Isothermal titration calorimetry (ITC) was used to determine the binding constant, stoichiometry, enthalpy, and entropy of beta-lactoglobulin/low- and high-methoxyl pectin (beta-lg-LM- and HM-pectin) complexes at 22 degrees C and at pH 4. The binding isotherms revealed the formation of soluble intrapolymer complexes (C1) further followed by their aggregation in interpolymer complexes (C2). The interaction between beta-lg and LM- or HM-pectin in C1 and C2 occurred spontaneously with a Gibbs free energy around -10 kcal/mol. The C1 were enthalpically driven, whereas enthalpic and entropic factors were involved in the C2 formation. Because ITC did not allow the dissociation of different enthalpic contributions, the values measured as pectin and beta-lg interacted could partially be attributed to conformational changes. The C1 had a binding stoichiometry of 8.3 and 6.1 beta-lg molecules complexed per LM- or HM-pectin molecule, respectively. The C2 had about 16.5 and 15.1 beta-lg molecules complexed per LM- and HM-pectin, respectively.

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Biopolymer interactions have many potential applications in pharmaceutical, cosmetic, nutraceutical, and functional food industries. Attractive interactions between proteins and polysaccharides can lead to the formation of complexes. Binding parameters of beta-lactoglobulin (beta-lg)/pectin complexes were determined using frontal analysis continuous capillary electrophoresis and the overlapping binding site model. At pH 4, approximately 23 beta-lg molecules were cooperatively complexed on low-methoxyl pectin, where each beta-lg molecule covered an average of 12 galacturonic acid residues. The calculated binding constant was 1431 M(-1). The interactions between pectin and four selected peptides located on the outer surface of the beta-lg were investigated in order to identify which part of the protein was likely to interact with the pectin. The peptide beta-lg 132-148, which corresponds to the alpha-helix zone, and the peptides beta-lg 76-83, 41-60, and 1-14 would be involved in the interaction with the pectin.

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