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

The two proteins, β-casein (β-CN) and β-lactoglobulin (β-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 pH 7 β-CN forms a structure that is described by two layers, a compact layer adjacent to the solid surface and a looser layer protruding into the solution. β-Lg adsorbs as a uniform layer. At lower pH both proteins adsorb more, with thicker layers, and β-Lg also adsorbs as a non-uniform layer. The adsorption of both proteins is irreversible. The merits of contrast variation are discussed and, in particular, the importance for the systems studied of the use of water of scattering length density 4.5 x 10^-6 Å^-2 is described. Owing to the large size of the proteins, this contrast, intermediate between those of D₂O and silicon, allows details masked by the higher critical angle of D₂O to be revealed.