Arabinogalactan from Western larch. Part IV. Polymeric products of partial acid hydrolysis

Under identical conditions of acid-catalyzed hydrolysis, disordered arabinogalactan (DAG) degrades about twice as fast as ordered arabinogalactan (AG). Size-exclusion chromatography (SEC) shows that the polymeric product from DAG becomes increasingly polydisperse as hydrolysis proceeds, whereas the polymeric product from AG persists as a narrow SEC peak. Polymeric products isolated from hydrolysates of AG and DAG at different stages of degradation, like original AG, are disordered with alkali and subsequently reordered by drying, which indicates that order is maintained in the degrading polysaccharide. Conformational restraints imposed by a network of hydrogen bonds in the ordered portion of the AG multiplex are a likely cause for its slower degradation, and this interpretation is supported by results from 1H NMR. After disordering, the partially hydrolyzed AG products return to the ordered state more readily than DAG returns to AG, and this preference for order increases with extent of degradation, suggesting that prior hydrolytic removal of non-ordered portions of the AG multiplex facilitates re-establishment of order from disorder. By contrast, the partially hydrolyzed DAG products show less tendency to reorder than DAG, the more so with increasing extent of degradation. Compositional analyses of the products from AG and DAG indicate little difference in the rate of initial rapid loss of Ara residues, consistent with the view that these residues are on the periphery of the molecule and not involved with order in the multiplex. Results of linkage analyses and of alkaline degradation analyses of these products indicate that maintenance of order during hydrolysis of AG coincides with preservation of 3,6-linked main-chain Gal p residues with single, terminal Gal p residues at C-6, from which it follows that these moieties are involved in the ordered network. Part of an earlier study, which led to the conclusion that hydrolysis of AG yields two distinct polymeric fragments, was repeated, and this conclusion is critically reassessed.