Adsorption of proteins from aqueous solutions on hydrophobic surfaces studied by neutron reflection

The two proteins, beta -casein (beta -CN) and beta -lactoglobulin (beta -Lg ), were adsorbed on hydrophobic silicon substrates from buffer solutions at pH 8, 7, 5 and 3. The structure, in terms of thickness and composition of the adsorbed species, was determined by means of neutron reflectivity. At p H 7 beta -CN forms a structure that is described by two layers, a compact l ayer adjacent to the solid surface and a looser layer protruding into the s olution. beta -Lg adsorbs as a uniform layer. At lower pH both proteins ads orb more, with thicker layers, and beta -Lg also adsorbs as a non-uniform l ayer. The adsorption of both proteins is irreversible. The merits of contra st variation are discussed and, in particular, the importance for the syste ms studied of the use of water of scattering length density 4.5x10(-6) Angs trom (-2) is described. Owing to the large size of the proteins, this contr ast, intermediate between those of D2O and silicon, allows details masked b y the higher critical angle of D2O to be revealed.