SCREENING FOR SYNERGISTIC INTERACTIONS IN DILUTE POLYSACCHARIDE SOLUTIONS

A simple viscometric approach has been used to screen for binding inte ractions between different polysaccharides in very dilute solution whe re exclusion effects should be negligible. The method involves prepari ng stock solutions to approximately the same, low, viscosity (eta(sp)a pproximate to 1), dialysing to identical ionic conditions, mixing in v arious proportions, and looking for departures from the initial common viscosity. Mixtures of xanthan or de-acetylated xanthan with locust b ean gum (LEG) or konjac glucomannan (KM) show massive enhancement of v iscosity, as anticipated from the formation of synergistic gels at hig her concentrations. However, no viscosity changes on mixing with LEG o r KM were observed for other conformationally ordered bacterial polysa ccharides (welan and rhamsan) or for alginate and pectin with sufficie nt Ca2+ to induce almost complete conversion to the dimeric 'egg box' form, demonstrating that conformational rigidity is not, in itself, su fficient for other polysaccharides to form heterotypic junctions with mannan or glucomannan chains. Interactions of carrageenans with LEG ap pear to depend on both conformation and the extent of aggregation Mixt ures of LEG with K+ kappa carrageenan in 100 mM KCI (which is known to promote extensive aggregation of double helices) gave erratic values for rotational viscosity and showed typical gel-like mechanical spectr a under low-amplitude oscillation. Disordered carrageenans (K+ kappa i n water and lambda in 100 mM KCI) showed no evidence of interaction wi th LEG. Negative results were also obtained for iota carrageenan under ionic conditions believed to promote ordering without significant agg regation (100 mM KCI). However, under conditions where limited aggrega tion might be expected (iota carrageenan in 90 mM CaCl2; Me(4)N(+) kap pa carrageenan in 150 mM Me(4)NI), significant reductions in viscosity were observed on mixing with LEG, which may indicate some intermolecu lar association but without the formation of an extended network struc ture. Copyright (C) 1996 Elsevier Science Ltd.