The effect of solution pH on the structure of lysozyme layers adsorbed at the silica-water interface studied by neutron reflection

We have studied the structure of lysozyme layers adsorbed at the silica-water interface using specular neutron reflection. The effect of pH on the adsorbed lysozyme layer was examined by manipulating the pH in two different cycles at two constant lysozyme concentrations of 0.03 and 1.0 g dm-3; the first cycle was started at pH = 4 followed by pH = 7 and then 8, before returning to 4; the second cycle was started at pH = 7 followed by a decrease to 4 and then back to 7. The neutron reflectivity profiles showed no hysteresis in either adsorbed amount or structure. There was less adsorption at pH = 4 than at pH = 7 for both lysozyme bulk concentrations. No variation of the reflectivity with time was found at the experimental resolution of about 5 min per measurement. The lysozyme structure at the interface at pH = 4 and pH = 7 was determined from reflectivity profiles at different isotopic compositions of the water. The thickness of the adsorbed layer at the lower concentration of 0.03 g dm-3 was found to be 30 ± 2 Å, suggesting sideways-on adsorption of the ellipsoidally shaped protein. At the higher concentration of 1.0 g dm-3 the thickness of the layer was found to be 60 ± 2 Å, suggesting bilayer adsorption with side-on orientation in each layer. These observations disagree with literature results from surface force and ellipsometric measurements which suggest that a side-on monolayer of 30 Å thickness is formed at dilute bulk concentrations, which switches to end-on adsorption of 45 Å thickness as the bulk concentration increases, eventually reaching a bilayer of 90 Å thickness when the bulk lysozyme concentration is further increased. The neutron measurements indicate that the adsorbed amount and the orientation of the globular protein are determined by the electrostatic repulsion between the lysozyme molecules within the layer.