Complex equilibria, speciation, and heteroprotein coacervation of lactoferrin and β-lactoglobulin

There has been a resurgence of interest in complex coacervation, a form of liquid-liquid phase separation (LLPS) in systems of oppositely charged macroions, but very few reports describe the somewhat anomalous coacervation between acidic and basic proteins, which occurs under very narrow ranges of conditions. We sought to identify the roles of equilibrium interprotein complexes during the coacervation of β-lactoglobulin dimer (BLG2) with lactoferrin (LF) and found that this LLPS arises specifically from LF(BLG2)2. We followed the progress of complexation and coacervation as a function of r, the LF/BLG molar ratio, using turbidity to monitor the degree of coacervation and proton release and dynamic light scattering (DLS) to assess the stoichiometry and abundance of complexes. Isothermal titration calorimetry (ITC) showed that initial complex formation is endothermic, but a large exotherm related to coacervate formation obscured other regions. On the basis of turbidimetry, proton release, and DLS, we propose a speciation diagram that presents the abundance of various complexes as a function of r. Although multiple species could be simultaneously present, distinct regions could be identified corresponding to equilibria among particular protein pairs.