The Effect of Solution pH on the Structural Conformation of Lysozyme Layers Adsorbed on the Surface of Water

Abstract

Neutron reflection has been used to study the effects of solution pH and ionic strength on the surface excess and layer thickness of lysozyme layers adsorbed at the air/water interface. All the measurements were made in null reflecting water (NRW) so that all the specular signal arose from the protein layers. At the low ionic strength of 0.02 M, the adsorption was found to reach a maximum at the protein isoelectric point (IP) of pH 11, with the effect of pH on the adsorbed amount and layer thickness being more pronounced at the higher lysozyme concentration. At the low lysozyme concentration of 0.03 g dm-3, the thicknesses of the adsorbed layers are 30 ± 3 Å over almost the entire pH range, close to the short axial length of the globular dimension of lysozyme, and the area per molecule is 1700 ± 200 Å2, suggesting the formation of a sideways-on monolayer. At the high lysozyme concentration of 1 g dm-3, a number of conformational transitions occur within the adsorbed layers with respect to pH and these variations correlate well with the change in the number of net charges within lysozyme with pH, suggesting that the preferred conformation of protein molecules is dominated by the combined effect of steric and electrostatic repulsion within the adsorbed layer. Subsequent measurements at the high ionic strength of 1 M showed no obvious variation in either layer thickness or surface excess with pH or with bulk protein concentration. The thickness was found to be constant at 30 ± 3 Å and the area per molecule to be 1500 ± 100 Å2, corresponding to the formation of a close-packed sideways-on monolayer. These results clearly show that salt addition has screened the charges within lysozyme molecules.