Replica-exchange molecular dynamics simulations of cellulose solvated in water and in the ionic liquid 1-butyl-3-methylimidazolium chloride

Molecular mechanism of cellulose dissolution in N-methyl morpholine-N-oxide: A molecular dynamics simulation study

N-methyl morpholine-N-oxide (NMMO) is the only commercialised solvent to dissolve cellulose and produce lyocell. However, the molecular mechanism of NMMO-induced cellulose solubilisation is unknown which limits further process development. In this work, and for the first time the complete dissolution process of a large cellulose bunch was simulated in NMMO monohydrate using long microsecond molecular dynamic simulations. The dissolution process was also simulated in 1-ethyl-3-methylimidazolium acetate (EmimAc) as an efficient ionic liquid in cellulose dissolution and the results were compared with the aqueous conditions. While the cellulose bunch showed a stable and insoluble structure in pure water, it was completely and efficiently dissolved in both NMMO monohydrate and EmimAc. It was shown that the dissolution time of cellulose in NMMO monohydrate is almost twice that in EmimAc, which is in agreement with the experimental observations. Although it is revealed that hydrogen bonding is the main driving force of cellulose dissolution in NMMO monohydrate, one cannot explain the complete molecular mechanism of NMMO-induced cellulose dissolution only by considering hydrogen bonds. A straightforward molecular mechanism was proposed, in which the interactions of NMMO molecules, not with cellulose, but with the other NMMO molecules play a critical role in the dissolution process.

Role and importance of solvents for the fractionation of lignocellulosic biomass

Lignocellulosic biomass is one of the most important renewable materials to replace carbon-based fossil resources. Solvent-based fractionation is a promising route for fractionation of biomass into its major components. Processing is governed by the employed solvent-systems properties. This review sheds light on the factors governing both dissolution and potential reactivities of the chemical structures present in lignocellulose, highlighting how proper understanding of the underlying mechanisms and interactions between solute and solvent help to choose proper systems for specific fractionation needs. Structural and chemical differences between the carbohydrate-based structural polymers and lignin require very different solvents capabilities in terms of causing and eventually stabilizing conformational changes and consequent activation of bonds to be cleaved by other active components in the. A consideration of potential depolymerization events during dissolution and energetic aspects of the dissolution process considering the contribution of polymer functionalities allow for a mapping of solvent suitability for biomass fractionation.

The role of urea in the solubility of cellulose in aqueous quaternary ammonium hydroxide

We examine the role of water and urea in cellulose solubility in tetrabutylammonium hydroxide (TBAH). Molecular dynamics simulations were performed for several different solvent compositions with a fixed cellulose fraction. For each composition, two simulations were carried out with cellulose fixed in each of the crystalline and the dissolved states. From the enthalpy and the entropy of the two states, the difference in Gibbs free energy (ΔG) and hence the spontaneity is determined. A comparison with solubility experiments showed a strong correlation between the calculated ΔG and the experimental measurements. A breakdown of the enthalpic and entropic contributions reveals the roles of water and urea in solubility. At high water concentration, a drop in solubility is attributed to both increased enthalpy and decreased entropy of dissolution. Water displaces strong IL–cellulose interactions for weaker water–cellulose interactions, resulting in an overall enthalpy increase. This is accompanied by a strong decrease in entropy, which is primarily attributed to both water and the entropy of mixing. Adding urea to TBAH(aq) increases solubility by an addition to the mixing term and by reducing losses in solvent entropy upon dissolution. In the absence of urea, the flexible [TBA]⁺ ions lose substantial degrees of freedom when they interact with cellulose. When urea is present, it partially replaces [TBA]⁺ and to a lesser extent OH⁻ near cellulose, losing less entropy because of its rigid structure. This suggests that one way to boost the dissolving power of an ionic liquid is to limit the number of degrees of freedom from the outset.

We examine the role of water and urea in cellulose solubility in tetrabutylammonium hydroxide (TBAH).

Unraveling the structural and molecular properties of 34-residue levans with various branching degrees by replica exchange molecular dynamics simulations

Levan has various potential applications in the pharmaceutical and food industries, such as cholesterol-lowering agents and prebiotics, due to its beneficial properties, which depend on its length and branching degree. A previous study also found that the branching degree of levan affected anti-tumor activities against SNU-1 and HepG2 tumor cell lines. Despite its promising potential, the properties of levans with different branching degrees are not well understood at the molecular level. In two models of the generalized Born implicit solvent (GB_HCT and GB_OBC1), we employed replica-exchange molecular dynamics simulations to explore conformational spaces of 34-residue levans (L₃₄) with branching degrees of zero (LFO_34B0), one (LFO_34B1), three (LFO_34B3) and five (LFO_34B5), as well as to elucidate their structural and molecular properties. To ensure a fair comparison of the effects of branching degree on these properties, we focused on analyzing the properties of the central 21-residue of the main chains of all systems. Our results show that all major representative conformations tend to form helix-like structures with kinks, where two-kink helix-like structures have the highest population. As branching degree increases, the population of helix-like structures with zero or one kink tends to increase slightly. As the number of kinks in the structures with the same branching degree increases, the average values of the lengths and angles among centers of masses of three consecutive turns of residue i, i+3, and i+6 tended to decrease. Due to its highest occurring frequencies, the O6 _(i)—H3O _(i+1) hydrogen bond could be important for helix-like structure formation. Moreover, hydrogen bonds forming among the branching residue (br), branching position (bp) and other residues of L_34B1, L_34B3 and L_34B5 were identified. The O1_(bp)—H3O_(br), O1_(br)—H3O_(br) and O5_(br)—H1O_(br) hydrogen bonds were found in the first-, second- and third-highest occurrence frequencies, respectively. Our study provides novel and important insights into conformational spaces and the structural and molecular properties of 34-residue levans with various branching degrees, which tend to form helix-like structures with kinks.

Towards a molecular understanding of cellulose dissolution in ionic liquids: anion/cation effect, synergistic mechanism and physicochemical aspects

This perspective summarizes mechanistic studies on cellulose dissolution in ionic liquids, highlighting the synergistic mechanism, physicochemical aspects and future research trends.

Cellulose is one of the most abundant bio-renewable materials on the earth and its conversion to biofuels provides an appealing way to satisfy the increasing global energy demand. However, before carrying out the process of enzymolysis to glucose or polysaccharides, cellulose needs to be pretreated to overcome its recalcitrance. In recent years, a variety of ionic liquids (ILs) have been found to be effective solvents for cellulose, providing a new, feasible pretreatment strategy. A lot of experimental and computational studies have been carried out to investigate the dissolution mechanism. However, many details are not fully understood, which highlights the necessity to overview the current knowledge of cellulose dissolution and identify the research trend in the future. This perspective summarizes the mechanistic studies and microscopic insights of cellulose dissolution in ILs. Recent investigations of the synergistic effect of cations/anions and the distinctive structural changes of cellulose microfibril in ILs are also reviewed. Besides, understanding the factors controlling the dissolution process, such as the structure of anions/cations, viscosity of ILs, pretreatment temperature, heating rate, etc., has been discussed from a structural and physicochemical viewpoint. At the end, the existing problems are discussed and future prospects are given. We hope this article would be helpful for deeper understanding of the cellulose dissolution process in ILs and the rational design of more efficient and recyclable ILs.

In silico insights into the solvation characteristics of the ionic liquid 1-methyltriethoxy-3-ethylimidazolium acetate for cellulosic biomass

Lignocellulosic biomass is a domestically grown, sustainable, and potentially carbon-neutral feedstock for the production of liquid fuels and other value added chemicals. This underutilized renewable feedstock has the potential to alleviate some of the current socio-economic dependence on foreign petroleum supplies while stimulating rural economies. Unfortunately, the potential of biomass has largely been underdeveloped due to the recalcitrant nature of lignocellulosic materials. Task-specific ionic liquids (ILs) have shown considerable promise as an alternative non-aqueous solvent for solvation and deconstruction of lignocellulose in the presence of metal chloride catalyst or enzymes. Recently it has been hypothesized that adding oxygen atoms to the tail of an imidazolium cation would alleviate some of the negative characteristics of the ILs by increasing mass transport properties, and decreasing IL deactivation of enzymes, while at the same time retaining favorable solvation characteristics for lignocellulose. Reported here are fully atomistic molecular dynamic simulations of 1-methyltriethoxy-3-ethylimidazolium acetate ([Me-(OEt)3-Et-IM(+)] [OAc(-)]) that elucidate promising molecular-level details pertaining to the solvation characteristics of model compounds of cellulose, and IL-induced side-chain and ring puckering conformations. It is found that the anion interactions with the saccharide induce alternate ring puckering conformations from those seen in aqueous environments (i.e.(1)C4), while the cation interactions are found to influence the conformation of the ω dihedral. These perturbations in saccharide structures are discussed in the context of their contribution to the disruption of hydrogen bonding in cellulosic architecture and their role in solvation.

The effect of pressure on cation-cellulose interactions in cellulose/ionic liquid mixtures

Cation-cellulose interactions in binary mixtures of [EMIM][OAc] and cellulose have been investigated using high-pressure infrared spectroscopy. At low concentrations of cellulose, almost no changes were observed in the imidazolium C(2)-H frequency; on the other hand, at high concentrations of cellulose, increases in the C(2)-H vibration frequency were observed under ambient pressure. As the pressure was elevated, the imidazolium C(2)-H absorption of the [EMIM][OAc]/cellulose mixtures underwent band-narrowing and blue-shifts in the frequency. These observations suggest that high pressures may strengthen the hydrogen bonds formed between C(2)-H and cellulose, possibly forcing the cellulose to dissociate clusters of ionic liquid through enhanced cation-cellulose interactions. In contrast to the cation-cellulose interaction results, the COO(-) absorption of the anion does not show dramatic changes under high pressures. Our results indicate the possibility of enhanced cation-cellulose interactions through pressure elevation, demonstrating that high pressures may have the potential to tune the relative contributions of cation-cellulose and anion-cellulose interactions in cellulose/ionic liquid mixtures.

Mechanisms of hydrogen bond formation between ionic liquids and cellulose and the influence of water content

We explore the complex network of transitions occurring between different hydrogen bonding states within ionic liquids and cellulose.

Elucidating the conformational energetics of glucose and cellobiose in ionic liquids

The energetics driving the conformational preference of the ω dihedral of glucose and the φ–ψ dihedrals of cellobiose solvated in imidazolium acetate ionic liquids and water are elucidated and compared.