CONFORMATIONAL AND RHEOLOGICAL TRANSITIONS OF WELAN, RHAMSAN AND ACYLATED GELLAN

Native ('high acyl') gellan adopts double helix geometry at a much hig her temperature than the deacylated polymer (commercial gellan gum), b ut the resulting gels are weaker, more elastic, and show no thermal hy steresis between formation and melting, indicating that acetyl groups, which are located on the periphery of the helix, prevent aggregation. On progressive removal of glyceryl substituents, which are located in the core of the helix and modify its geometry, the disorder-order tra nsition becomes broader (i.e. less co-operative) and moves to a lower temperature. Eventually a second transition appears at the position ch aracteristic of the fully deacylated polymer. Comparison of the relati ve magnitudes of the two transitions with the proportion of residual g lycerate indicates that conversion from 'high acyl' to 'deacetylated' geometry requires six consecutive repeating units devoid of glyceryl g roups. In welan and rhamsan, the double helix is stabilised to tempera tures above 100 degrees C by incorporation of, respectively, monosacch aride and disaccharide sidechains in the ordered structure. Both have 'weak gel' properties similar to those of xanthan. However, 'true' gel s are formed when the helix structure is dissociated and regenerated ( by dissolving welan in dimethyl sulphoxide and adding water, or by hea ting and cooling deacylated rhamsan in aqueous solution). Our interpre tation of this behaviour is that the native structures of both polymer s are perfect double helices, with exact pairing of strands along the full length of the participating chains. Dissociation of these 'perfec t' structures allows development of a crosslinked network by individua l chains forming shorter helices with more than one partner. Copyright (C) 1996 Elsevier Science Ltd