Pectin-mucin interactions: Insights from fluorimetry, thermodynamics and dual (static and dynamic) quenching mechanisms.

Binary systems of citrus peel pectin (a major food carbohydrate) and mucin (a principal oral-gastrointestinal glycoprotein) are studied, as to understand the interactions and thermodynamics between food and biofluids during oral processing and digestion. The fluorimetry emission spectra of mucin were quenched by pectin addition at 293, 301, 310 and 318 K, indicating direct contact between the two macromolecular populations. A red shift, suggesting pectin-induced alterations on mucin conformation, has been observed at 318 K. Intensity-based Stern - Volmer plots fitted second-order polynomial equations, suggesting the coexistence of both static and dynamic quenching, while the increase of the slopes with temperature points to the predominance of dynamic phenomena. Time-resolved fluorescence measurements also point to dynamic quenching related to transient interactions, rather than to specific bonding. Thermodynamic analysis yields negative free energy changes in all cases, with positive changes for enthalpy and large positive values for TΔS. These are in agreement with the Stern - Volmer analysis, suggesting the predominance of transient, dynamic (here entropic) interactions. These provide an image of mucin interacting with pectin macromolecules during the oral processing and digestion of foods, and can relate to the texture, flavor (e.g. astringency) and bioavailability of polysaccharide-based foods.