STRUCTURAL CONFORMATION OF LYSOZYME LAYERS AT THE AIR WATER INTERFACESTUDIED BY NEUTRON REFLECTION/

The adsorption of chicken egg white lysozyme at the air/water interfac e has been studied by specular neutron reflection. The variation of th e total thickness of the lysozyme layer at the surface of water under varying solution conditions has been determined. The use of mixed H2O and D2O allowed the determination of the extent of immersion of the la yer in water at all concentrations. The measured layer thickness combi ned with the globular dimensions of lysozyme suggests that the adsorbe d lysozyme molecules retain their globular structure with no significa nt denaturation. Measurements were made over a lysozyme concentration range of 9 x 10(-4) g dm(-3) to 4 g dm(-3) at pH 7 and at an ionic str ength of 0.02 M. The thickness of the layer was determined by measurin g neutron reflectivities in null reflecting water (NRW) where the sign al is only from the adsorbed protein layer. Below 0.1 g dm(-3) the sur face coverage increases with bulk concentration but the thickness of t he layer is constant at 30 +/- 3 Angstrom, suggesting that lysozyme is adsorbed sideways-on. As the bulk concentration increases, the layer thickness gradually increases to a value of 47 +/- 3 Angstrom(2) at a bulk concentration of 1 g dm(-3), suggesting that the molecules switch from sideways-on to longways-on orientations. The area per molecule a t 1 g dm(-3) was found to be 950 +/- 50 Angstrom(2) which is close to the limit of 30 x 30 Angstrom(2) for a saturated layer of longways-on molecules. The extent of mixing of the layer with water was determined directly by measuring reflectivity profiles in mixed H2O and D2O. A t wo layer model was found to be appropriate with an upper layer in air and a lower layer fully immersed in water. The thickness of the layer in air was found to vary from 15 +/- 5 Angstrom at the lowest bulk con centration to 9 +/- 3 Angstrom at the highest concentration studied. T he results show that as the total layer thickness increases with bulk concentration the fraction of the layer immersed in water increases fr om 50 to 85%. At the highest concentration of 4 g dm(-3) the adsorbed layer is better described by a two layer model consisting of a close p acked top layer of thickness 47 +/- 3 Angstrom and a loosely packed su blayer of 30 +/- 3 Angstrom.