Highly uneven distribution of O-acetyl groups in the acidic d-xylan of Mimosa scabrella (bracatinga)

Abstract

The acidic O-acetyl- d-xylan of Mimosa scabrella contains xylose, 4- O-methylglucuronic acid, and O-acetyl groups in the molar ratios of 60:7:33 and, when subjected to controlled Smith degradation, provided xylosyl-, xylobiosyl-, xylotriosyl-, and xylotetraosyl-glycerol, as well as glycerol. Their proportions differed greatly from that which would arise from xylopyranosyl units substituted regularly with O-acetyl groups. In order to avoid ambiguities arising from resistance of some of the xylosyl units by substitution with 4- O-methylglucuronic acid, the O-acetylxylan was converted into a partly O-methylated polysaccharide having O-methyl groups with similar positions of substitution. Two successive Smith degradations were carried out, and the two sets of partly O-methylated derivatives resulting were identified by f.a.b.-m.s. The highly uneven distribution of O-acetyl groups along the xylan main-chain was confirmed, with the highest number of consecutive O-acetyl and 4- O-methylglucuronic acid substituents being six, and five O-acetyl substituents alone. 4- O-Methylglucuronic acid units were not substituted.