X-Ray diffraction of fibres prepared from oriented gels has become a standard method for producing molecular models for the association between like polysaccharides within the junction zones of gels l. Although the preparation of the fibres requires that the gels are stretched, and at least partially dehydrated, studies of single polysaccharide systems have shown, in all but one instance, that the method provides reliable models for the ordered polysaccharide structure present in the hydrated state'. Recently, this technique has been applied to study binary polysaccharide gels in order to investigate proposed intermolecular binding between different polysaccharides -* '. Such studies of xanthan-galactomannan gels2y3 have provided evidence for xanthan-galactomannan binding in the binary systems xanthan-carob and xanthan-tara.
The primary structure of xanthan is a (l-4)-linked P-o-glucan backbone (cellulose) substituted through O-3 on alternate glucosyl residues with a charged trisaccharide side-chains,9. The side chains are considered to modify the normal backbone geometry, leading to a helical structure with five-fold symmetrylOJ1. Galactomannans consist of a (1+4)-linked /3-o-mannan backbone incompletely substituted at O-6 by galactosyl side-chains l*. Early workers attributed gelation to an intermolecular binding between the xanthan helix and unsubstituted regions of the galactomannan backbone 12-14. Recent mixing experiments2T3 suggest that intermolecular binding (as revealed by X-ray diffraction studies) and gelation occurs only if the two polymers are mixed under conditions which denature the xanthan helix. Such studies have led to the proposal 2,3 that intermolecular binding involves co-crystallisation of sections of the denatured xanthan molecule with segments of the galactomannan chains. Since D-glucose and D-mannose differ only in the orientation of HO-2, an interaction between the cellulosic and mannan backbones provides a stereochemically acceptable basis for xanthan-galactomannan binding. On the basis of the above model, it might be expected that xanthan would gel upon admixture with other polysaccharides whose structures are stereochemically