Adsorption dynamics and interfacial properties of α-lactalbumin in native and molten globule state conformation at air–water interface

The effect of conformation of α-lactalbumin on the dynamics of adsorption as well as on the properties of adsorbed layer at air–water interface were investigated. The surface hydrophobicity of molten globule conformation of α-lactalbumin (prepared by lowering pH to 2) was found to increase by 15-fold compared to the protein in its native form (pH 7) with increased flexibility as evidenced by a much smaller hysteresis area of spread surface pressure isotherms. This higher surface hydrophobicity resulted in (i) higher surface activity of α-lactalbumin in its molten globule conformation compared to the native form inspite of a much higher electrical energy barrier due to much higher net charge and (ii) diffusion controlled adsorption for much longer times and surface pressures than the native form. The diffusion coefficient of α-lactalbumin in molten globule state was lower than that for native form (2.22×10−10 m2 s−1 vs 4.65×10−10 m2 s−1) and increased for the former at higher ionic strength. The area per molecule during the dynamics of adsorption was found to be the same at a fixed surface pressure for both the conformations and lower than the corresponding value for fully denatured protein thus indicating that α-lactalbumin does not fully unfold at the interface in both the conformations. Even though α-lactalbumin adsorbed much more in its molten globule conformation, it gave poor interfacial rheological properties compared to the native form. Ionic strength did not influence the relationship between the interfacial elasticity and surface concentration for both the conformations.