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.