Heteroprotein complex of soy protein isolate and lysozyme: Formation mechanism and thermodynamic characterization

The electrostatic assembly between heteroproteins is one of the most important applications of associative interaction. The present study investigated the effects of pH and NaCl on the formation rules, phase behavior, and thermodynamic changes of soy protein isolate (SPI)/lysozyme (LYS) complexes. The hydrodynamic diameter of SPI significantly increased as pH was lowered to 6.5, whereas that of LYS significantly increased at pH ≥ 7.5, as a result of self-aggregation. Electrostatic assembly between SPI and LYS completely complies with the charge compensation, producing nanoscale (about 200 nm) and microscale (about 5 μm) complexes at different mixing ratio. However, NaCl can lower ζ-potential of SPI and LYS, and weaken the electrostatic interaction between heteroproteins. The presence of 200 mM NaCl can completely inhibit the electrostatic assembly between SPI and LYS (1 mg/mL). Not only electrostatic interactions but also hydrogen bonds participated in the complex. As the observation of phase-contrast mode, the pH, stoichiometry, NaCl, and assembling time significantly affected size and shape of SPI/LYS complexes, whereas all SPI/LYS complexes exhibited solid-liquid phase separation on the time scale of 0–60 min. Isothermal titration calorimetry analysis indicated that SPI/LYS complex was thermodynamically favored (ΔG < 0), both the exotherm (ΔH < 0) and the entropy gain contributed to SPI/LYS assembling while the entropy gain might be the dominant term. Moreover, the absolute values of ΔG, ΔH, and ΔS significantly decreased with increasing NaCl concentration.