Adsorption and displacement of a globular protein on hydrophilic and hydrophobic surfaces

Abstract

The adsorption of the globular protein β-lactoglobulin on hydrophilic and hydrophobic surfaces has been investigated. Spectroscopic ellipsometry and Fourier transform infrared spectroscopy in the attenuated total reflection mode were used in our study. β-lactoglobulin adsorbed in surface concentrations less than or equal to those corresponding to a closely packed monolayer of molecules. The amount of β-lactoglobulin removed from hydrophilic surfaces upon elution with buffer solution, as a fraction of the final amount reached upon adsorption, was observed to decrease at longer adsorption times. This provides evidence that the conformation of the adsorbed protein changes relatively slowly once adsorbed, leading to more irreversibly adsorbed states with stronger binding to the surface. The fraction of protein irreversibly adsorbed to the surface was generally higher on hydrophobic surfaces than hydrophilic surfaces, confirming the general principle that globular proteins exhibit stronger binding to hydrophobic surfaces than hydrophilic ones. The displacement of pre-adsorbed layers of the protein from hydrophobic and hydrophilic surfaces by the non-ionic surfactant octaethylene glycol monododecyl ether (C 12E 8) was also studied. We observed that the non-ionic surfactant caused partial displacement of adsorbed protein from hydrophobic surfaces. The displacement kinetics also reveal that the protein layers are more strongly bound to hydrophobic surfaces at longer than at shorter adsorption times, showing that slow protein binding changes and ageing effects are also important at hydrophobic surfaces.