Structural characterisation of an anti-complementary arabinogalactan from the roots of Angelica acutiloba Kitagawa.

An anti-complementary arabinogalactan (AGIIb-1), isolated from the roots of Angelica acutiloba Kitagawa, has been subjected to methylation analysis, digestion with alpha-L-arabinofuranosidase, controlled Smith-degradation, and partial acid hydrolysis. AGIIb-1 consisted of arabinose, galactose, rhamnose, galacturonic acid, and glucuronic acid in the molar ratios 1.8-2.2:1.0:0.2-0.3:0.2-0.4:0.1. AGIIb-1 contained mainly an arabino-3,6-galactan moiety, and most of the Ara was present as alpha-L-arabinofuranosyl residues in the non-reducing terminals and the highly polymerised and branched side-chains which were attached mainly to positions 3 and 6 of (1----6)- and (1----3)-linked Gal, respectively. Some Ara-containing chains were also attached to (1----4)-linked Gal residues. The 13C-n.m.r. data for AGIIb-1 showed that the Galp was beta. Mild acid hydrolysis of AGIIb-1 yielded several linear and highly branched arabino-oligosaccharides, a neutral arabinogalactan, and two acidic arabinogalactans. Some arabino-oligosaccharides contained a (1----4)-linked Arap at the reducing terminal. The neutral arabinogalactan contained (1----3)-, (1----4)-, and (1----6)-linked and 3,6-di-O-substituted Gal, whereas the acidic arabinogalactans contained, in addition, non-reducing terminal GlcA, (1----4)-linked GalA, and 2,4-di-O-substituted Rha. The anti-complementary activity was decreased when AGIIb-1 was partially hydrolysed with mild acid (10mM HCl, 100 degrees, 10 min), but treatment with exo-alpha-L-arabinofuranosidase markedly enhanced the activity.