Desorption of lysozyme layers by sodium dodecyl sulfate studied with the surface force technique

The effect of sodium dodecyl sulfate (SDS) on preadsorbed lysozyme layers on mica was investigated using a surface force technique and electron spectroscopy for chemical analysis. Lysozyme was adsorbed onto mica surfaces from a 0.2 mg/mL protein solution containing 1 mM NaCl at pH 5.6, which nearly neutralizes the surface charge on the mica, and it was found that the adsorption was irreversible with respect to dilution with I mM NaCl solution. Addition of SDS (0.83-4 mM) to the hulk solution does not induce large-scale desorption of the adsorbed protein layers; the layer thickness remains the same, 4.8 +/-, 0.4 nm, up to 4 mM. However, addition of SDS results in a pronounced gradual increase in the interfacial charge manifested in the long-range double-layer repulsion. At 6 mM, the surfactant binding to the protein layer does change the layer properties. An initial surfactant-induced denaturation of the adsorbed protein occurs, whereby the thickness of the layers decreases to 2.7 +/- 0.3 nm, yet the interfacial charge remains high, similar in magnitude to the charge of mica. When SDS is introduced at the critical micelle concentration, the protein layers are completely desorbed; we thus conclude that the desorption results from the combined effect of protein denaturation and the increased complex charge.