Coacervates of lysozyme and β-casein

Abstract

Complexes are formed when positively charged lysozyme (LYZ) is mixed with negatively charged caseins. Adding b-casein (BCN) to LYZ leads to flocculation even at low addition levels. Titrating LYZ into BCN shows that complexes are formed up to a critical composition (x = [LYZ]/([LYZ] + [BCN]). The formation of these complex coacervates increases asymptotically toward the molar charge equivalent ratio (xcrit), where the size of the complexes also seems to grow asymptotically. At xcrit, insoluble precipitates of charge-neutral complexes are formed. The precipitates can be re-dispersed by adding NaCl. The value of xcrit shifts to higher values on the LYZ side with increasing salt concentration and pH. Increasing the pH, de-protonates the BCN and protonates the LYZ, and therefore, charge neutrality will shift toward the LYZ side. xcrit increases linearly from 0.2 at no salt to 0.5 at 0.5 M NaCl. It ends abruptly at a salt concentration of 0.5 M after which a clear mixed solution remains. Away from the charge equivalent ratio, it seems that the buildup of charges limits the complex size. A simple scaling law to predict the size of the complex is proposed. By assuming that surface charge density is constant or can reach only a maximum value, it follows that scattering intensity is proportional to |(1 x/xcrit)| 3 where x is the mole fraction of one protein and xcrit the value of the mole fraction at the charge equivalent ratio. Both scattering intensity and particle size seem to obey this simple assumption. For BCN–LYZ, the buildup occurs only at the LYZside in contrast to lactoferrin which forms stable complexes on either side of xcrit. The reason that the complexes are formed at the BCN side only may be due to the small size of LYZ, which induces a bending energy in the BCN on adsorption.