STRUCTURE

The monosaccharide building units of gellan gum are glucose, glucuronic acid and rhamnose in the molar ratios of 2:1:1. The primary structure, a tetrasaccharide repeating unit is shown in Fig.1. In the native form of the polysaccharide, there are approximately one and a half O-acyl groups per repeating unit. Originally the O-acyl substituent was thought to be O-acetyl, resulting in the various forms of gellan gum being referred to as high-and low-acetyl, and so on. Recent studies by Kuo et al. suggest that gellan gum contains both O-acetyl and O-L-glyceryl substituents on the 3-linked glucose unit, the former tentatively assigned to the 6-position and the latter to the 2-position. Analysis of samples in our laboratory following this work has indicated that glycerate substitution predominates over acetate. Undoubtedly, these bulky glycerate groups hinder chain association and account for the significant changes in gel texture that accompany deacylation. Inclusion of bulkier substituents on the gellan gum backbone has an even more dramatic impact on properties. For example, welan gum and rhamsan gum, which have the same backbone as gellan gum but are substituted by mono- and disaccharide side-chains, respectively, have no similarity to gellan gum in solution behavior.
The shape or conformation adopted by the gellan gum molecule as a result of this primary structure has been under investigation for a number of years using X-ray crystallography. The quality of early diffraction patterns was not adequate to permit a detailed structural analysis. Subsequent work at Bristol, although resulting in high-quality diffraction patterns from well-oriented polycrystalline samples, also failed to produce a structure consistent with the X-ray data. A recent re-examination of the Bristol data has indicated that gellan gum forms an extended intertwined, three-fold, left-handed parallel double helix. Molecular shape in the solid state is usually an indicator of molecules associate in solution. The mechanism whereby gellan gum molecules associate in solution is believed to involve ion-mediated aggregation of double helices.

 Fig. 1. High Acyl Gellan gum tetrasaccharide repeat unit