Polysaccharide gels helix forms

Using computer modeling, jointly with x-ray fiber diffraction data, the molecular architectures of two different gel-forming polysaccharides have been examined. Preliminary results indicate that the neutral and doubly branched capsular polysaccharide from Rhizobium trifolii can form a 2-fold single helix of pitch 1.96 nm or a half-staggered, 4-fold doublehelix of pitch 3.92 nm. The molecules are likely to be stabilized by main chain — side chain interactions. 

Rees et al. (25) have extensively examined the solution conformation and interactions of a number of polysaccharides, especially carrageenan fractions. Kappa-carrageenan is unusual as it forms gels when a solution of its potassium salt is cooled. Rees conceives that double helix formation occurs in solution and, interlocking helices develop in the gel state. 

Curdlan is a bacterial polysaccharide made by Agrobacterium biovar [87,88,89]. It is a linear (1 3)- -glucan (MW 73,000) that forms a triple helix. Curdlan is insoluble in cold water. When aqueous dispersions of curdlan are heated, two types of gels form. First curdlan dissolves. When the solution reaches 55-66 °C, then is cooled, a reversible gel forms. The gel melts when held at about 60 °C. When the thermoreversible gel is heated to a temperature above 80 °C, an irreversible gel forms. Heating to higher temperatures results in stronger irreversible gels. Transition temperatures are a function of concentration. 

Since polysaccharides have polar groups as well as some hydrophobic features, there can be inter residue interactions in a polysaccharide as well, but these attractions are expressed as interactions between different chains giving rise to such phenomena as crystallization which is prominently expressed in chitin and in cellulose. Other types of polysaccharides exhibit multistrand helix formation as in the case of sceleroglucan whose tendency for triple helix formation leads to unusual physical properties. Still other types of polysaccharides can form cross links in a more random way leading to gel formation as in the case of pectin, agarose and alginates. Chain interactions in other polysaccharides such as hyaluronan, chondroitin or... 

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