Screening for synergistic interactions in dilute polysaccharide solutions

Abstract

A simple viscometric approach has been used to screen for binding interactions between different polysaccharides in very dilute solution where exclusion effects should be negligible. The method involves preparing stock solutions to approximately the same, low, viscosity (ηsp ≈ l), dialysing to identical ionic conditions, mixing in various proportions, and looking ror departures from the initial common viscosity. Mixtures of xanthan or de-acetylated xanthan with locust bean gum (LBG) or konjac glucomannan (KM) show massive enhancement of viscosity, as anticipated from the formation of synergistic gels at higher concentrations. However, no viscosity changes on mixing with LBG or KM were observed for other conformationally ordered bacterial polysaccharides (welan and rhamsan) or for alginate and pectin with sufficient Ca2+ to induce almost complete conversion to the dimeric ‘egg box’ form, demonstrating that conformational rigidity is not, in itself, sufficient for other polysaccharides to form heterotypic junctions with mannan or glucomannan chains. Interactions of carrageenans with LBG appear to depend on both conformation and the extent of aggregation. Mixtures of LBG with K+ kappa carrageenan in 100 mM KCl (which is known to promote extensive aggregation of double helices) gave erratic values for rotational viscosity and showed typical gel-like mechanical spectra under low-amplitude oscillation. Disordered carrageenans (K+ kappa in water and lambda in 100 mM KC1) showed no evidence of interaction with LBG. Negative results were also obtained for iota carrageenan under ionic conditions believed to promote ordering without significant aggregation (100 mM KCl). However, under conditions where limited aggregation might be expected (iota carrageenan in 90 mM CaC12; Me4N+ kappa carrageenan in 150 mM Me4NI), significant reductions in viscosity were observed on mixing with LBG, which may indicate some intermolecular association but without the formation of an extended network structure.