Order-disorder transition, polysaccharides

Macromolecular conformations and reversible order-disorder and disorder-order transitions are highly sensitive to solvent, temperature, pressure, pH, water activity, and metal ions. Polyanions are distinguished from neutral molecules by their sensitivity to electrolytes. Whereas synthetic polymers do not normally dissolve or disperse spontaneously, some polysaccharides may do so in water (hydration), given their strong hydrophilicity. 

According to this sequence, the monovalent cations Rb+, Cs+ and K+ induce the transition at lower temperatures. Unlike other polyelectrolytes, the disorder-order transition of K-carrageenan is also sensitive to the presence of iodide as a result of electrostatic repnilsion between the anion and the sulfate group of K-carrageenan without aggregation of double helices to form a gel network Because of this particular feature, the iodide ion has been used to study the mechanism of gelation of this polysaccharide (Slootmaekers et al., 1988 Viebke et al., 1995,1998 Meunier et al., 2001 Takemesa Nishinari, 2004). 

Order-disorder conformational transitions very often occur on changing physical and/or chemical conditions of polysaccharide solutions. DMSO, for example, is the solvent, which is commonly used as co-solvent for stabilizing or destabilizing ordered solution conformations. Schizophyllan, a triple helical polysaccharide with a [P-D-(l-3)-glc] backbone exhibits a highly cooperative order-disorder transition in aqueous solution. When small quantities of DMSO are added to aqueous solutions the ordered state is remarkably stabilized, as has been observed in the heat capacity curves by means of the DSC technique. ... 

Heparin.—Evidence for a calcium-induced co-operative conformational transition of heparin has been reported. The characteristics of the complex allow suggestions to be made regarding the backbone structure of the glycosaminoglycan. It appears to be an example of a polysaccharide undergoing a cation-induced co-operative intramolecular disorder-order process. The chiroptical properties of heparin in salt-free water solutions at various degrees of neutralization and for different counter-ions have been reported. O.r.d. and c.d. data are discussed on the basis of a randomly coiled structure for macromolecules composed... 

Hirao, T., Sato, T., Teramoto, A., Matsuo, T., and Suga, H., Solvent effects on the co-operative order-disorder transition of aqueous solutions of schizophyl-lan, a triple helical polysaccharide, Biopolymers, 29, 1867, 1990. 

Morris, E. R., Rees, D. A., Young, G., Walkinshaw, M. D., and Darke, A., Order-disorder transition for a bacterial polysaccharide in solution. A role for polysaccharide conformation in recognition between xanthomonas pathogen and its plant host, J. Mol. Biol., 110,1,1977. 

The smart response capability of polysaccharides upon application of stress allows polar and apolar domains to easily form or disintegrate due to variation of the order/disorder ratio at the molecular level. This finally results in variation of specific crystallinity index or crystalline/amorphous ratio at the macroscopic level. Significant variation of interactive properties may even be achieved by minor variation of branching characteristics, which changes surface/volume ratio and, hence, preferences for inter- or intramolecular stabilization. Additionally, a rather effective response option is variation of relative percentages of molar and mass fractions by limited degradation/ reorganization or precipitation/dissolution transition. 

Pyruvic acid has been identified in approximately half of seventy-seVen Klebsiella species investigated. On the basis of o.r.d. measurements and their interpretation from Moffitt-Yang plots, a helical superstructure is proposed for Klebsiella capsular polysaccharides of serotypes Kl, K5, K6, K8, Kll, K56, and K57. Measurements of the temperature dependence of the specific optical rotation reveal, in all cases, co-operative order-disorder transitions at temperatures between 25 and 40 C. 

In general ordered conformations are promoted by favourable non-covalent interactions, inflexible secondary structure, and efficiency of packing, and inhibited by loss of conformational entropy, energy of hydration, intermolecular electrostatic repulsion, structural irregularities, and branching. The balance of these opposing drives is often delicate, and may be tipped by relatively small perturbations. For example, thermally induced order-disorder transitions, which may or may not be accompanied by a gel-sol transition, have been observed for a number of polysaccharide systems (8,13,14), and show the sharp temperature profile characteristic of a co-operative process. 

Viebke, C. (2005) Order- disorder transition ofxanthan gum. In S. Dumitriu (ed.) Polysaccharides Structural Diversity and Functional Versatility, 2nd edn. Marcel Dekker, New York, p. 459. 

Figure 2.6. The viscosity/temperature behaviour of two polysaccharides showing the order-disorder transition at T. This transition is much clearer for the heteropolysaccharide (from Ash et al, 1983).

Rees, D. A., Williamson, F.,B., Frangou, S. A., Morris, E.R. (1982). Fragmentation and Modification of r-Carrageenan and Characterisation of the Polysaccharide Order-Disorder Transition in Solution. European Journal of Biochemistry, 122, 71-79. 

---