Arabinogalactan from Western larch .2. A reversible order-disorder transition

At the molecular level, larch arabinogalactan (AG) fragments into subunits when exposed to mild alkali or heat. This process is accompanied by a large increase in NMR spectral resolution, especially for NMR signals associated with main-chain units, and by the disappearance of H-1 NMR signals probably due to persistent hydrogen bonds. Analysis by size-exclusion chromatography (SEC) shows that smaller AG species undergo the transition in preference to larger ones, and the transition can be reversed by drying from or freezing an aqueous solution of the ''fragments''. These observations indicate that the fragmentation is a dissociation of a molecular assembly accompanied by an order-disorder transition, analogous to the known triple helix --> single-chain random coil transition of (1 --> 6)-branched (1 --> 3)-beta-glucans. The small amount of uronic acid units in AG and the consequent ion-exclusion effect in SEC provide a probe to study this transition, and results provide evidence in support of the concept that the transition involves dissociation of a tripler.