Cellulose tends to orient itself into a sheet-like network of highly ordered, densely packed molecules. These sheets stack upon themselves and bond together very tightly due to interactions between hydrogen atoms—somewhat like sheets of chicken wire stacked together and secured by loops of bailing wire. This stacking and bonding arrangement prevents enzymes from directly attacking most of the individual cellulose molecules and isolating the sugar chains within them. The new process reduces the strength of the hydrogen bonds in the network—as if the bailing wire in the bound chicken-wire analogy had been removed and replaced more loosely with thread.

This, in turn, significantly reduces the tightness of the cellulose network and leaves it more vulnerable to conversion into sugar by fungi-derived cellulolytic enzymes. This new approach can thus significantly reduce the enzyme loading necessary to achieve economical conversion of lignocellulosic biomass, the researchers note in a paper published in the Journal of the American Chemical Society. The study also provides fundamental insights into the nature of cellulose recalcitrance.

Several different approaches are being developed to pretreat lignocellulose so that its complex architecture can be disrupted, thereby making its cellulosic component more accessible to water and enzymes for an accelerated conversion to glucose. These approaches can be classified into two major categories of thermochemical pretreatments: the first type facilitates removal of hemicellulose and/or lignin and increases enzyme accessibility to embedded crystalline cellulose fibrils (with little or no major change to cellulose crystallinity), whereas the second type proceeds further to disrupt cellulose crystallinity and hence increase glycosidic bond accessibility.

Most thermochemical pretreatments fall within the first category (e.g., dilute acid, ammonia fiber expansion (AFEX), steam explosion, hot water, organosolv), except the ones that use ionic liquids or concentrated acids (like 85% phosphoric acid). However, complexities associated with both the cost-effective utilization and recycling of these chemicals have so far prevented rapid commercialization of either of these approaches.

...A route to improve cellulose conversion is to engineer enzymes with enhanced specific activities for crystalline cellulose hydrolysis. However, despite significant advances in our understanding of the molecular-scale enzymatic mechanisms driving cellulose deconstruction, engineering highly efficient cellulases remains a major challenge. An alternative approach relies on the use of inexpensive and easily recoverable chemicals to alter the cellulose crystal structure in order to increase its rate of depolymerization.

—Chundawat et al.

The approach taken by the team involves a structural conversion between crystalline forms of cellulose catalyzed by ammonia to enhance the depolymerization kinetics. This ammonia-based pretreatment produces cellulose III_I (without any relevant loss of crystallinity) with enzymatic hydrolysis rates the closest to those of amorphous cellulose among all reported cellulose allomorphs.

Ammonia transformed the naturally occurring crystalline allomorph I_β to III_I, which led to a decrease in the number of cellulose intrasheet hydrogen bonds and an increase in the number of intersheet hydrogen bonds. This rearrangement of the hydrogen bond network within cellulose III_I, which increased the number of solvent-exposed glucan chain hydrogen bonds with water by ~50%, was accompanied by enhanced saccharification rates by up to 5-fold (closest to amorphous cellulose) and 60–70% lower maximum surface-bound cellulase capacity. The enhancement in apparent cellulase activity was attributed to the “amorphous-like” nature of the cellulose III_I fibril surface that facilitated easier glucan chain extraction.

Subtle alterations within the cellulose hydrogen bond network provide an attractive way to enhance its deconstruction and offer unique insight into the nature of cellulose recalcitrance. This approach can lead to unconventional pathways for development of novel pretreatments and engineered cellulases for cost-effective biofuels production.

—Chundawat et al.

In addition to LANL, the GLBRC, and Michigan State University, the paper included collaborators from American University and the US Department of Agriculture’s Forest Products Laboratory in Madison, Wisconsin.

Resources

• Shishir P. S. Chundawat, Giovanni Bellesia, Nirmal Uppugundla, Leonardo da Costa Sousa, Dahai Gao, Albert M. Cheh, Umesh P. Agarwal, Christopher M. Bianchetti, George N. Phillips Jr., Paul Langan, Venkatesh Balan, S. Gnanakaran, Bruce E. Dale (2011) Restructuring the Crystalline Cellulose Hydrogen Bond Network Enhances Its Depolymerization Rate. Journal of the American Chemical Society 133 (29), 11163-11174 doi: 10.1021/ja2011115