Enhanced enzymatic saccharification of kenaf powder after ultrasonic pretreatment in ionic liquids at room temperature

This study demonstrates for the first time that the enzymatic hydrolysis of cellulose is drastically enhanced following ultrasonic pretreatment of lignocellulosic material in ionic liquids (ILs) when compared to conventional thermal pretreatment. Five types of ILs, 1-buthyl-3-methylimidazolium chloride (BmimCl), 1-allyl-3-methylimidazolium chloride (AmimCl), 1-ethyl-3-methylimidazolium chloride (EmimCl), 1-ethyl-3-methylimidazolium diethyl phosphate (EmimDep), and 1-ethyl-3-methylimidazolium acetate (EmimOAc) were tested. Cellulose saccharification ratio was about 20% for kenaf powders pretreated in BmimCl, AmimCl, EmimCl, and EmimDep by conventional heating at 110 °C for 120 min. Conversely, 60-95% of cellulose was hydrolyzed to glucose, subsequent to ultrasonic pretreatment in the same ILs for 120 min at 25 °C. The cellulose saccharification ratio of kenaf powder in EmimOAc was 86% after only 15 min of the ultrasonic pretreatment at 25 °C, compared to only 47% in that case of thermal pretreatment in the IL.     

Increased enantioselectivity in the enzymatic hydrolysis of amino acid esters in the ionic liquid 1-butyl-3-methyl-imidazolium tetrafluoroborate

The initial rate and enantioselectivity of enzymatic asymmetric hydrolysis of amino acid esters were examined in methylimidazolium-based ionic liquids with anions including tetrafluoroborate, chloride, bromide and bisulfate and in typical organic solvents. Papain displayed much higher enantioselectivity but lower activity in phosphate buffer solution of 1-butyl-3-methylimidazolium tetrafluoroborate BMIM·BF₄ than in other media tested (i.e. E=100, V ₀=0.21 mM min^-1 in BMIM·BF₄, E=2, V ₀=0.43 mM min^-1 in phosphate buffer, E=14-92, V ₀=0.22-0.25 mM min^-1 in organic solvents for D,L-phenylglycine methyl ester). The influence of BMIM·BF₄ on enzyme activity and enantioselectivity also varied with the substrate and the enzyme used. All of the enzymes assayed showed no activity or low enantioselectivity in the ILs with anions including chloride, bromide and bisulfate.     

A profile of the in vitro anti-tumor activity of imidazolium-based ionic liquids

The anti-cancer activity and cytotoxicity of imidazolium-based ionic liquids has been determined for the first time via NCI’s in vitro 60 human tumor cell lines. The preliminary SAR showed that the chain length of alkyl substitution at N-3 position of imidazole ring plays crucial role towards anti-tumor activity and cytotoxicity of these ionic liquids. The ionic liquids with alkyl substitution of C-12 chain length were found to be effective against all 60 tumor cell lines and show very low cytotoxicity in most of the cases. Further increase in chain length resulted in enhanced growth inhibition of tumor cell lines as well as high cytotoxicity. Interestingly, active compounds 1-dodecyl-3-methylimidazolium chloride (8), 1-dodecyl-3-methylimidazolium tetrafluoroborate (9), 1-hexadecyl-3-methylimidazoium chloride (10), 1-octadecyl-3-methylimidazolium chloride (11), 1-octadecyl-3-methylimidazolium hexafluorophosphate (12), 1-octadecyl-3-methylimidazolium bis(triflic)imide (13) and 1-octadecyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (14) were highly active against leukemia cell lines, especially compounds 13 and 14 where the cytotoxicity was also very low as given by LC₅₀ >100 μM in all six leukemia cell lines.     

The study of factors affecting the enzymatic hydrolysis of cellulose after ionic liquid pretreatment

Cellulose resource has got much attention as a promising replacement of fossil fuel. The hydrolysis of cellulose is the key step to chemical product and liquid transportation fuel. In this paper a serials of chloride, acetate, and formate based ionic liquids were used as solvents to dissolve cellulose. The cellulose regenerated from ILs was characterized by FTIR and X-ray powder diffraction. From the characterization and analysis, it was found that the original close and compact structure has changed a lot. After enzymatic hydrolysis, different kinds of ionic liquids (ILs) have different yields of the reducing sugar (TRS). They are 100%, 90.72%, and 88.92% from 1-butyl-3-methylimidazolium chloride ([BMIM]Cl), 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]), 1-butyl-3-methylimidazolium formate ([BMIM][HCOO]) respectively after enzymatic hydrolysis at 50 °C for 5 h. The results indicated that the yields and the hydrolysis rates were improved apparently after ILs pretreatment comparing with the untreated substrates.     

Improving enzymatic hydrolysis of wheat straw using ionic liquid 1-ethyl-3-methyl imidazolium diethyl phosphate pretreatment

This study aims to establish a cellulose pretreatment process using ionic liquids (ILs) for efficient enzymatic hydrolysis. The IL 1-ethyl-3-methyl imidazolium diethyl phosphate ([EMIM]DEP) was selected in view of its low viscous and the potential of accelerating enzymatic hydrolysis, and it could be recyclable. The yield of reducing sugars from wheat straw pretreated with this IL at 130 °C for 30 min reached 54.8% after being enzymatically hydrolyzed for 12 h. Wheat straw regenerated were hydrolyzed more easily than that treated with water. The fermentability of the hydrolyzates, obtained after enzymatic saccharification of the regenerated wheat straw, was evaluated using Saccharomyces cerevisiae. This microbe could ferment glucose efficiently, and the ethanol production was 0.43 g/g glucose within 26 h. In conclusion, the IL [EMIM]DEP shows promise as pretreatment solvent for wheat straw, although its cost should be reduced and in-depth exploration of this subject is needed.     

Cationic effect of imidazolium-based ionic liquid on the stability of myoglobin

The conformational change of myoglobin (Mb) during guanidine hydrochloride (GuHCl)-induced protein unfolding in the presence of various ionic liquids (ILs) in phosphate buffer was investigated using both the Soret band absorption and the fluorescence of tryptophan measurements. The GuHCl-induced denaturation midpoints of Mb derived from the absorption and fluorescence spectra were almost similar in the presence of 150 mM ILs with the same cation 1-butyl-3-methylimidazolium (Bmim⁺) but different anions (BF₄⁻, NO₃⁻, Cl⁻, and Br⁻) in phosphate buffer. In addition, the denaturation midpoints of Mb in the presence of ILs were little lower than those in the absence of ILs in phosphate buffer. For the sake of clarity and comparison, we also measured the GuHCl-induced denaturation midpoints of Mb in the presence of 150 mM sodium salts with different anions (BF₄⁻, NO₃⁻, Cl⁻, and Br⁻) in phosphate buffer and found that their corresponding denaturation midpoints of Mb were almost similar to those observed in the absence of sodium salts in phosphate buffer. These experimental data indicate that Bmim⁺ cation can promote the unfolding of Mb. Further experiments revealed that the denaturation ability of ILs increases with increasing alkyl chain length of imidazolium cation of ILs and that hydroxyl-substituted imidazolium cation could also promote the unfolding of Mb.     

α-Rhamnosidase and β-glucosidase expressed by naringinase immobilized on new ionic liquid sol-gel matrices: Activity and stability studies

Novel ionic liquid (IL) sol-gel materials development, for enzyme immobilization, was the goal of this work. The deglycosylation of natural glycosides were performed with α-l-rhamnosidase and β-d-glucosidase activities expressed by naringinase. To attain that goal ILs with different structures were incorporated in TMOS/Glycerol sol-gel matrices and used on naringinase immobilization.The most striking feature of ILs incorporation on TMOS/Glycerol matrices was the positive impact on the enzyme activity and stability, which were evaluated in fifty consecutive runs. The efficiency of α-rhamnosidase expressed by naringinase TMOS/Glycerol@ILs matrices increased with cation hydrophobicity as follows: [OMIM] > [BMIM] > [EMIM] > [C₂OHMIM] > [BIM] and [OMIM] ≈ [E₂-MPy] ? [E₃-MPy]. Regarding the imidazolium family, the hydrophobic nature of the cation resulted in higher α-rhamnosidase efficiencies: [BMIM]BF₄ ? [C₂OHMIM]BF₄ ? [BIM]BF₄. Small differences in the IL cation structure resulted in important differences in the enzyme activity and stability, namely [E₃-MPy] and [E₂-MPy] allowed an impressive difference in the α-rhamnosidase activity and stability of almost 150%. The hydrophobic nature of the anion influenced positively α-rhamnosidase activity and stability. In the BMIM series the more hydrophobic anions (PF₆⁻, BF₄⁻ and Tf₂N⁻) led to higher activities than TFA. SEM analysis showed that the matrices are shaped lens with a film structure which varies within the lens, depending on the presence and the nature of the IL.The kinetics parameters, using naringin and prunin as substrates, were evaluated with free and naringinase encapsulated, respectively on TMOS/Glycerol@[OMIM][Tf₂N] and TMOS/Glycerol@[C₂OHMIM][PF₆] and on TMOS/Glycerol. An improved stability and efficiency of α-l-rhamnosidase and β-glucosidase expressed by encapsulated naringinase on TMOS/Glycerol@[OMIM][Tf₂N] and TMOS/Glycerol@[C₂OHMIM][PF₆] were achieved. In addition to these advantageous, with ILs as sol-gel templates, environmental friendly processes can be implemented.     

Dialkyl phosphate-related ionic liquids as selective solvents for xylan

Herein we describe a possibility of selective dissolution of xylan, the most important type of hemicellulose, from Eucalyptus globulus kraft pulp using ionic liquids (ILs). On the basis of the IL 1-butyl-3-methylimidazolium dimethyl phosphate, which is well-known to dissolve pulp, the phosphate anion was modified by substituting one oxygen atom for sulfur and selenium, respectively. This alteration reduces the hydrogen bond basicity of the IL and therefore prevents dissolution of cellulose fibers, whereas the less ordered xylan is still dissolved. (1)H NMR spectra of model solutions and Kamlet-Taft parameters were used to quantify the solvent polarity and hydrogen bond acceptor properties of the ILs. These parameters have been correlated to their ability to dissolve xylan and cellulose, which was monitored by (13)C NMR spectroscopy. It was found that the selectivity for xylan dissolution increases to a certain extent with decreasing hydrogen-bond-accepting ability of anions of the ILs.     

Study of thermodynamic properties of imidazolium-based ionic liquids and investigation of the alkyl chain length effect by molecular dynamics simulation

In this paper, structural and dynamical properties of five imidazolium-based ionic liquids (ILs) [amim]Br (a = methyl, ethyl, butyl, pentyl, hexyl) were studied by molecular dynamics simulations. United atom force field (UAFF) has been used for the representation of the interaction between ions. Good agreement with experimental data was obtained for the simulated density based on the UAFF. The calculated densities gradually decrease with an increase in the length of alkyl side chain, which is a result of weakening the electrostatic interaction between ions. The simulated heats of vaporisation are higher than that of non-ILs and decrease with an increase in temperature. Radial distribution function (RDF) was employed to analyse the local structure of ILs. Cation–anion RDFs show that the anions are well organised around the cation in two shells (0.41 and 0.6 nm). The velocity autocorrelation functions of the anion and cations show that the relaxation time increased with an increase in the length of the alkyl side chain. The diffusion coefficients of ions were calculated by mean square displacement of the centre of mass of the ions at 400 K. The calculated diffusion coefficients using UAFF agree well with other all atom force fields. Also diffusion coefficients decrease with an increase in the length of the alkyl side chain. The calculated transference numbers show that the cation contributes more than anion in the electrical current. The diffusion coefficients increase with temperature.     

Amide functionalised dithiocarbamate ruthenium(II) bis-bipyridyl receptors: A new class of redox-responsive anion sensor

The synthesis of new amide functionalised ruthenium(II) bis-bipyridyl dithiocarbamate receptor molecules is described. These hosts have been shown to sense the binding of anions electrochemically. Proton NMR titration studies in dmso-d₆:MeCN-d₃ (1:1) solvent mixtures indicate that the receptors selectively bind dihydrogen phosphate. A single crystal X-ray structure of one receptor reveals the crucial role of amide-anion hydrogen bonding interactions in the binding of sulphate. Cyclic and square wave voltammetric investigations demonstrate that the receptors can sense the binding of anions electrochemically. The addition of dihydrogen phosphate induced the largest cathodic perturbation of the metal centred Ru(II)/(III) dithiocarbamate redox couple (ΔE = 180 mV).     

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.     

Quantitative determination of cellulose dissolved in 1-ethyl-3-methylimidazolium acetate using partial least squares regression on FTIR spectra

Rapid and quantitative measurements of cellulose concentrations in ionic liquids (ILs) are difficult. In this study, FTIR operated in attenuated total reflectance (ATR) mode was investigated as a tool to measure cellulose concentration in 1-ethyl-3-methylimidazolium acetate ([emim][OAc]) and the spectra were subjected to partial least squares (PLS) regression for the quantitative determination of cellulose content. Additionally, the spectra were subjected to 7 data preprocessing methods to reduce physical effects in the spectra. Peak normalization was found to be the technique that most improved the prediction of dissolved cellulose in [emim][OAc]. When peak normalization was used for data preprocessing, a model for the quantitative estimation of cellulose content between 0 wt.% and 4 wt.% with an error of 0.53 wt.% was generated. The methods described here provide the basis for a rapid and facile technique for the determination of dissolved cellulose content in [emim][OAc].     

Influence of protic ionic liquids on the structure and stability of succinylated Con A

We report the synthesis of a series of ionic liquids (ILs) from various ions having different kosmotropicity including dihydrogen phosphate (H(2)PO(4)(-)), hydrogen sulfate (HSO(4)(-)) and acetate (CH(3)COO(-)) as anions and chaotropic cation such as trialkylammonium cation. To characterize the biomolecular interactions of ILs with protein, we have explored the stability of succinylated Con A (S Con A) in the presence of these aqueous ILs, which are varied combinations of kosmotropic anion with chaotropic cation such as triethylammonium dihydrogen phosphate [(CH(3)CH(2))(3)NH][H(2)PO(4)] (TEAP), trimethylammonium acetate [(CH(3))(3)NH][CH(3)COO] (TMAA), trimethylammonium dihydrogen phosphate [(CH(3))(3)NH][H(2)PO(4)] (TMAP) and trimethylammonium hydrogen sulfate [(CH(3))(3)NH][HSO(4)] (TMAS). Circular dichroism (CD) and fluorescence experiments have been used to characterize the stability of S Con A by ILs. Our data distinctly demonstrate that the long alkyl chain IL TEAP is a strong stabilizer for S Con A. Further, our experimental results reveal that TEAP is an effective refolding enhancer for S Con A from a thermally denatured protein structure.     

A criterion for proper cosolvents used for ionic liquids: The Lewis acidic and basic dual nature of propylene carbonate

Probe of the Lewis acidic property of propylene carbonate (PC) has been initiated by temperature-dependent ¹⁹F chemical shifts recorded for 1-butyl-3-methylimidazolium hexafluorophosphate/PC (BMIPF₆/PC) mixtures. Investigation of hyperconjugative interactions by natural bond orbital method reveals the trend of electron-accepting capabilities from anion, PC > dimethyl sulfoxide (DMSO) > N,N-dimethyl formamide (DMF) > acetonitrile (AN). Comparison of solvent-induced ionic association of BMIPF₆ in the four basic aprotic organic solvents, made by monitoring concentration- and temperature-dependent-ratios of solvent diffusion coefficients to BMI⁺-containing components, D_sol/D_BMI, shows that the extent of original ionic association can be reduced by solvent dilution in the same decreasing order, PC ∼ DMSO > DMF > AN. In DMF and AN, two basic solvents having similar dielectric constants, the greater Lewis acid of DMF accounts for its better ability to dissociate aggregates of BMIPF₆. By replacing ethanol with 2,2,2-trifluoroethanol (TFE), the poor miscibility of BMIPF₆ with ethanol can be greatly enhanced due to the CF₃ group, capable of adapting electron density from anions via hyperconjugative interactions. In BMIPF₆/TFE mixtures, values of measured on ¹⁹F resonances show larger aggregates than measured on ¹H resonances, indicating charged clusters prefer anionic to cationic states. This observation can be explained by varying ionic states concerning with unbalanced charged aggregates between positive and negative ones.     

Effect of kosmotropicity of ionic liquids on the enzyme stability in aqueous solutions

This paper examined the effect of several pyridinium and imidazolium-based ionic liquids (ILs) on the protease stability in aqueous solutions. In general, the enzyme was found quite active at low concentrations of hydrophilic ILs. In aqueous environment, the enzyme was stabilized by the kosmotropic anions (such as CF3COO- and CH3COO-) and chaotropic cations (such as [BuPy]+ and [EMIM]+), but was destabilized by chaotropic anions (such as tosylate and BF4-) and kosmotropic cations (such as [BMIM]+).     

Surface structure of ionic liquids under an external electric field

Abstract

Surface structure and properties play an important role in many applications of ionic liquids (ILs). ILs can form unique surface structures that are very different from the bulk. In imidazolium-based ILs, for example, polar groups form ordered layer structure, while cationic alkyl chains are bundled together and point out from the surface. In many applications, ILs work under an external electric field. However, the effect of external electric field on the surface structure of ILs is still not clear. Here by using coarse-grained molecular dynamics simulation, we found that an electric field as strong as 1 V/nm is required to alter the surface structure of 1-dodecyl-3-methylimidazolium nitrate. Under a strong external electric field, layered structure disappears, and instead a sparser, more homogeneous and less orientationally ordered surface develops.     

Investigations about dissolution of cellulose in the 1-allyl-3-alkylimidazolium chloride ionic liquids

In this work, the 1-allyl-3-alkylimidazolium chloride ionic liquids were synthesized and characterized by increasing carbon atoms (n ≤ 6) of alkyl chains on a cationic 3-imidazole ring. The results indicated that 1-allyl-3-alkylimidazolium chloride with asymmetrical structure on the two sides of a cationic 3-imidazole ring (i.e., n = 1, 2, 6) exhibited alkalinity and lower thermal stabilities, and showed better solubility to the cellulose samples at 60-120 °C than those with symmetrical structures (n = 3, 4). The cellulose samples treated by 20% (w/w) ethylenediamine solution showed better solubility in 1-allyl-3-ethyl, hexyl-imidazolium chloride ionic liquids than that treated with 20% (w/w) NaOH solution at 5 °C for 72 h. XRD and TG analysis indicated that 0 0 2 plane apparent crystallite size as well as thermal stability of the regenerated cellulose samples from the ionic liquids decreased significantly compared with the untreated cellulose samples.     

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.     

Theoretical study on the interactions between methanol and imidazolium-based ionic liquids

To better understand the property of the binary systems composing of imidazolium salt, [emim]⁺Aˉ (A=Clˉ, Brˉ, BF₄ˉ, and PF₆ˉ) and methanol, we have investigated in detail the interactions of methanol molecule with anions Aˉ, cation [emim]⁺, and ion pair [emim]⁺Aˉ of several ionic liquids (ILs) based on 1-ethyl-3-methylimidazolium cation by performing density functional theory calculations. It is found that H-bonds are universally involved in these systems, which may play an important role for the miscibility of methanol with imidazolium-based ILs. The interaction mechanisms of methanol molecule with anion and cation are found to be different in nature: the former mainly involves LP_X-s_{O - H}^* \sigma_{{O - H}}^{*} interaction, while the latter relates with the decisive orbital overlap of the type of LP_O-s_{C - H}^* \sigma_{{C - H}}^{*} . Based on the present calculations, we have provided some reasonable interpretations for properties of the binary mixtures of ILs and alcohol and revealed valuable information for the interaction details between ILs and alcohols, which is expected to be useful for the design of more efficient ILs to form superior solvent system with alcohol.     

Dissolution of cellulose in phosphate-based ionic liquids

Two kinds of alkylimidazolium salts containing dimethyl phosphate or diethyl phosphate were obtained as room temperature ionic liquids synthesized by one step, and both of them have the ability to dissolve untreated cellulose. Especially, 1-ethyl-3-methylimidazolium diethylphosphonate ([EMIM]DEP) could obtain 4 wt% cellulose solution within 10 min under 90. The effects of dissolution temperature on cellulose dissolution time and degree of polymerization were investigated. As dissolution temperature increased, dissolution time was greatly reduced. Both the original and regenerated cellulose samples were characterized with wide-angle X-ray diffraction, thermogravimetric analysis and scanning electron micrograph. The results showed that the crystalline structure of cellulose was converted to cellulose II from cellulose I in native cellulose. It was also found that the regenerated cellulose had good thermal stability with [EMIM]DEP ionic liquid.