Isolation from a softwood xylan of oligosaccharides containing two 4- O-methyl- d-glucuronic acid residues

Abstract

Partial hydrolysis of a larch arabino(4- O-methylglucurono)xylan afforded two series of oligouronides composed of 4- O-methyl- d-glucuronic acid and d-xylose residues. The first series included aldouronic acids up to the aldopentaouronic acid. Methylation analysis indicated that the aldopentao- and aldotetrao-uronic acids were mixtures of isomers. One aldotetraouronic acid was isolated and identified as O-β- d-Xyl p-(1 → 4)- O-β- d-Xyl p-(1 → 4)- O-(4- O-Me-α- d-GlcA p)-(1 → 2)- d-Xyl. The two isomeric aldotriouronic acids were separated from each other. The acids of the second series, which were composed of two uronic acids and 2-4 d-xylose residues, were identified as follows: O-β- d-Xyl p-(1 → 4)- O-(4- O-Me-α- d-GlcA p)-(1 → 2)- O-β- d-Xyl p-(1 → 4)- O(4- O-Me-α- d-GlcA p)-(1 → 2)- O-β- d-Xyl p-(1 → 4)- d-Xyl, O-(4- O-Me-α- d-GlcA p)-(1 → 2)- O-β- d-Xyl p-(1 → 4)- O-(4- O-Me-α- d-GlcA p)-(1 → 2)- O-β- d-Xyl p-(1 → 4)- O-β- d -Xyl p-(1 → 4)-D-Xyl, O-(4- O-Me-α- d-GlcA p)-(1 → 2)- O-β- d-Xyl p-(1 → 4)- O-(4- O-Mec-α- d-GlcA p)-(1 → 2)- O-β- d-Xyl p-(1 → 4)-D-Xyl, and O-(4- O-Me-α- d-GlcA p)-(1 → 2)- O-β- d-Xyl p-(1 → 4)- O-(4- O-Me-α- d-GlcA p)-(1 → 2)-D-Xyl. The first three compounds were new acidic oligosaccharides. The 4- O-methyl- d-glucuronic acid in the second series was present in a larger proportion than in the first series, indicating that a large proportion of the uronic acid side-chains were located on two contiguous D-xylose residues in the backbone of the softwood xylan.