THE ADSORPTION OF LYSOZYME AT THE SILICA-WATER INTERFACE - A NEUTRON REFLECTION STUDY

The adsorption of lysozyme (chicken egg white) from aqueous solution o n to the hydrophilic silica surface and the variation of interfacial s tructure with solution conditions have been studied by neutron reflect ion. The accurate determination of the adsorbed layer thicknesses in c ombination with the dimension of the globular structure of lysozyme al lows us to postulate the mean structural conformation of the lysozyme molecules within the adsorbed layer. It was found that the adsorption was completely reproducible with respect to lysozyme concentration, bu t it was irreversible. The effect of ionic strength on the adsorption of lysozyme was examined at pH 7 and at a bulk lysozyme concentration of 0.03 g dm(-3). The adsorbed layer was not affected by changes in io nic strength when the total ionic strength was below 0.05 M, but above this concentration addition of NaCl gradually reduced the amount of l ysozyme adsorbed. Complete removal of adsorbed lysozyme was achieved w hen the total ionic strength was above 0.5 M. The effect of solution p H on the amount of lysozyme adsorbed was characterized by varying the pH in cycles at fixed lysozyme concentrations. Adsorption was found to be completely reversible with respect to pH over a wide protein conce ntration range. The level of surface excess was dominated by the elect rostatic repulsion between lysozyme molecules within the adsorbed laye rs, rather than the attraction between the surface and lysozyme. The l ysozyme layer structure along the surface normal was characterized by varying the isotopic composition of the water. At pH 7 a monolayer 30 +/- 2 Angstrom thick was formed when the lysozyme concentration was be low 0.03 g dm-3, indicating that the lysozyme was adsorbed with its lo ng axis parallel to the surface (sideways-on). At higher concentration s the thickness of the layer changed to 60 +/- 2 Angstrom, suggesting the formation of a bilayer of lysozyme molecules in the sideways-on co nfiguration. When the lysozyme concentration is above 1 g dm-3 the sur face excess within the inner layer is sufficiently high that repulsion within the adsorbed layer becomes significant and the molecules start to tilt towards longways-on adsorption. At pH 4, the electrostatic re pulsion between the adsorbed molecules is stronger than at pH 7, resul ting in a lower surface excess and a tilting away from the sideways-on configuration at lower surface concentration. (C) 1998 Academic Press .