Cellulose Recalcitrance
As described earlier in the chapter, recalcitrance is the natural resistance to degradation in lignocellulosic biomass materials. Understanding the recalcitrance is the key to developing efficient cellulosic biomass depolymerization and pretreatment methods. A number of researchers have approached this problem by studying the structure of cellulose by experimental methods as well as computational methods [4-7]. Experimental methods are severely limited due to the crystalline nature and insolubility of cellulose. X-ray crystallography is the most widely applied experimental technique in cellulose structural studies [8-10].
Naturally occurring cellulose, or cellulose I, has two distinct crystal phases, these are monoclinic ip and the triclinic Ia phase [11]. The ip crystal phase is the more stable of the two, as heating of the Ia phases causes irreversible conversion to ip [12]. Nishiyama and coworkers have recently resolved the atomic structures of both crystalline forms of cellulose [8, 9]. The two allomorphs consist of polymerized cellobiose chains arranged in parallel to form flat sheets. These sheets are stacked on top of each other to form the full three-dimensional crystal structures [8, 9].
Molecular modeling using computational methods is one of the established techniques to study molecular architecture and deconstruction of cellulose. As cellulose is insoluble in common organic solvents, only a limited number of special solvents like ionic liquids are capable of dissolving cellulose to any appreciable extent. Since the 2002 report on use of 1-”butyl-3-methylimidazolium salts [13], which are room temperature ionic liquids for the dissolution of cellulose, a number of researchers have studied the cellulose-ionic liquid system by computational methods [14-17]. These studies are aimed at revealing the molecular basis of recalcitrance and the development of efficient cellulose depolymerization and biomass pretreatment techniques.
In one recent approach to explain the mechanism and evaluate the energetics of 1-”~butyl-3-methylimidazolium chloride (BMIMCl) ionic liquid pretreatment of cellulose, Cho and coworkers used computational methods. In this case, gradual peeling off of a cellulose chain out of a microfibril or a bundle of cellulose chain was used as the model, as shown in Figure 4.9 [4].
In this study they 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 BMIMCl ionic liquid. The computational study showed that, for this deconstruction process, calculated free-energy cost in BMIMCl is about 2 kcal/mol per glucose residue less than the energy cost in water. To unravel the molecular origin of solvent-induced differences, they devised a coarse graining scheme to dissect force interactions in simulation models by a force-matching method. The results established that solvent-glucan interactions are dependent on the deconstruction
iM«»TH¥-*S *t4hh-iv
Figure 4.9 A molecular model for peeling off a cellulose chain from a microfibril. (Reprinted with permission from reference [4]; copyright 2011 American Chemical Society).
state of cellulose. Cho and coworkers found that 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. Additionally, they proved that chloride anions strongly interact with hydroxyl groups, and the coupling of cations to side chains and linker oxygens is stronger in the peeled-off state. Furthermore, Cho and Gross identified the coarse-grain analysis of force interactions in configuring cations to target side chains and linker oxygens as a useful design strategy for pretreatment of ionic liquids [4].
In another approach to understanding the molecular origin of cellulose recalcitrance, Chu and coworkers characterized the interaction network and solvation structures of cellulose microfibrils via all-atom molecular dynamics simulations [7]. In this study the network was divided into three components: intrachain, interchain, and intersheet interactions. Analysis of their spatial dependence and interaction energetics indicated that intersheet interactions are the most robust and strongest component and do not display a noticeable dependence on solvent exposure. The major finding is that intersheet interactions, which involve C-H-O pseudo hydrogen bonds and van der Waals interactions acting in concert, are the strongest and most robust component in the interaction network. Furthermore, Gross and Chu found that structural fluctuations of intersheet interactions are spatially homogeneous, with no variation with respect to closeness to solvent-exposed surfaces. Although the interaction energy of each individual C-H-O hydrogen bonds is less than that of O-H-O hydrogen bonds (both of which are electrostatic
Figure 4.10 Average number of hydrogen bonds per glucose unit for each hydrogen bond class in the interior of cellulose Ia and Ip forms. (Reprinted with permission from reference [7]; copyright 2010 American Chemical Society).
in nature), the addition of van der Waals interactions makes intersheet interactions stronger than interchain interactions.
In a comparison of the two crystalline forms of cellulose, a and в types, Gross and Chu found that cellulose Ip differs from Ia in having a higher number of intersheet hydrogen bonds. Their results on the average number of hydrogen bonds per glucose residue in intrachain, interchain, and intersheet interactions is shown in Figure 4.10. For both the Ip and Ia microfibrils, the total number of hydrogen bonds per glucose is highest in the interior and decreases toward the surface due to both a lack of bonding partners and decreases in individual bond occupancy from finite packing and solvent effects. Comparing Ip and Ia microfibrils indicates that each glucose residue in Ip has two more hydrogen bonds in the interior than the residues in Ia (24.5 versus 22.5), and this result is consistent with the higher stability observed for the Ip phase of cellulose [12]. Gross and Chu’s molecular dynamics simulations shows that the two allomorphs only differ in the number of intersheet hydrogen bonds. Therefore, they conclude that intersheet hydrogen bonds are the main cause of the different stabilities between Ip and Ia cellulose [12]. In conclusion, this result highlights the importance of intersheet interactions in giving rise to the recalcitrance of cellulose.