Distribution of uronate residues in alginate chains in relation to alginate gelling properties — 2: Enrichment of β-d-mannuronic acid and depletion of α-l-guluronic acid in sol fraction

Abstract Alginate, a copolymer comprised of β- d -mannuronic and α- 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°C, 24°C and 50°C from Ca-, and Sr-alginate gel beads were found to be depleted in α- l -guluronic acid and enriched in β- d -mannuronic acid compared to the starting material. For Sr gels, leached material is found to be almost devoid of α- 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 α- l -guluronic acid residues are required to form stable junction zones for Sr- and Ca-induced gelation respectively.