Characterization of polysaccharide -surfactant interaction
The hypocholesterolemic effect of certain polysaccharides has been attributed to their ability to bind bile acids. The purpose of this study was to better understand bile acid - polysaccharide interactions by systematically characterizing the interactions between selected polysaccharides (chitosan and pectin) and anionic surfactants (sodium dodecyl sulfate (SDS) and sodium taurocholic acid (NaTCA)) using isothermal titration calorimetry (ITC), surfactant selective electrode (SSE) and turbidity measurements.
Initially, the influence of environmental conditions (pH, ionic strength and temperature) on the properties of SDS and NaTCA in buffer solutions was characterized. The CMC's (critical micelle concentrations) were largely independent of temperature and pH, but decreased appreciably as the ionic strength increased. In general, the micellization behavior of NaTCA was different from that of SDS because of their different molecular structures. The influence of environmental conditions on the interactions between SDS and NaTCA with pectin and chitosan were then studied.
SDS bound strongly to chitosan and formed insoluble complexes, which was attributed to electrostatic attraction. For SDS-chitosan interactions, temperature did not have a large affect on T1 (onset binding), T2 (surfactant concentration at polymer saturation) or CMC* (effective CMC in the presence of polymer). Strong binding only occurred at pH values where the chitosan was cationic (pH 3 and 5), but not when it was uncharged (pH 7). Salt (0 to 200 mM) decreased the CMC* because of the depression of the CMC of free SDS in solution. SDS bound weakly to pectin and formed soluble complexes, which was attributed to hydrophobic interactions.
The general characteristics of NaTCA-chitosan interactions were fairly similar to those of SDS-chitosan interactions. The binding interaction was exothermic at all temperatures studied (10 to 50°C), suggesting that it was electrostatic in origin. The T1, T2 and CMC* values were influenced by salt and pH as described for SDS. In addition, only a weak binding interaction was observed between pectin and NaTCA.
This study provides information that may lead to the rational design of polysaccharide-based food ingredients with beneficial nutritional and functional characteristics.