed by polyene antibiotic amphotericin B in a lipid bilayer were studied in various electrolyte solutions. Nonpermeant magnesium and sulphate ions were used to independently vary the concentration of monovalent anions and cations as well as to maintain the high ionic strength of the two solutions separated by the membrane. Under certain conditions the channels select very strongly for anions over cations. They are permeable to small inorganic anions. However, in the absence of these anions the channels are practically impermeable to any cation. In the presence of a permeant anion the contribution of monovalent cations to channel conductance grows with an increase in the anion concentration. The ratio of cation-to-anion permeability coefficients is independent of the membrane potential and cation concentration, but it does depend linearly on the sum of concentrations of a permeant anion in the two solutions. These results are accounted for on the assumption that a cation can enter only an anion-occupied channel to form an ionic pair at the center of the channel. The cation is also assumed to slip past the anion and then to leave the channel for the opposite solution. This model with only few parameters can quantitatively describe the concentration dependences of conductance and zero current potential under various conditions.     

Salt or ion bridges in biological systems: a study employing quantum and molecular mechanics

Equilibrium geometries and binding energies of model "salt" or "ion" bridge systems have been computed by ab initio quantum chemistry techniques (GAUSSIAN82) and by empirical force field techniques (AMBER2.0). Formate and dimethyl phosphate served as anions in the model compounds while interacting with several organic cations, including methyl ammonium, methyl guanidinium, and divalent metal ion (either Mg2+ or Ca2+) without and with an additional chloride; and a divalent metal ion (either Mg2+ or Ca2+), chloride, and four water molecules of hydration about the metal ion. The majority of the quantum chemical computations were performed using a split-valence basis set. For the model compounds studied we find that the ab initio optimized geometries are in remarkably good agreement with the molecular mechanics geometries. Several calculations were also performed using diffuse fractions. The formate anion binds these model cations more strongly than does dimethyl phosphate, while the organic cation methyl ammonium binds model anions more strongly than does methyl guanidinium. Finally, in model compounds including organic anions, Mg2+ or Ca2+ and four molecules of water, and a chloride anion, we find that the equilibrium structure of the magnesium complex involves a solvent separated ion pair (the magnesium ion is six coordinate), whereas the calcium ion complex remains seven coordinate. Molecular mechanics overestimates binding energies, but the estimates may be close enough to actual binding energies to give useful insight into the details of salt bridges in biological systems.     

Flexibility and enzyme activity of NADH oxidase from Thermus thermophilus in the presence of monovalent cations of Hofmeister series

Recently, we have shown that anions of Hofmeister series affect the enzyme activity through modulation of flexibility of its active site. The enzyme activity vs. anion position in Hofmeister series showed an unusual bell-shaped dependence. In the present work, six monovalent cations (Na(+), Gdm(+), NH(4)(+), Li(+), K(+) and Cs(+)) of Hofmeister series with chloride as a counterion have been studied in relation to activity and stability of flavoprotein NADH oxidase from Thermus thermophilus (NOX). With the exception of strongly chaotropic guanidinium cation, cations are significantly less effective in promoting the Hofmeister effect than anions mainly due to repulsive interactions of positive charges around the active site. Thermal denaturations of NOX reveal unfavorable electrostatic interaction at the protein surface that may be shielded to different extent by salts. Michaelis-Menten constants for NADH, accessibility of the active site as reflected by Stern-Volmer constants and activity of NOX at high cation concentrations (1-2 M) show bell-shaped dependences on cation position in Hofmeister series. Our analysis indicates that in the presence of kosmotropic cations the enzyme is more stable and possibly more rigid than in the presence of chaotropic cations. Molecular dynamic (MD) simulations of NOX showed that active site switches between open and closed conformations [J. Hritz, G. Zoldak, E. Sedlak, Cofactor assisted gating mechanism in the active site of NADH oxidase from Thermus thermophilus, Proteins 64 (2006) 465-476]. Enzyme activity, as well as substrate binding, can be regulated by the salt mediated perturbation of the balance between open and closed forms. We propose that compensating effect of accessibility and flexibility of the enzyme active site leads to bell-shaped dependence of the investigated parameters.     

Cation permeability and cation-anion interactions in a mutant GABA-gated chloride channel from Drosophila.

To investigate the structural basis of anion selectivity of Drosophila GABA-gated Cl(-) channels, the permeation properties of wild-type and mutant channels were studied in Xenopus oocytes. This work focused on asparagine 319, which by homology is one amino acid away from a putative extracellular ring of charge that regulates cation permeation in nicotinic receptors. Mutation of this residue to aspartate reduced channel conductance, and mutation to lysine or arginine increased channel conductance. These results are consistent with an electrostatic interaction between this site and permeating anions. The lysine mutant, but not the arginine mutant, formed a channel that is permeable to cations, and this cannot be explained in terms of electrostatics. The lysine mutant had a 25-mV reversal potential in solutions with symmetrical Cl(-) and asymmetrical cations. The permeability ratio of K(+) to Cl(-) was determined as 0. 33 from reversal potential measurements in KCl gradients. Experiments with large organic cations and anions showed that cation permeation can only be seen in the presence of Cl(-), but Cl(-) permeation can be seen in the absence of permeant cations. Measurements of permeability ratios of organic anions indicated that the lysine mutant has an increased pore size. The cation permeability of the lysine-containing mutant channel cannot be accounted for by a simple electrostatic interaction with permeating ions. It is likely that lysine substitution causes a structural change that extends beyond this one residue to influence the positions of other channel-forming residues. Thus protein conformation plays an important role in enabling ion channels to distinguish between anions and cations.     

Alteration of the groove width of DNA induced by the multimodal hydrogen bonding of denaturants with DNA bases in its grooves affects their stability

BackgroundDenaturants, namely, urea and guanidinium chloride (GdmCl) affect the stability as well as structure of DNA. Critical assessment of the role of hydrogen bonding of these denaturants with the different regions of DNA is essential in terms of its stability and structural aspect. However, the understanding of the mechanistic aspects of structural change of DNA induced by the denaturants is not yet well understood.MethodsIn this study, various spectroscopic along with molecular dynamics (MD) simulation techniques were employed to understand the role of hydrogen bonding of these denaturants with DNA bases in their stability and structural change.Results and conclusionIt has been found that both, GdmCl and urea intrude into groove region of DNA by striping surrounding water. The hydrogen bonding pattern of Gdm⁺ and urea with DNA bases in its groove region is multimodal and distinctly different from each other. The interaction of GdmCl with DNA is stabilized by electrostatic interaction whereas electrostatic and Lennard-Jones interactions both contribute for urea. Gdm⁺ forms direct hydrogen bond with the bases in the minor groove of DNA whereas direct and water assisted hydrogen bond takes place with urea. The hydrogen bond formed between Gdm⁺ with bases in the groove region of DNA is stronger than urea due to strong electrostatic interaction along with less self-aggregation of Gdm⁺ than urea. The distinct hydrogen bonding capability of Gdm⁺ and urea with DNA bases in its groove region affects its width differently. The interaction of Gdm⁺ decreases the width of the minor and major groove which probably increases the strength of hydrogen bond between the Watson-Crick base pairs of DNA leading to its stability. In contrast, the interaction of urea does not affect much to the width of the grooves except the marginal increase in the minor groove width which probably decreases the strength of hydrogen bond between Watson Crick base pairs leading to the destabilization of DNA.General significanceOur study clearly depicts the role of hydrogen bonding between DNA bases and denaturants in their stability and structural change which can be used further for designing of the guanidinium based drug molecules.     

p-Chloromercuribenzenesulfonic acid stimulation of chloride-dependent sodium and potassium transport in human red blood cells

The organic mercurial p-chloromercuribenzensulfonic acid (PCMBS) reversibly increases fluxes of sodium and potassium across the human red blood cell membrane. We examined the effect of different monovalent anions on cation fluxes stimulated by PCMBS. A substantial portion of the fluxes of both cations was found to have a specific anion requirement for chloride or bromide, and was not observed when chloride was replaced by nitrate, acetate or methylsulfate. The chloride-dependent component of the cation fluxes was only observed when the cells were exposed to PCMBS concentrations of 0.5 mM or greater. Furosemide (1 mM) did not inhibit the PCMBS-stimulated cation fluxes. The observed anion specificity is directly associated with the transport process rather than PCMBS binding to the membrane. A portion of the potassium transport stimulated by PCMBS appears to involve K+-K+ exchange; however, Na+ + K+ cotransport is not stimulated by this sulfhydryl reagent.     

α-Chitin dissolution,N-deacetylation and valorization in deep eutectic solvents

Chitin displays a highly rigid structure due to the vast intra- and intermolecular hydrogen bonding, thus hindering its dissolution and deacetylation using most solvents. Deep eutectic solvents (DESs) are special and environmentally friendly solvents composed of a hydrogen bond acceptor and a hydrogen bond donor. This allows them to dissolve chitin by disturbing its natural hydrogen bonding while establishing new bonds, hence turning the polymer more susceptible to solvents. Therefore, four distinct DESs (choline chloride-lactic acid ([Ch]Cl:LA), choline chloride:oxalic acid ([Ch]Cl:OA), choline chloride:urea ([Ch]Cl:U) and betaine-glycerol (Bet:G)) were applied in chitin dissolution, being the most performant ones further applied in its homogenous N-deacetylation with NaOH. In this work, a milder and more biocompatible approach was carried out by using 30 wt% NaOH at 80°C, instead of the typical ≥40 wt% NaOH at temperatures ≥100°C. Herein, the reaction process took up to 18 hours, being the results analyzed through ATR-FTIR. Chitin was converted into chitosan with a 70-80% degree of deacetylation (DDA) in a short period while using homogenous conditions. These promising results provide the first proof of concept of the ability of Bet:G and [Ch]Cl:LA-based DESs to be used as a greener approach for the chitin homogeneous N-deacetylation.     

The interaction of various lanthanide ions and some anions with phosphatidylcholine vesicle membranes. A 31P NMR study of the surface potential effects

The interaction of various lanthanide ions with vesicles of phosphatidylcholine from egg yolk has been followed by 31P NMR at 30 degrees C. From known magnetic properties of these ions, separation of the paramagnetic shift into a pure contact and a pseudo-contact part was carried out. Binding curves for the contact contribution (F curves) were obtained from vesicles in solutions of sodium salts with monovalent anions over a wide concentration range. These curves should be insensitive to any conformational effects due to ion binding. Indication of a conformational change in the lipid head group at low ion binding was obtained by studying the ratio between the contact and the pseudo-contact contributions. Besides the adsorption of lanthanide ions, specific anion binding to the surface was introduced to account for the enhanced chemical shifts (Cl- < Br- < NO3-). The results were analyzed in terms of the theory for the diffuse double layer (Gouy-Chapman-Grahame) with equilibrium conditions for the adsorbing cations and anions. Simulations of the titration curves furnished parameters for the ion-lipid interactions. The synergism between the cations and anions follows from the potential effects. Comparison of results with lanthanide ions and Ca2+ indicates that the anion adsorption probably depends on the nature of the adsorbed cation. Lanthanide ion binding to L-glycerophosphorylcholine is not influenced by sodium salts. The binding constant for this complex is weaker than with phosphatidylcholine. The chemical shifts for the lanthanide ion complexes with these two phosphorus compounds seem to be about the same.     

A density functional theory study on the interactions between dibenzothiophene and tetrafluoroborate-based ionic liquids

The interactions between dibenzothiophene (DBT) and N-butyl-N-methylimidazolium tetrafluoroborate ([BMIM][BF₄]), N-butyl-N-methylmorpholinium tetrafluoroborate ([Bmmorpholinium][BF₄]), N-butyl-N-methylpiperdinium tetrafluoroborate ([BMPiper][BF₄]), N-butyl-N-methylpyrrolidinium tetrafluoroborate ([BMPyrro][BF₄]), and N-butylpyridinium tetrafluoroborate ([BPY][BF₄]) were investigated using density functional theory approach. Geometric, electron, and topological properties were analyzed using natural bond orbital, atoms in molecules theory, and noncovalent interaction methods in order to understand intermolecular interactions between DBT and ionic liquids. The result shows that hydrogen bond and van der Waals interactions are widespread in all the ionic liquids-DBT systems. Ion-π interactions between DBT and cation or anion are also observed, while π⁺-π interactions are only found in the [BMIM][BF₄]-DBT and [BPY][BF₄]-DBT systems. The order of interaction energy is [BPY][BF4]-DBT > [BMIM][BF₄]-DBT >> [BMPiper][BF₄]-DBT > [BMPyrro][BF₄]-DBT > [BMmorpholinum][BF₄]-DBT. The energies between DBT and the two ionic liquids containing aromatic cations are significantly higher.     

The effects of acetate anion on cellulose dissolution and reaction in imidazolium ionic liquids

Quantum mechanical calculations were carried out to determine the mechanisms for the superiority of the imidazolium acetate-based ionic liquids to the corresponding chloride-based ionic liquids. Our results indicate that the imidazolium cation can react with the acetate anion to generate a carbene, a highly reactive intermediate. The carbene produced then reacts with cellulose to facilitate its dissolution in the ionic liquid solvents in addition to the stronger hydrogen bonds formed between the acetate anion and the hydroxyl groups on cellulose. The mechanisms for the imidazolium cation and acetate anion reactions involve the initial ion pairing of the cation and anion via hydrogen bonding and electrostatic interactions. The hydrogen bond formed between the C2–H on the imidazolium cation and COO⁻ of the anion facilitates the transfer of the H⁺ to the anion to form a carbene intermediate.