The Lysozyme−Sodium Dodecyl Sulfate System Studied by Dynamic and Static Light Scattering

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The interaction between lysozyme and sodium dodecyl sulfate (SDS) was investigated by dynamic and static light scattering. The lysozyme−SDS system was studied at two different pH values (3.3 and 9.0), at which the net positive charges of lysozyme are 16 and 6, respectively. Furthermore, a comparison was made between the native lysozyme−SDS system and the reduced lysozyme (i.e., lacking its four disulfide bridges)−SDS system at pH 3.3. The concentration of SDS was varied from 0 to 0.55 M. The lowest molar SDS/lysozyme ratio was ≈55 because lower ratios result in a precipitate, making the samples unsuitable for measurement. Under the conditions used, (free) SDS micelles and lysozyme−SDS complexes coexist. Based on characteristic differences in the relaxation time distributions, the results obtained at low SDS concentration (<0.1 M) are discussed separately from those obtained at high SDS concentration (>0.25 M). At low SDS concentration, the hydrodynamic radii for the lysozyme−SDS complex in the three different systems are 3.23 ± 0.04 nm (native lysozyme−SDS, pH 3.3), 3.0 ± 0.1 nm (reduced lysozyme−SDS, pH 3.3), and 2.61 ± 0.06 nm (native lysozyme−SDS, pH 9.0). The hydrodynamic radius of (native) lysozyme at pH 3.3 is 1.82 ± 0.01 nm, and that of reduced lysozyme is slightly smaller (i.e., 1.79 ± 0.01 nm). The rather small hydrodynamic radii of the complexes cannot be accounted for by an unfolded lysozyme molecule. Reducing the disulfide bridges does not result in an increase, but a small decrease of the hydrodynamic radius. The small size can possibly be explained by a complex containing a micelle-like aggregate bound to the protein. At high SDS concentrations, two relaxation modes exist, both representing diffusional species and both containing lysozyme. It is assumed that two different complexes coexist: complex 1 is similar to that obtained at low SDS concentrations and has a compact structure, and complex 2 is a larger complex in which lysozyme probably has a more open, expanded structure, presumably caused by the binding of a greater amount of SDS. The hypothesis of a more expanded lysozyme molecule corresponds well with the change in the observed refractive index increment (∂n/∂clyz) values. The number of lysozyme molecules in the complex does not seem to depend on the SDS concentration and is determined, for the three systems studied in this article, to be ∼1.