Researchers Discover Potential Weaknesses in Structure of Lignocellulose; Insight Could Lead to Cost-Effective Strategies for Biomass Conversion
|Click to enlarge. Source: Los Alamos National Laboratory.|
In separate papers published in Biophysical Journal and recently in an issue of the journal Biomacromolecules, Los Alamos National Laboratory researchers identify potential weaknesses among sheets of cellulose molecules comprising lignocellulosic biomass, the inedible fibrous material derived from plant cell walls. The insight ultimately could lead to a cost-effective and energy-efficient strategy for turning biomass into alternative fuels.
Cellulose is biosynthesized in plant cells by the polymerization of glucose into long chains (green, dark blue in diagram at right). As the chains are produced, they are assembled into sheets (light blue) that stack on top of each other through van der Waals forces to form nanometer-thick crystalline microfibrils (blue rods) in the cell wall (gold). The microfibrils are encrusted in other polysaccharides and lignin. Cellulose stability is maintained by networks of hydrogen bonds (yellow dashes) within the sheets.
Not only are the fibers incredibly strong, but they are incredibly resistant to the action of enzymes (cellulases) that can crack the fibers back into their simple-sugar components. The ability to economically and easily break cellulose into sugars is desirable because the sugars can be used to create fuel alternatives.
Working with researchers from the US Department of Agriculture and the Centre de Recherches sur les Macromolécules Végétales in France, Los Alamos researcher Paul Langan used neutrons to probe the crystalline structure of highly crystalline cellulose.
Langan and his colleagues found that although cellulose generally has a well-ordered network of hydrogen bonds holding it together, the material also displays significant amounts of disorder, creating a different type of hydrogen bond network at certain surfaces. These differences make the molecule potentially vulnerable to an attack by cellulases.
Moreover, in this month’s Biophysical Journal, Los Alamos researchers Tongye Shen and Gnana Gnanakaran describe a new lattice-based model of crystalline cellulose. The model predicts how hydrogen bonds in cellulose can shift to remain stable under a wide range of temperatures. This plasticity allows the material to swap different types of hydrogen bonds but also constrains the molecules so that they must form bonds in the weaker configuration described by Langan and his colleagues.
Most important, Shen and Gnanakaran’s model identifies hydrogen bonds that can be manipulated via temperature differences to potentially make the material more susceptible to attack by enzymes that can crack the fibers into sugars for biofuel production.
Funding for the project comes from Laboratory-Directed Research and Development (LDRD), which is the premier source of internally directed research-and-development funding at Los Alamos National Laboratory. The LDRD program invests in high-risk, potentially high-payoff projects at the discretion of the Laboratory Director. Strategic investments of the LDRD program help position Los Alamos to anticipate and prepare for emerging national security challenges.
Tongye Shen and S. Gnanakaran (2009) The Stability of Cellulose: A Statistical Perspective from a Coarse-Grained Model of Hydrogen-Bond Networks. Biophysical Journal, Volume 96, Issue 8, 3032-3040, doi: 10.1016/j.bpj.2008.12.3953
Masahisa Wada, Laurent Heux, Yoshiharu Nishiyama and Paul Langan (2008) X-ray Crystallographic, Scanning Microprobe X-ray Diffraction, and Cross-Polarized/Magic Angle Spinning 13C NMR Studies of the Structure of Cellulose IIIII. Biomacromolecules, 10 (2), pp 302–309 doi: 10.1021/bm8010227