Surfactants and mixtures of surfactants have wide application with proteins, including: formulation of cleaning agents, extraction and separation of membrane proteins, stabilization of biopharmaceuticals and formation of foams and emulsions.  Surfactant interactions with proteins form complexes that influence protein structure, activity, aggregation, and binding to surfaces.  While the interactions of individual ionic and nonionic surfactants with proteins have been well characterized, detailed studies of protein structure in surfactant mixtures is still an emerging field. We hypothesize that mixed surfactant systems offer the ability to specifically tune interactions between surfactants, micelles, and proteins, impacting protein structure and function, and thus application and formulation.  In this work, we investigated the interaction of binary mixtures of the anionic surfactant sodium dodecyl sulfate (SDS) and the amphoteric surfactant N,N-dimethyldodecylamine N-Oxide (DDAO) with the model protein β-Lactoglobulin (βLG) using a variety of biophysical techniques including pyrene fluorescence, circular dichroism and small-angle X-ray scattering (SAXS). The critical micelle concentration was unchanged for mixed surfactant samples in the presence of βLG, except above 0.85 mole SDS per mole total surfactant where a higher CMC was found, potentially due to surfactant binding to βLG as is observed for pure SDS.  Unfolding of βLG is observed in 2 mM DDAO and 1.1 mM SDS of the neat surfactants; however, higher concentrations of the mixed surfactants were required to unfold βLG.  Simple linear combination of SAXS scattering curves revealed that βLG structure was most stable in mixtures of 75% DDAO and 25% SDS where 5 mM total surfactant was required to unfold the protein.  We are currently investigating the reason for the improved stability of βLG with the surfactant mixtures.  Two potential explanations are less free surfactants in mixtures or that DDAO and SDS form mixed micelles with less favorable interactions with βLG, reducing unfolding.

Enhanced protein stability enabled by mixtures of ionic and amphoteric surfactants compared to each alone

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