Studying the adsorption of protein at the oil-water interface

Objective. To characterize the oil-water interfacial adsorption of folded and unfolded protein using rheology and pendant drop techniques. Methods. Phosphate buffer pH 7 (ionic strength 0.05M) was used as the water phase and Miglyol 812 (coconut oil; medium chain triglycerol) as the oil phase. Bovine serum albumin (BSA) dissolved in the water phase (0.8 mM and 0.4 mM) was mixed with 0-50% (v/v) solutions of thermally unfolded BSA (0.15 mM) and interfacial properties were determined. Oscillatory shear measurements, using a rheometer with double-wall-ring geometry, were conducted at 0.1 Hz and strain of 0.1% (within the linear viscoelastic regime). Interfacial tension measurements were performed using an aqueous drop volume of 50 μL (needle diameter 1.83 mm) which was lowered into the oil phase. Results. BSA formed a viscoelastic film at the oil-water interface. The presence of thermally unfolded protein in the bulk water phase delayed the interfacial film formation but giving similar elastic and viscose moduli after 1h. After ten minutes, the surface tension for the folded protein alone [7.63±0.25 (mN/m) (0.4 mM) and 9.81±0.29 (mN/m) (0.2 mM)] was higher than for the folded protein in the presence of the unfolded protein [7.20±0.04 (mN/m) (0.4 mM) and 8.49±0.20 (mN/m) (0.2 mM)]. The magnitude and time course of the observed differences in rheological properties and interfacial tension suggest that the interfacial behaviour between BSA solution and the oil phase was not governed by the folded protein concentration alone. In fact, the presence of unfolded protein in the solution also affects the properties of the viscoelastic film. Conclusion. The combination of rheology and pendant drop techniques was shown to be useful for the characterization of the physical behavior of protein at the oil-water interface.