INTERACTION BETWEEN ADSORBED LAYERS OF LYSOZYME STUDIED WITH THE SURFACE FORCE TECHNIQUE

The surface force technique was employed to investigate the adsorption of positively charged lysozyme onto negatively charged mica surfaces in 10(-3)M NaCl at pH 5.6 at lysozyme concentrations ranging from 0.00 2 to 0.2 mg/mL. At equilibrium the adsorbed lysozyme nearly neutralize s the surface charge of the mica at all bulk lysozyme concentrations i nvestigated. Prior to charge neutralization the decay length of the lo ng-range force is consistent with the electrostatic double-layer force predicted by the DLVO theory. At low concentration, 0.002 mg/mL, a de nsely packed side-on oriented layer adsorbs on the mica surface. Above 0.02 mg of lysozyme/mL, a rather thick layer is adsorbed onto the sur face. It consists of an inner, strongly bound layer of both side-on an d end-on adsorbed proteins and an outer layer of weakly adsorbed prote ins. An adhesion force is established upon contact of the adsorbed pro tein layers. The force measured between one lysozyme-coated surface an d one bare mica surface is attractive at short separations. It was dem onstrated that, at a concentration of 0.02 mg/mL, lysozyme adsorbs ''i rreversibly'' with respect to dilution with 10(-3) M NaCl.