Structure of a Freestanding Film of β-Lactoglobulin

Stable Newton black films of β-lactoglobulin, i.e., a soluble protein, with large lateral dimensions (≈1.5 cm 2 ) have been obtained close to the isoelectric point of the protein. The structure of these freestanding films has been determined for the first time by X-ray reflectivity and consists of three layers of protein molecules. The striking shape of the electron density profile at both air/film interfaces indicates a specific organization of the protein molecules located at the air/film interfaces. However, the protein molecules included in the central core of the film are expected to retain their native structure. The effect of different factors (pH, electrolyte concentration, and protein concentration) on the film structure have been studied. The optimal solution conditions for black homogeneous film formation were defined. Moreover, the gas permeability coefficients and the contact angles of the protein films have been determined by the diminishing bubble method. A low gas permeability coefficient and high contact angles indicate a very high packing of the protein molecules within the film.