The Interaction of ?-Lactoglobulin with N,N-Dimethyldodecylamine N-Oxide and Sodium Dodecyl Sulfate

Surfactants are amphiphilic molecules that lower the surface tension between two phases. In aqueous solution, they are known to interact with proteins, forming protein-surfactant complexes which can affect protein structure, kinetic activity, aggregation behavior and binding to surfaces. As such, surfactants and mixtures of surfactants have been used with proteins for a variety of applications including formulation of cleaning agents, membrane protein extraction and separation, stabilization of biopharmaceuticals and foam formation. While the interactions of individual ionic and nonionic surfactants with proteins have been well characterized, in-depth study of surfactant mixtures in the presence of protein is still an emerging field. We expect combinations of surfactants to form mixed micelles that take on properties of each surfactant, resulting in altered interaction with proteins.

We used a variety of biophysical characterization techniques to investigate the behavior of mixtures of anionic surfactant sodium dodecyl sulfate (SDS) and amphoteric surfactant N,N-dimethyldodecylamine N-Oxide (DDAO) at varying molar ratios. Pyrene fluorescence showed that the critical micelle concentration (CMC) of surfactant mixtures is lower than the CMC of both pure SDS and pure DDAO, with little variation over a range of 0.10-0.65 mole SDS per mole total surfactant. Additionally, we investigated the interactions between DDAO-SDS mixtures and the model protein β-lactoglobulin (βLG). The surfactant concentrations at which micelles form were unchanged in the presence of βLG except above 0.85 mole SDS per mole total surfactant, where surfactant aggregates formed on the protein, similar to pure SDS. βLG is known to unfold in 2mM DDAO and 1.09mM SDS, yet circular dichroism and small angle X-ray scattering (SAXS) showed βLG was structurally stable in mixtures at higher surfactant concentrations. Simple linear combination of SAXS scattering curves revealed that βLG was most stable in mixtures of 75% DDAO and 25% SDS up to 5mM total surfactant. These results indicate that DDAO and SDS indeed form mixed micelles in solution with less favorable interactions with βLG which improves the stability of the protein in comparison to pure DDAO or SDS.