Conformational and rheological transitions of welan, rhamsan and acylated gellan

摘要

Native ('high acyl') gellan adopts double helix geometry at a much higher temperature than the deacylated polymer (commercial gellan gum), but the resulting gels are weaker, more elastic, and show no thermal hysteresis 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 transition becomes broader (i.e. less co-operative) and moves to a lower temperature. Eventually a second transition appears at the position characteristic of the fully deacylated polymer. Comparison of the relative magnitudes of the two transitions with the proportion of residual glycerate indicates that conversion from 'high acyl' to 'deacetylated' geometry requires six consecutive repeating units devoid of glyceryl groups. In welan and rhamsan, the double helix is stabilised to temperatures above 100℃ by incorporation of, respectively, monosaccharide and disaccharide sidechains in the ordered structure. Both have 'weak gel' properties similar to those of xanthan. However, 'true' gels are formed when the helix structure is dissociated and regenerated (by dissolving welan in dimethyl sulphoxide and adding water, or by heating and cooling deacylated rhamsan in aqueous solution). Our interpretation of this behaviour is that the native structures of both polymers are perfect double helices, with exact pairing of strands along the full length of the participating chains. Dissociation of these 'perfect' structures allows development of a cross-linked network by individual chains forming shorter helices with more than one partner.