Dynamics of penetration of dipalmitoyl-phosphatidyl-choline (DPPC) monolayers by β-casein

In this work we have analyzed the dynamics of the penetration of β-casein into dipalmitoyl-phosphatidyl-choline (DPPC) monolayers and the surface rheological characteristics (under dilatation and shear conditions) of mixed films formed by DPPC penetrated by β-casein. The penetration of β-casein into a DPPC monolayer was followed by measurements of the surface pressure (π) increase at a constant surface area (A) after the injection of β-casein into the aqueous phase, underneath the DPPC monolayer previously spread at the air-water interface, at different initial surface pressures (πi) and at pH 5, 7, and 9. The evolution of surface pressure with time corresponds to an adsorption mechanism with three kinetic steps: (i) penetration of β-casein from underneath the DPPC monolayer, (ii) accumulation of the protein at the interface, and (iii) progressive penetration of β-casein into the DPPC monolayer. The results reflect the fact that β-casein-DPPC electrostatic interactions and the DPPC monolayer structure have an effect on the incorporation of β-casein into DPPC monolayers at the air-water interface. The penetration of β-casein is facilitated at acidic or neutral pH, but is hindered at basic pH. The surface viscoelastic properties under dilatational (using a ring trough with an isotropic area deformation) and shear (using an automatic Du Noüy ring oscillatory apparatus) conditions of a DPPC monolayer penetrated by β-casein are complementary and also demonstrated the complexity of the penetration mechanisms, due to the effect of electrostatic interactions between DPPC and β-casein as a function of the aqueous phase pH.