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

Water-soluble hemicelluloses were extracted from milled aspen wood (Populus tremula) employing microwave oven treatment at 180 degreesC for 10 min. Th e final pH of this extract was 3.5. From this extract oligo- and polysaccha rides were isolated and subsequently fractionated by size-exclusion chromat ography. The structures of the saccharides in three of the fractions obtain ed were determined by H-1 and C-13 NMR spectroscopy, using homonuclear and heteronuclear two-dimensional techniques. The polysaccharides present in th e two fractions eluted first were O-acetyl-(4-O-methylglucurono)xylans. The average degree of acetylation of the xylose residues in these compounds wa s 0.6. The structural element -->4)[4-O-Me-alpha -D-GlcpA-(1-->2)][3-O-Ac]- beta -D-Xylp -(1--> could also be identified. On the average, these two xyl ans were composed of the following (1-->4)-linked beta -D-xyIopyranosyl str uctural elements: unsubstituted (50 mol%), 2-O-acetylated (13 mol%), 3-O-ac etylated (21 mol%), 2,3-di-O-acetylated (6 mol%) and [MeGIcA alpha-(1-->2)] [3-O-acetylated] (10 mol%). Most of the 4-O-methylglucuronyl and acetyl sub stituents in the isolated polysaccharides survived the microwave oven treat ment. The third fraction, eluted last, contained acetylated xylo-oligosacch arides, with minor contamination by an acetylated mannan. In the case of th ese xylo-oligosaccharides, the average degree of acetylation was 0.3. (C) 2 000 Elsevier Science Ltd. All rights reserved.