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Naturally occurring cellulose iscrystalline as a consequence ofthe strong interactions between the glucan chains that comprise itand therefore is insoluble in most solvents. One of the few solventsystems able to dissolve cellulose is lithium chloride (LiCl) dissolvedin N,N-dimethylacetamide (DMA).By an integrated application of all-atom molecular dynamics (MD) simulations,reaction path optimization, free-energy calculations, and a force-matchinganalysis of coarse-grained atomistic simulations, we establish thatDMA-mediated preferential interactions of Li+cations andCl–anions with glucan chains enable cellulose dissolutionin LiCl/DMA. The relatively weak solvation of Li+, Cl–, and glucan chains by DMA results in strong effectiveinteractions of Li+and Cl–ions withthe glucans, leading to cellulose dissolution. The small size of theLi+cations allows them to strongly couple to multipleinteraction sites on the glucan chains of cellulose, including thespatially restricted regions around the ether linkages connectingneighboring glucose residues. Li+cations were thus identifiedas the main component responsible for driving cellulose dissolution.The mechanism for explaining the solubility of cellulose in the LiCl/DMAsystem deduced from the analysis of atomistic-scale simulations conductedin this work is also consistent with most of the empirical observationsrelated to cellulose dissolution in salt/amide solvent systems. [ABSTRACT FROM AUTHOR]

A Staggered LATtice (SLAT) model is developed for modelingcellulosemicrofibrils. The simple representation of molecular packing and interactionsemployed in SLAT allows simulations of structure fluctuations andphase transition of cellulose microfibrils at sufficiently long andlarge scales for comparison with experiments. Glucan chains in themicrofibril are modeled as connected monomers, each correspondingto a cellobiose subunit, and the surrounding space around the celluloseis composed of solvent cells. Interaction parameters of monomer–monomerinteractions were parametrized based on the results of atomistic moleculardynamics simulations. The monomer–solvent interaction was optimizedto give a melting temperature of ∼695 K for the 36-glucan chainmodel cellulose microfibril, which is consistent with the estimationbased on experimental data. Monte Carlo simulations of the SLAT modelalso capture experimentally measured X-ray diffraction patterns ofcellulose as a function of temperature, including the region of meltingtransition, as well as predict the highly flexible regions in themicrofibril. Beyond the diameter of ∼3 nm, we found that meltingtemperature of the cellulose microfibril is not significantly shiftedby changing the thickness. On the other hand, a slight decrease inthe degree of polymerization of glucan chains is shown to enhancestructure fluctuations through the ends of glucan chains, i.e., thedefect sites, and thereby significantly reduce the melting temperature.Analysis of the sizes, densities, and lifetimes of defect structuresin the microfibril indicates a significant extent of fluctuationson the surfaces even at room temperature and that defect statisticsare strong but distinct functions of temperature and solvent quality.The SLAT model is the first of its kind for simulating cellulosicmaterials, and this work shows that it can be used to incorporateinformation obtained from atomistic simulations and experimental datato enable the aforementioned findings through computation. [ABSTRACT FROM AUTHOR]

Cellulose is present in biomass as crystalline microfibrils held together by a complex network of intermolecular interactions making it difficult to initiate its hydrolysis and conversion to fuels. While cellulose is insoluble in water and most organic solvents, complete dissolution of cellulose can be achieved in certain classes of ionic liquids (ILs). The present study was undertaken to analyze the thermodynamic driving forces of this process and to understand how the anions and cations comprising an IL interact with the different moieties of glucose residues to cause dissolution. All-atom molecular dynamics (MD) simulations were performed at two extreme states of cellulose dissolution: a crystalline microfibril and a dissociated state in which all the glucan chains of the microfibril are fully separated from each other by at least four solvation shells. MD simulations of the two states were carried out in water and in the IL 1-butyl-3-methylimidazolium chloride (BmimCl) to provide a comprehensive analysis of solvent effects on cellulose dissolution. The results reveal two important molecular aspects of the mechanism of cellulose dissolution. The first is that the perturbation of solvent structures by the dissolved glucan chains can be a crucial factor in determining solubility, particularly for the insolubility of cellulose in water at 300 K. Second, both the Cl–and the Bmim誉 of BmimCl interact with the moieties of glucan residues that form intersheet contacts, the most robust component of the interaction network of crystalline cellulose. Cl–anions can form hydrogen bonds (HBs) with the hydroxyl groups of glucan chains from either the equatorial or the axial directions. For Bmim熫抢, the calculated density profiles reveal that the contacts with glucan chains along the axial directions are closer than those along the equatorial directions. On the basis of the results of atomistic MD simulations, we propose that interacting with glucan chains along axial directions and disrupting the intersheet contacts of cellulose is an important ability of cellulose pretreatment solvents. [ABSTRACT FROM AUTHOR]

Pretreatment for deconstructing the multifaceted interaction network in crystalline cellulose is a limiting step in making fuels from lignocellulosic biomass. Not soluble in water and most organic solvents, cellulose was found to dissolve in certain classes of ionic liquids (ILs). To elucidate the underlying mechanisms, we simulated cellulose deconstruction by peeling off an 11-residue glucan chain from a cellulose microfibril and computed the free-energy profile in water and in 1-butyl-3-methylimidazolium chloride (BmimCl) IL. For this deconstruction process, the calculated free-energy cost/reduction in water/BmimCl is ∼2 kcal/mol per glucose residue, respectively. To unravel the molecular origin of solvent-induced differences, we devised a coarse graining scheme to dissect force interactions in simulation models by a force-matching method. The results establish that solvent—glucan interactions are dependent on the deconstruction state of cellulose. Water couples to the hydroxyl and side-chain groups of glucose residues more strongly in the peeled-off state but lacks driving forces to interact with sugar rings and linker oxygens. Conversely, BmimCl demonstrates versatility in targeting glucose residues in cellulose. Anions strongly interact with hydroxyl groups, and the coupling of cations to side chains and linker oxygens is stronger in the peeled-off state. Other than enhancing anion—hydroxyl group coupling, coarse-grain analysis of force interactions identifies configuring cations to target side chains and linker oxygens as a useful design strategy for pretreatment ILs. Furthermore, the state dependence of solvent—glucan interactions highlights specific stabilization and/or frustration of the different structure states of cellulose as important design parameters for pretreatment solvents. [ABSTRACT FROM AUTHOR]