Effect of Structural Features of Pectin on Its Complexation with Lysozyme

The interaction between lysozyme (Lys) and pectins with different net and local charges and with block (BD) and random (RD) distributions of methoxyl groups is studied via dynamic light scattering, turbidimetry, high-resolution optical microscopy, and electrophoretic mobility measurements. RD 16.2, RD 38.2, BD 16.9, and BD 33.1 form water-soluble complexes at pH 5.1, ionic strength I = 0.01, and q 3 × 10–3. The corresponding parameters for RD 66.5 and BD 68.2 are higher: q ≅ 3 × 10–3 up to q ≅ 0.01, and q higher than 0.01. Optical microscopy shows that the complexes between Lys and BDs are large (15–20 μm) for all studied values of the degree of methoxylation (DM), while the average size of RD-based complexes is substantially lower (from 0.6 to 3 μm) and depends on the DM. The BD-based complexes have a gel-like morphology irrespective of the DM, whereas particles of RD-based complex have the form of both coacervate drops and gel-like particles, depending on the DM. The threshold value for ionic strength Iset above which Lys does not form complexes with RD and BD is found to be 0.11. That being so, the values for Iset and pHset for the BD/Lys systems are independent of the pectin DM, while for the RD/Lys systems these parameters diminish considerably with an increase in the DM. The dependence of complexation of Lys and pectin on I has a non-monotonic character with a maximum at I = 0.03 and 0.06 for RD and BD, respectively. Our understanding of the effect that the total charge of a pectin molecule and its distribution along the chain has on the complexation of pectin and a protein is important for our ability to predict the stability of the food product structure.