STOICHIOMETRY AND CONFORMATION OF XANTHAN IN SYNERGISTIC GELATION WITH LOCUST BEAN GUM OR KONJAC GLUCOMANNAN - EVIDENCE FOR HETEROTYPIC BINDING

Synergistic gels of xanthan or deacetylated xanthan (DX) with locust b ean gum (LEG) or konjac glucomannan (KM) melt and set at similar to 60 degrees C, with no thermal hysteresis, Gelation occurs with the xanth an component in either its ordered or its disordered form and, with KM as cosynergist, is accompanied by large enthalpy changes (Delta H) in DSC. Gel modulus (G') and Delta H increase linearly with increasing r atio of KM:DX up to similar to 1:1, with little further change at high er ratios. Liquid-like character (tan delta) passes through a sharp mi nimum at about the same composition. Mixed gels of KM with unmodified xanthan show similar behavior, but the maximum value of Delta H is low er, and the proportion of KM required to achieve this maximum is highe r. The heat changes (per gram of xanthan or DX) depend only on mixing ratio, not on total concentration, arguing strongly for stoichiometric binding rather than an exclusion mechanism. With LEG in place of KM, the sol-gel transition is much wider and gives no discernible peaks in DSC. The minimum in tan delta with varying composition, however, is s till evident, again arguing for a binding process, and the moduli are higher (similar to 3x). Gels incorporating KM show evidence of structu ral rearrangement after their initial formation (maxima in the tempera ture dependence of G''; shoulders in tan delta and in DSC); no such ef fects are seen for LEG. In the light of previous X-ray diffraction stu dies in the condensed phase, it is suggested that initial gelation inv olves heterotypic junctions between xanthan or DX and KM or LEG, with both components in a 2(1) conformation, but that junctions involving K M convert to a more compact 6-fold arrangement at lower temperature.