Rheological and microstructural properties of heteroprotein complex coacervates formed by lactoferrin and osteopontin

Lactoferrin (LF) and osteopontin (OPN) form heteroprotein complex coacervates (HPCCs) within limited conditions of pH, ionic strength, total protein concentration (TPC), and protein stoichiometry. In this study, it was determined using dynamic oscillatory rheology, that the coacervates exhibit frequency-dependent, viscoelastic behaviour whereby the storage modulus (G′) was less than the loss modulus (G″) for all frequencies studied (≤100 rad/s). By reducing the TPC and/or ionic strength of the system, an increase in the protein concentration and strength of the coacervate was achieved. The G′ and G″ of the coacervates were largely independent of temperature <50 °C, but above this temperature, significant changes occurred due to protein denaturation. Using microscopic techniques, it was also determined that both proteins co-localise throughout the coacervate phase in a highly ordered, structural network of protein and water. The resulting freeze-dried coacervate powder particles had a honeycomb-like appearance due to an intricate network of porous channels formed during the drying process. This physicochemical characterisation of LF-OPN HPCCs has increased scientific understanding of their rheological and microstructural properties which can now be understood, predicted, and controlled for subsequent biochemical analysis.