Protein folding at the air-water interface studied with x-ray reflectivity

We report the results of x-ray reflectivity measurements of thin films form ed by different water-soluble proteins at the air-aqueous solution interfac e. It is demonstrated that glucose oxidase, alcohol dehydrogenase, and urea se molecules denaturate at the air-aqueous solution interface to form 8- to 14-Angstrom-thick peptide sheets. X-ray reflectivity data indicate that th e spreading of a lipid monolayer at the aqueous solution surface before pro tein injection does not prevent proteins from unfolding. On the other hand, crosslinking of proteins results in intact enzyme lavers at the subphase s urface. A model that involves interaction of glucose oxidase molecules with a phospholipid monolayer is proposed. In this model, an observed decrease of the lipid electron density in the protein presence is explained in terms of "holes'' in the monolayer film caused by protein molecule adsorption.