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Unraveling Lignocellulosic Biomass to Produce BiofuelsLignocellulosic biomass is inedible fibrous material derived from plant cell walls. Its main cellulose component is an abundant potential source of natural sugars that could be used for producing biofuels, such as ethanol and butanol. However, energy-efficient and cost-effective processes for breaking up the plant cell wall and releasing these sugars are needed. To address this problem, LANL scientists focused on understanding the interactions between atoms that hold cellulose together. Plants biosynthesize cellulose by joining glucose molecules into long chains, then assembling the chains into sheets within which the chains are held together by hydrogen bonds (Figure 1). These sheets stack on top each other through weaker van der Waals and hydrophobic forces to form nanometer thick crystalline microfibrils, which are often millimeters in length. The complexes of proteins that produce cellulose act as a biological spinneret, exuding high tensile strength microfibrils into the plant cell wall. The microfibrils are encrusted in other polysaccharides and lignin. The crystalline nature of cellulose (the highly regular arrangement of atoms within microfibrils) and the regularity and strength of its hydrogen bonds are expected to be key in making cellulose resistant to chemical attack and preventing depolymerizing enzymes, called cellulases, from pulling the cellulose chains away from the surfaces of the microfibrils. 
Figure 1. Schematic of the biosynthesis of cellulose. The hydrogen bonds between two chains, colored green and blue, are represented by broken yellow lines. Oxygen atoms, colored red, and hydrogen atoms, colored grey, are the key players in these hydrogen bonds. The crystalline microfibrils, represented as blue rods, are exuded into the plant cell wall (shown in gold). LANL scientists report some surprising details about the nature of this hydrogen bonding. In the first report Paul Langan (B-8) and colleagues from the U.S. Department of Agriculture and the Centre de Recherche sur les Macromolecule Vegetale in France used neutron crystallography, molecular dynamics, and quantum mechanical calculations to reveal the presence of hydrogen bonding disorder in highly crystalline cellulose. The significant amounts of disorder have characteristics of a completely different type of hydrogen bond network at surfaces. The result is highly relevant to the interaction of cellulose with cellulases (Figure 2). Reference: “Neutron Crystallography, Molecular Dynamics, and Quantum Mechanics Studies of the Nature of Hydrogen Bonding in Cellulose”, Biomacromolecules 9, 3133 (2008). LDRD-DR supported the LANL work. 
Figure 2. Surface chain I from the molecular dynamics study. The top of the model is the crystallite surface, and the lower part of the model is adjacent to the crystallite interior. Hydrogen bonds in yellow correspond to bonds that are characteristic of scheme A, and those in magenta on the top side correspond to scheme B. At the left end of the model, there was substantial deviation from two-fold symmetry, and the O6 atom has a gg orientation. Dotted blue lines correspond to arrangements that are not part of either A or B. In the second report, Tongye Shen and Gnana Gnanakaran (both in T-6) developed a new lattice based model for cellulose to predict the nature of plasticity of the hydrogen bonding network arising from frustration and redundancy in available hydrogen bonds (Figure 3). These microscopic properties are revealed in a statistical mechanical model at the resolution of explicit hydrogen bonds, which take into account both intra-chain and inter-chain hydrogen bonds in naturally occurring cellulose crystals. A fundamental insight gained from these calculations is that hydrogen bonding frustration and redundancy are intrinsic properties of cellulose. This plasticity allows cellulose to remain stable over a wide range of temperatures by swapping between different types of hydrogen bonds. The scientists identified critical inter-chain and intra-chain hydrogen bonds that can be manipulated towards rational destruction crystalline cellulose. Reference: “The Stability of Cellulose: A Statistical Perspective from a Coarse-Grained Model of Hydrogen-Bond Networks”, Biophysical Journal 96, in press. LDRD-DR and the Center for Nonlinear Studies supported the research. 
Figure 3. An illustration of the sheet structure of cellulose I-B. Cellulose chains (solid vertical rods) are linear collection of monomers (solid circles), which can be potentially linked using hydrogen bonds residing intrachain positions (vertical dot-dashed lines) and interchain positions (horizontal dashed lines). The assembly (or disassembly) process is expressed by the formation (or disruption) of hydrogen bonds at these positions. A small part (four monomers) of the sheet structure is viewed with an atomistic representation. The LANL work revealed some unexpected insights into the fundamental properties of a material for next generation biofuels. The result of these initial studies suggests that hydrogen bond disorder and plasticity are important factors in the stability of cellulose to degradation. This finding has lead scientists to identify specific hydrogen bonds to be targeted for rational degradation of cellulose. |
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