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Influence of the overall charge and local charge density of pectin on the complex formation between pectin and β-lactoglobulin

Sperber, Bram L.H.M., Schols, Henk A., Cohen Stuart, Martien A., Norde, Willem, Voragen, Alphons G.J.
Food hydrocolloids 2009 v.23 no.3 pp. 765-772
pectins, lactoglobulins, electrical charges, chemical reactions, physicochemical properties, biopolymers, electrical properties, light scattering, titration, pH, titratable acidity, ionic strength, electrostatic interactions
The complex formation between @b-lactoglobulin (@b-lg) and pectin is studied using pectins with different physicochemical characteristics. Pectin allows for the control of both the overall charge by degree of methyl-esterification as well as local charge density by the degree of blockiness. Varying local charge density, at equal overall charge is a parameter that is not available for synthetic polymers and is of key importance in the complex formation between oppositely charged (bio)polymers. LMP is a pectin with a high overall charge and high local charge density; HMP"B and HMP"R are pectins with a low overall charge, but a high and low local charge density, respectively. Dynamic light scattering (DLS) titrations identified pH"c, the pH where soluble complexes of @b-lg and pectin are formed and pH"@f, the pH of phase separation, both as a function of ionic strength. pH"c decreased with increasing ionic strength for all pectins and was used in a theoretical model that showed local charge density of the pectin to control the onset of complex formation. pH"@f passed through a maximum with increasing ionic strength for LMP because of shielding of repulsive interactions between @b-lg molecules bound to LMP, while attractive interactions were repressed at higher ionic strength. Potentiometric titrations of homo-molecular solutions and mixtures of @b-lg and pectin showed charge regulation in @b-lg-pectin complexes. Around pH 5.5-5.0 the pK"as of @b-lg ionic groups are increased to induce positive charge on the @b-lg molecule; around pH 4.5-3.5 the pK"a values of the pectin ionic groups are lowered to retain negative charge on the pectin. Since pectins with high local charge density form complexes with @b-lg at higher ionic strength than pectins with low local charge density, pectin with a high local charge density is preferred in food systems where complex formation between protein and pectin is desired.