Sunday, April 29, 2007

Characterization of the cell-wall microfibril structure from native and pretreated biomass using atomic force microscopy

Shi-You Ding1, Michael Selig1, Tina Jeoh2, David K. Johnson1, and Michael E. Himmel2. (1) Chemical and Biosciences Center, National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401, (2) Chemical and Biosciences Center, National Renewable Energy Lab, 1617 Cole Blvd., Golden, CO 80401

The plant cell-wall microfibril, the primary target substrate of bioconversion, is believed to consist of a cellulose-elementary-fibril core surrounded by a hemicellulose sheath forming a macromolecular composite that is approximate 3-5 nm in diameters and ten to hundreds microns in length depending on different cell types. Understanding the microfibril structure and changes that occur during the processes of biomass handling, pretreatment, and enzyme hydrolysis is critical to overcome biomass recalcitrance.  Nanometer resolution is required to characterize the microfibril structure. In addition, sample preparation involving dehydration and chemical staining must be minimized to avoid potential alteration of the microfibril molecular structure.  Previous studies have suggested the hypothesis that pretreatment chemistry removes and/or relocates non-cellulosic polymers, thus increasing the accessibility of cellulase enzymes and therefore enhancing enzyme digestibility. However, over-pretreatment may cause rearrangement (aggregation) of the microfibril network, resulting in reduction of enzyme digestibility.  To test this hypothesis, atomic force microscopy (AFM) imaging in an aqueous environment was used to measure the microfibril structure from corn stover. We focused on measuring the diameters, lengths, and arrangements of the microfibrils from native and pretreated biomass with various conditions including dilute acid, ammonia fiber expansion, hydrogen peroxide, and organic solvent delignification.