Contribution of Secondary Structure Changes to the Surface Activity of Proteins

A major difference between proteins and surfactants is that proteins are capable of changing their structure during refolding processes in the adsorbed state. It is often reported that these interfacial structure changes increase the surface activity of proteins. In order to investigate this, the surface activity of 5 proteins was determined in foam fractionation experiments, where pH and ionic strength were adjusted separately to gain the maximum surface activity for each protein. Infrared Reflection Absorption Spectroscopy was performed for each protein to analyze the changes in secondary structure after adsorption. In order to investigate quick structural changes, transmission Fourier Transform Infrared spectroscopy was performed to gain information about the secondary structure of the dissolved, non-adsorbed proteins. It was found that most proteins maintain a native-like conformation when adsorbed at the interface. With increasing time, most of the proteins investigated increased the amount of β-sheets at the interface. This slow process went along with a slow increase in surface pressure. A correlation between structural changes on molecular level and surface activity on macroscopic level could not be found. However, the absolute increase of surface pressure at the very beginning of the adsorption process correlated with the surface activity of the proteins, suggesting that the slow processes on molecular level do not have a considerable impact on macroscopic surface activity.