Characterization of acetylated 4- O-methylglucuronoxylan isolated from aspen employing 1H and 13C NMR spectroscopy

Abstract

Water-soluble hemicelluloses were extracted from milled aspen wood ( Populus tremula) employing microwave oven treatment at 180 °C for 10 min. The final pH of this extract was 3.5. From this extract oligo- and polysaccharides were isolated and subsequently fractionated by size-exclusion chromatography. The structures of the saccharides in three of the fractions obtained were determined by 1H and 13C NMR spectroscopy, using homonuclear and heteronuclear two-dimensional techniques. The polysaccharides present in the two fractions eluted first were O-acetyl-(4- O-methylglucurono)xylans. The average degree of acetylation of the xylose residues in these compounds was 0.6. The structural element →4)[4- O-Me-α- d-Glc pA-(1→2)][3- O-Ac]-β- d-Xyl p-(1→ could also be identified. On the average, these two xylans were composed of the following (1→4)-linked β- d-xylopyranosyl structural elements: unsubstituted (50 mol%), 2-O-acetylated (13 mol%), 3-O-acetylated (21 mol%), 2,3-di-O-acetylated (6 mol%) and [MeGlcA α-(1→2)][3-O-acetylated] (10 mol%). Most of the 4- O-methylglucuronyl and acetyl substituents in the isolated polysaccharides survived the microwave oven treatment. The third fraction, eluted last, contained acetylated xylo-oligosaccharides, with minor contamination by an acetylated mannan. In the case of these xylo-oligosaccharides, the average degree of acetylation was 0.3.