Surfactant Interactions with Zein Protein

Interactions of a model surfactant, sodium dodecyl sulfate (SDS), with a water-insoluble model protein, zein, were investigated to gain an understanding of the effects, such as skin irritation and protein denaturation, of surfactants that are common in personal-care products. To elucidate the mechanisms of such effects, the zein protein interaction with SDS in aqueous solutions was investigated using a multipronged approach involving a range of techniques, such as UV−visible and fluorescence spectroscopy, TOC (total organic carbon analysis), light scattering, and viscosimetry. The zein protein solubilization increases with an increase in the SDS concentration. Solubilization of zein occurs in two distinct stages followed by a complete unfolding of the protein. In the first stage ([SDS] ∼ 4 mM; critical complexation concentration), SDS is incorporated into the globular zein structure, forming small hydrophobic microdomains. From the pyrene fluorescence lifetime decay measurements, the aggregation number of SDS in such hydrophobic microdomains was found to be markedly lower than the aggregation number of pure SDS micelles in the bulk solution. The vibrational fine structure of pyrene fluorescence, however, showed the core of SDS−zein complex micelles to be more hydrophobic than that of the SDS micelles. In the second stage ([SDS] ∼ 200 mM; unfolding concentration), the protein unfolds, as is evidenced by viscosity and dynamic light scattering measurements.