Complex coacervates of lactotransferrin and β-lactoglobulin

Oppositely charged proteins should interact and form complex coacervates or precipitates at the correct mixing ratios and under defined pH conditions.

The cationic protein lactotransferrin (LF) was mixed with the anionic protein β-lactoglobulin (B-Lg) at a range of pH and mixing ratios. Complexation was monitored through turbidity and zeta potential measurements.

Complexation between LF and B-Lg did occur and complex coacervates were formed. This behaviour for globular proteins is rare. The charge ratio's of LF:B-Lg varies with pH due to changing (de) protonation of the proteins. Nevertheless we found that the complexes have a constant stoichiometry LF:B-Lg=1:3 at all pH's, due to charge regularization. At the turbidity maximum the zeta potential of complexes is close to zero, indicating charge neutrality; this is required when the complexes form a new concentrated liquid phase, as this must be electrically neutral. Complexes were formed in pH region 5-7.3. On addition of salt (NaCl) complexation is diminished and disappears at a salt concentration of about 100 mMol. The coacervate phase has a very viscous consistency. If we consider the proteins as colloidal particles then the formed complex coacervate phase may have a structure that resembles a molten salt comparable to, for example, AlCl3.