Tertiary and quaternary structure in aqueous polysaccharide systems which model cell wall cohesion: Reversible changes in conformation and association of agarose, carrageenan and galactomannans

Certain polysaccharides which form gels by a mechanism that is known or suspected to involve the cross-linking of molecular chains in multiple helix formation have been subjected to selective chain-cleavage by Smith degradation at helix-terminating residues. The products retain the ability to convert to helix but do not now form a network and gel properties are thus abolished. Addition of certain galactomannans to such fragmented products from agarose and κ-carrageenan can evidently re-establish the network because characteristic bulk properties re-appear. Similar interactions can be demonstrated with the native (non-fragmented) polysaccharides. These systems have been studied by optical rotation and its temperature dependence, by interactions with dye using ultraviolet and circular dichroism spectroscopy, and by methods based on chemical fractionation and analysis. It is shown that ordered binding can occur between the agarose or carrageenan helix and parts of the galactomannan backbone that contain contiguous unsubstituted mannose residues. This association of unlike polysaccharide entities is regarded as “polysaccharide quaternary structure” and its influence can illustrate “ligand induction of polysaccharide tertiary structure.” Stereochemical analogies between the galactomannan backbone and polysaccharides such as cellulose and peptidoglycan, suggest that the binding might mimic biological cohesion between skeletal and gel phases of natural cell walls.