Structure and denaturation of adsorbed lysozyme at the air–water interface

Adsorption of lysozyme at the surface of a buffer solution at 25 °C, pH 7, and ionic strength 0.1 is studied under different denaturing conditions using on X-ray reflectometry technique. When the lysozyme is fully denatured with urea and dithiothreitol (DTT), its measured adsorption profile is very well explained by the scale law ( z −4/3) profile theoretically predicted for polymer adsorption. When no denaturing agent is present, a monolayer is also produced, but the adsorption profile cannot be explained by a monolayer of nondenatured lysozyme; furthermore, it is close to the one obtained for lysozyme partially denatured with urea. A PMIRRAS study of native lysozyme adsorbed at the air–buffer interface shows that the secondary structure of the protein is modified: most of the α-helices are replaced by β-sheets. In contrast, when the lysozyme is adsorbed below a monolayer of oleic acid at the air–buffer interface, that is, on a hydrophilic interface, the protein forms a monolayer whose thickness, 3.0 nm, is equal to one dimension of crystallized lysozyme. Under such conditions, the adsorbed protein is not denatured. Thus the hydrophobic nature of the air–water interface yields partial denaturation of the protein upon adsorption, but the disulfur bridges and β-sheets prevent total denaturation.