Unit structure of the anti-complementary arabinogalactan from Angelica acutiloba Kitagawa

Mild acid hydrolysis of an anti-complementary arabinogalactan (AGIIb-1), isolated from the roots of Angelica acutiloba Kitagawa, gave one neutral (N-I) and two acidic arabinogalactans (A-I and A-II) and one neutral arabinan (N-II). Likewise, the product (AF-AGIIb-1) of digestion with exo-alpha-L-arabinofuranosidase gave four carbohydrate units. Methylation analysis showed that N-I was a (1----6)-linked galactan with unbranched short side-chains of Araf attached at position 3 and that A-I and A-II contained, in addition, 4-linked Galp. Methylation analysis and oligosaccharide analysis showed that A-I and A-II also contained highly branched Ara chains possessing Araf side-chains attached at positions 3 of some 4- or 5-linked Ara and that a small proportion of Arap was present in each acidic unit. Base-catalysed beta-elimination and oligosaccharide analysis indicated that A-1 and A-II also contained a rhamnogalacturonan moiety in which 2,4-disubstituted Rha residues were attached to 4-substituted GalpA through position 2 of Rha. Methylation analysis, 1H- and 13C-n.m.r. studies, and enzymic hydrolysis showed N-II to be a highly branched arabinan containing a backbone of (1----5)-linked alpha-L-Araf with alpha-L-Araf side-chains attached to positions 3.