Structural Characterization of the Heteroxylans from Poplar and Switchgrass

Heteroxylans are polysaccharides with a backbone composed of 1,4-linked β-D-xylosyl residues. In hardwoods some of these xylosyl residues are substituted at O-2 with 4-O-methyl α-D-glucuronic and occasionally with α-D-glucuronic acid. In grasses, the xylan backbone is predominantly substituted with α-L-arabinofuranosyl residues (most often at O-3, but sometimes at O-2). Grass heteroxylan backbone residues may also have small amounts of α-D-glucuronic acid and/or 4-O-methyl α-D-glucuronic acid at O-2. Heteroxylans have a role in maintaining the structural integrity of the cell walls that comprise the bulk of lignocellulosic biomass. Moreover, differences in the molecular features of these hemicellulosic polysaccharides, including their degree of polymerization, degree of branching and spatial arrangement of side chains along the xylan backbone, have been correlated to altered cell wall properties (Izydorczyk MS, Biliaderis CG, Carbohydr Polym 28:33-48, 1995) and the ease with which biomass can be enzymatically converted to fermentable sugars. Thus, understanding the relationship between heteroxylan structure and biomass properties is required to engineer bioenergy crops with improved processing characteristics. In this chapter we describe some of the analytical methods we routinely use to perform in-depth structural analysis of heteroxylans from poplar and switchgrass biomass.