Arabinogalactan from Western larch, Part III: alkaline degradation revisited, with novel conclusions on molecular structure

Abstract

Alkaline degradation of larch arabinogalactan (AG) involves rapid peeling of the (1→3)-galactan main chain from the reducing end. The products are the original side chains attached to galactometasaccharinic (GalMS) acids derived from main-chain residues. These products have been separated by GPC and studied by compositional, methylation and NMR analyses. Results confirm that in a typical AG molecule most main-chain residues carry a side chain on C-6. About half of these side chains are β-(1→6)-linked Gal p dimers, and about a quarter are single Gal p residues. The rest contain three or more residues and include most of the Ara (arabinose) found in the polysaccharide. About one-fifth of the original Ara is consumed by the peeling reaction, and Ara groups at the non-reducing end of the main chain are proposed, predominantly as arabinobiosyl groups [β- l-Ara p-(1→3)-L-Ara f-(1→.. In general for the larger side chains abundance decreases with size, while branching and Ara content increase with size. Most terminal Ara f residues occur in three- and four-residue side chains, while most arabinobiosyl groups are found in side chains of more than three residues. The Ara probably occurs only as these monomeric or dimeric groups, and since no Ara is found in side chains smaller than three residues, this implies that there are no Ara branches attached directly to the main chain.