BINDING OF SODIUM DODECYL-SULFATE TO BOVINE SERUM-ALBUMIN LAYERS ADSORBED AT THE SILICA-WATER INTERFACE

One of the main difficulties in studying the interaction of protein an d a surfactant at an interface is to establish the structural distribu tions of each component in the mixed layer. We demonstrate in this wor k that the distributions of the surfactant, protein, and water adsorbe d at the hydrophilic solid/aqueous solution interface can be separatel y and unambiguously determined by using specular neutron reflection co mbined with deuterium labeling of the surfactant and water. Adsorption of bovine serum albumin (BSA) from a 0.15 g dm(-3) solution onto a hy drophilic silicon oxide surface produced a densely packed uniform laye r with a thickness of 35 +/- 3 Angstrom. The binding of sodium dodecyl sulfate (SDS) onto this preadsorbed BSA layer was studied at a consta nt SDS concentration of 1 x 10(-4) M with different isotopic compositi ons of SDS and water. The surface excesses of BSA, SDS, and the struct ural distributions of BSA, SDS, and water were obtained by simutaneous ly fitting a single structural model to the set of measured neutron re flectivity profiles. The results show that at this SDS concentration t he structural profiles of the interfacial components are approximated well as uniform layer distributions. The binding of SDS results in an expansion of the preadsorbed BSA layer from 35 +/- 3 Angstrom in the a bsence of SDS to 50 +/- 5 Angstrom, suggesting considerable structural deformation of the protein. The weight ratio of SDS to BSA in the mix ed layer was found to be 0.43, in close agreement with the literature value for the binding of SDS onto denatured protein in the bulk, sugge sting that the protein in the adsorbed complex is also denatured.