THE ROLE OF SIDE-CHAINS IN THE CR3+-INDUCED GELATION OF XANTHAN AND XYLINAN (ACETAN) VARIANTS

The effect of the length and chemical composition of the side-chains in comb-like branched polysaccharides on gelation with trivalent metal ions has been studied using xanthan and xylinan (acetan) with intact and truncated side-chains. Partial or complete removal of the terminal beta-D-mannose, or removal of up to 22% of the trisaccharide side-chains of xanthan using partial acid hydrolysis, has only small effects on the Cr3+-induced gelation. In contrast, replacement of beta-D-mannose by the trisaccharide alpha-L-Rhap-(1 --> 6)-beta-D-Glcp-(1 --> 6)-alpha-D-Glcp-(1 --> 4)- to yield the polysaccharide xylinan totally inhibits the gelation with Cr3+ ions. Removal of the trisaccharide by partial acid hydrolysis, which leads to a series of polymers with structures converging towards the partially hydrolysed xanthans, restores the gelling ability with Cr3+ ions. These observations seem to support the gelation model where Cr3+-glucuronic acid interactions are involved in the cross-linking of chains. It is further suggested that this interaction can be suppressed due to steric hindrance caused by the bulky side-chains in xylinan.