Heteroprotein complex coacervation: bovine β-lactoglobulin and lactoferrin

Lactoferrin (LF) and β-lactoglobulin (BLG), strongly basic and weakly acidic bovine milk proteins, form optically clear coacervates under highly limited conditions of pH, ionic strength I, total protein concentration C(P), and BLG:LF stoichiometry. At 1:1 weight ratio, the coacervate composition has the same stoichiometry as its supernatant, which along with DLS measurements is consistent with an average structure LF(BLG2)2. In contrast to coacervation involving polyelectrolytes here, coacervates only form at I < 20 mM. The range of pH at which coacervation occurs is similarly narrow, ca. 5.7-6.2. On the other hand, suppression of coacervation is observed at high C(P), similar to the behavior of some polyelectrolyte-colloid systems. It is proposed that the structural homogeneity of complexes versus coacervates with polyelectrolytes greatly reduces the entropy of coacervation (both chain configuration and counterion loss) so that a very precise balance of repulsive and attractive forces is required for phase separation of the coacervate equilibrium state. The liquid-liquid phase transition can however be obscured by the kinetics of BLG aggregation which can compete with coacervation by depletion of BLG.