Small angle neutron scattering study of lysozyme-sodium dodecyl sulfate aggregates

The lysozyme-sodium dodecyl sulfate-water system features several interesting aggregation phenomena and is also of interest as it constitutes a model system for mixtures of a charged colloid with an oppositely charged surfactant, as both colloid and surfactant are pure and monodisperse compounds. The structure of such mixed protein - surfactant systems has been investigated by means of SANS contrast variation experiments. Two interesting issues of protein-surfactant aggregation are discussed. First, a new set of data on the structure of the protein-surfactant complex in solution is added to the discussion of whether the model of "beads on a necklace", "protein decorated micelles", or "flexible helix" is most appropriate. It is our conclusion that the compact globule of
ysozyme does not fit well into any of the mentioned models. Instead, transient clusters of lysozyme-SDS aggregates are proposed for the L-1 phase and more strongly bound locally linear clusters for the gel phase. Second, the structure and formation of a homogeneous, transparent gel at room temperature is analyzed and compared to the well-studied heat-set globular gels. The gel structure in different ionic strengths, lower than the one caused by naturally occurring buffer salts has also been analyzed and it seems that small amounts of salt render the system a more repulsive character than salt-free conditions. In addition to the equilibrated samples studied with the contrast variation technique, the complexation has been studied over time.