DISTRIBUTION OF URONATE RESIDUES IN ALGINATE CHAINS IN RELATION TO ALGINATE GELLING PROPERTIES .2. ENRICHMENT OF BETA-D-MANNURONIC ACID AND DEPLETION OF ALPHA-L-GULURONIC ACID IN SOL FRACTION

Alginate, a copolymer comprised of beta-D-mannuronic and alpha-L-guluronic acid isolated from seaweeds and extruded by certain bacteria, forms a gel in combination with several divalent cations. Polymer chains leaching out at 7-degrees-C, 24-degrees-C and 50-degrees-C from Ca-, and Sr-alginate gel beads were found to be depleted in alpha-L-guluronic acid and enriched in beta-D-mannuronic acid compared to the starting material. For Sr gels. leached material is found to be almost devoid of alpha-L-guluronic acid triplets for the Sr gels, and a significant reduction in such triplets compared to the starting material was observed for the polymers leached from the Ca-gels. The leached material was also found to comprise the low-molecular-weight tail of the starting material molecular weight distribution, the truncation towards lower molecular weight was larger for the more block-like alginate. The chemical composition of the leaching out from Sr-gels and Ca-gels suggests that 3 and 8 +/- 2 contiguous alpha-L-guluronic acid residues are required to form stable junction zones for Sr- and Ca-induced gelation respectively.