Molecular recognition processes at functionalized lipid surfaces - a neutron reflectivity study

The specific binding of proteins to functionalized monolayers on aqueous subphases has been characterized by neutron reflectivity measurements. As a model for the investigation of a recognition process on a molecular length scale, streptavidin and biotin were used because of the high specific affinity between them. Reflectivities from the aqueous surfaces covered with biotinylated monolayers before and after the adsorption of proteins were collected with a novel fixed wavelength liquid surface neutron reflectometer. In quantitative terms, binding was found to occur at a biotin surface concentration as low as 1 unit/1250 Å2. The data are consistent with the formation of a monomolecular protein layer at the interface, directly underneath the lipid monolayer. The thickness of the protein film is 44 ± 2 Å, comparable in size to the smallest axis of the unit cell of three-dimensional streptavidin crystals (48 Å). It is found that the composition of the functionalized surface monolayer critically influences the two-dimensional aggregration behavior of the protein underneath.