Structural analysis of arabinoxylans isolated from ball-milled switchgrass biomass

Ball-milled alcohol-insoluble residue (AIR) was prepared from switchgrass (Panicum virgatum var Alamo) and sequentially extracted with 50 mM ammonium oxalate buffer, 50 mM sodium carbonate, 1 M KOH containing 1% NaBH(4), and 4 M KOH containing 1% NaBH(4). Arabinoxylan was the most abundant component of the 1 M KOH-extracted fraction, which was treated with endoxylanase to generate oligosaccharides. Gel-permeation chromatography of these oligosaccharides produced three size-homogeneous oligosaccharide fractions with molecular weights of 678, 810, and 1074 Da, corresponding to 5, 6, and 8 pentose units, respectively. Detailed structural analysis of these oligosaccharides was performed using methylation analysis, multiple-step mass spectrometry (ESIMS(n)), and 1D and 2D NMR spectroscopy. The preferred gas-phase fragmentation pathways were identified for these oligosaccharides, providing extensive sequence information that was completely consistent with structures determined by ab initio NMR analysis. These results demonstrate the high information content of ESIMS(n) analysis when applied to cell-wall-derived oligosaccharides and provide standard data that will facilitate the analysis of cell-wall polysaccharide fragments with a sensitivity that is sufficient for the analysis of samples obtained from dissected tissues as well as other small samples.