Protein exposed hydrophobicity reduces the kinetic barrier for adsorption of ovalbumin to the air-water interface

Using native and caprylated ovalbumin, the role of exposed hydrophobicity on the kinetics of protein adsorption to the air-water interface is studied. First, changes in the chemical properties of the protein upon caprylation were characterized followed by measurement of the changes in adsorption kinetics. No change in the molecular structure of ovalbumin was observed upon caprylation. However, aggregation of the protein was observed when more than three capryl chains were coupled per protein. A batch of caprylated ovalbumin with an average coupling of four capryl chains per protein was separated into a monomeric and an aggregated protein fraction. The exposed hydrophobicity of the monomeric and the aggregated species was measured using 8-anilino-1-naphthalenesulfonic acid fluorescence. The exposed hydrophobicity of the monomeric fraction was significantly higher than that of the nonmodified protein. The changes in adsorption kinetics were studied by measuring the increase in surface load (Gamma) and in surface pressure (Pi) as a function of time (t) using an ellipsometer and a Wilhelmy plate, respectively. It was found that the increase of surface load in time (even at low surface coverage) is much lower than the value that was calculated from diffusional transport. This shows that the adsorption of native ovalbumin is barrier limited. The adsorption kinetics of the caprylated protein follow the calculations from diffusional transport more closely, which shows that the energy barrier for adsorption of caprylated ovalbumin is much lower than for the native protein. The surface pressure at a certain surface load (Pi-Gamma) was not affected by the modification, indicating that the effect of increased hydrophobicity is limited to the adsorption process.