The influence of borate and calcium on the gel formation of a sulfated polysaccharide from Ulva lactuca.

The water-soluble, sulfated polysaccharide isolated from Ulva lactuca, forms a soft gel when dialysed against sea-water, provided the polysaccharide concentration is 1% or higher and the pH above 7.5. The components of sea-water needed for this gel formation are boric acid and calcium ions. At concentration normally found in sea-water, neither borate nor calcium ions alone produce a gel. It is assumed that the gel formation involves the formation of borate-polysaccharide complexes leading to intermolecular linkages which are stabilized by calcium ions in an unknown manner. This assumption is supported by experiments with extraction of this polysaccharide from the dry seaweed and by the results obtained with the isolated polysaccharide. Agreement between these experiments leads to the conclusion that the sulfated polysaccharide (in vivo) most probably occurs as a gel stabilized by the borate and calcium ions in sea-water.