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

Abstract

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 ( π i) 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.