MOLECULAR-ORIGIN FOR RHEOLOGICAL CHARACTERISTICS OF XANTHAN GUM

The molecular basis for the rheological characteristics of xanthan, a polysaccharide produced by the plant pathogen Xanthomonas campestris, was studied. Possible modes of intramolecular associations in xanthan between an alternate hydroxyl group at C-3 and the adjacent hemiacetal oxygen atom of the D-glucosyl residues, and between the methyl group of the acetyl residue and the adjacent hemiacetal oxygen atom of the D-glucosyl residue, were proposed. Possible binding sites for D-mannose-specific interaction between xanthan and galactomannan have also been proposed. The interaction may take place between the hemiacetal oxygen atom of the inner D-mannose side-chain of xanthan and the hydroxyl group at C-2 of the D-mannose residue of the galactomannan molecule with hydrogen bonding. Such analogous counterparts may play a dominant role in the interaction. The univalent cation (K+) on the carboxyl group of the intermediate D-glucuronic acid in the side-chain of xanthan may also take part in the interaction by electrostatic force of attraction. The interaction between the extracellular bacterial polysaccharide and typical galactomannan components of the plant cell wall may play a part in the host-pathogen relationship, and may provide the existence of D-mannose-specific binding sites used for cell recognition by the pathogen.