Composition of β-lactoglobulin/lactoferrin heteroprotein coacervates

A complex coacervate is a co-assembled soft material with numerous potential practical applications in food and non-food sectors. It results from the interaction of oppositely charged polyelectrolytes in solution leading to a liquid-liquid phase separation. Examples of complex coacervation between oppositely charged polyelectrolytes is numerous but the complex coacervation involving two oppositely charged proteins (heteroprotein coacervation) was only described recently and exhibits some specificities: it is observed in a narrow range of pH, ionic strength, protein concentration and stoichiometry1. A better knowledge of the mechanism of formation and stabilization of such system is essential to design material with optimized technological properties. In most heteroprotein coacervates studied up to now, the protein molar ratio in the coacervate was constant in the whole domain of formation of the coacervate and it was suggested that it corresponds to the molar ratio of an identified heteroprotein building unit. Surprisingly, in the case of -lactoglobulin( -LG)/lactoferrin(LF) system, the molar ratio in -LG/LF coacervates increased from 4 to almost 8 when the -LG/LF molar ratio in the mixing solution increased suggesting the coexistence of different building units with different -LG/LF molar ratio. By combining protein quantification, fluorescence recovery after photobleaching and solid NMR measurements in the coacervates and docking simulation we suggest that -LG/LF coacervates are formed from 3 different building units with different -LG/LF molar ratio. Change in the proportion of these building units in the coacervates give a plausible explanation of the observed change in -LG/LF molar ratio in the coacervate when changing protein molar ratio in the mixing solution.