The influence of sidechains on the calculated dimensions of three related bacterial polysaccharides

Conformational and configurational features of acidic polysaccharides and their interactions with calcium ions: a molecular modeling investigation

Modeling simulations have been performed on the four regular glycuronans: alpha-D-(1--->4) polygalacturonic, alpha-L-(1--->4) polyguluronic, beta-D-(1--->4) polymannuronic, and beta-D-(1--->4) polyglucuronic acids. The goal of this study was to characterize the similarities and differences in conformational and configurational behavior as well as in calcium binding in order to progress in the understanding of the physicochemical properties of the parent polysaccharides of industrial interest, namely pectin, alginate and glucuronan. This required the evaluation of the accessible conformational space for the disaccharide subunits of the four homopolymers, using the flexible residue protocol of the MM3 molecular mechanics procedure. The results were used to access the configurational statistics of representative polysaccharide chains, as well as for the determination of the regular polysaccharide helices and their conformational transitions. The surfaces of all regular helices likely to occur for each polyuronide were explored for cation binding using the GRID procedure. Both alpha-D-(1--->4) polygalacturonate and alpha-L-(1--->4) polyguluronate chains exhibit a high specificity for calcium binding, and have well-defined chelation sites. In contrast, beta-D-(1--->4) polymannuronate and beta-D-(1--->4) polyglucuronate chains do not display any stereospecificity for calcium binding. The results gathered from molecular modeling lead to a clear understanding of the different structural features that are displayed by the four ionic polymers.

Vacuum ultraviolet circular dichroism of gellan-family polymer films from water and dimethyl sulfoxide

The denaturing effect of dimethyl sulfoxide (Me2SO) on the conformation of the gellan-welan-rhamsan family of microbial polysaccharides is directly demonstrated by circular dichroism (CD). The three polysaccharides display strikingly similar CD spectra (140-210 nm) for films cast from Me2SO. The disrupting effect of Me2SO on gellan and welan conformations has previously been reported by others on the basis of light-scattering and viscosity studies. Films cast from aqueous solutions at room temperature show more-intense CD bands, both at 182 nm, as is also observed for aqueous solutions, and in the 150-175 nm region. These features correspond to the ordered helical chains found by X-ray diffraction studies of similarly prepared films.

Influence of the method of purification on some solution properties of welan gum

The origin and some consequences of the presence of aggregates and impurities (deriving from the fermentation procedure) on the solution properties of welan gum have been studied. For this purpose, a method of purification was developed for a fermentation welan broth. It was verified that this method is ineffective for the dissociation of aggregates in the commercial product. This demonstrates the irreversible nature of the intermolecular interactions of welan samples isolated by precipitation in the presence of impurities. The purified products were characterized (Mw, [eta], kH) and the results were compared to those obtained from commercial samples of welan. The influence of the method of purification on the rheological properties was analysed.

Structure-function relationships in microbial exopolysaccharides

Sufficient well-characterized microbial exopolysaccharides are now available to permit extensive studies on the relationship between their chemical structure and their physical attributes. This is seen even in homopolysaccharides with relatively simple structures but is more marked when greater differences in structure exist, as are found in several heteropolysaccharides. The specific and sometimes unique properties have, in the case of several of these polymers, provided a range of commercial applications. The existence of "families" of structurally related polysaccharides also indicates the specific role played by certain structures and substituents; the characteristics of several of these microbial polysaccharide families will be discussed here. Thus, microbial exopolysaccharides frequently carry acyl groups which may profoundly affect their interactive properties although these groups often have relatively little effect on solution viscosity. Xanthan with or without acylation shows marked differences in synergistic gelling with plant gluco- and galacto-mannans, although the polysaccharides with different acylation patterns show similar viscosity. Similarly "gelrite" from the bacterium originally designated as Auromonas (Pseudomonas)elodea is of greater potential value after deacetylation, when it provides a valuable gelling agent, than it is as a viscosifier in the natural acylated form. The Klebsiella type 54 polysaccharide only forms gels when it, too, has been chemically deacetylated to give a structure equivalent to the Enterobacter XM6 polymer. Both these polysaccharides form gels due to the enhanced interaction with cations following deacylation and to the conformation adopted after removal of the acyl groups. Recent work in our laboratory suggests that deacetylation of certain bacterial alginates also significantly increases ion binding by these polysaccharides, making them more similar in their properties to algal alginates even although the alginates from some Pseudomonas species lack poly-L-guluronic acid sequences. The existence within families of polysaccharides of types in which monosaccharides are altered within a specific structure, or with varying side-chains, also gives an indication of the way in which such substituents affect the physical properties of the polymers in aqueous solution.

X-ray and computer modeling studies on gellan-related polymers: molecular structures of welan, S-657, and rhamsan

The primary structures of the four bacterial polysaccharides gellan, welan, S-657, and rhamsan are the same with respect to their backbones, but have different side-chains. This difference has a profound influence on their behavior in aqueous media. Solutions of gellan gum form stable aqueous gels under appropriate ionic conditions. By contrast, welan, S-657, and rhamsan do not gel but give very viscous solutions over a wide range of thermal, pH, and salt conditions. X-Ray fiber diffraction analysis and computer modeling of these branched polysaccharides demonstrate that they all have the same half-staggered, double-helical conformations as in the unbranched gellan, suggesting, therefore, that the side chains are responsible for diminishing gelling behavior. Depending on the size and location, the side chains shield the carboxylate groups to varying degrees; this shielding is substantial in welan and S-657, but less in rhamsan. In all cases, side-chain-main-chain interactions within the double helix prevent the carboxylate-mediated aggregation of double helices that is necessary for the gelation.

Polyelectrolyte and rheological studies on the polysaccharide welan

This paper investigates the polyelectrolyte behaviour of welan in aqueous solution. From conductivity and activity measurements it is demonstrated that a double-helical conformation is adopted irrespective of pH and ionic strength. The intrinsic pK of its carboxylic groups is found to be abnormally low (pKO = 2.2) and no conformational transition was observed during neutralization. The viscometric behaviour was studied as a function of degree of neutralization, polymer concentration, shear rate and ionic strength. The linear relation [eta] = f(Cs-1/2) allows the determination of B, the stiffness parameter from Smidsrød, whose value characterizes a very stiff molecule.