(27r) Enhanced Protein Stability Enabled By Mixtures of Ionic and Amphoteric Surfactants Compared to Each Alone

The interaction between proteins and surfactants is import in the formulation of cleaning agents, the extraction and separation of membrane proteins, the stabilization of biopharmaceuticals, and the creation of foams and emulsions. This interaction profoundly impacts the structure, stability, activity, and surface-binding abilities of the protein. Studies on the effects of single ionic or nonionic surfactants on the stability and conformation of proteins have been extensive, but research on the interactions between proteins and surfactant mixtures is still in its early stages. We propose that surfactant-surfactant interactions in binary mixtures may be tailored to present favorable environments for maintaining protein structure. In this work, we examined the conformational stability of the model protein β-Lactoglobulin (βLG) in the presence of binary mixtures of the anionic surfactant sodium dodecyl sulfate (SDS) and the amphoteric surfactant N,N-dimethyldodecylamine N-Oxide (DDAO). To characterize surfactant and protein interactions, we used a variety of biophysical methods including pyrene fluorescence, circular dichroism (CD), 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 lower CMC was found, potentially due to free SDS monomers binding to βLG as is observed for pure SDS. Unfolding of βLG is observed above 2 mM DDAO and above 1.1 mM SDS for the neat surfactants; however, higher concentrations of the mixed surfactants were required to unfold βLG. Protein secondary and tertiary structure as measured by CD were found to be in tact at 20 mM total surfactant in a mixture of 90% DDAO and 10% SDS. Simple linear combination of SAXS scattering curves revealed that βLG structure was stable in mixtures of 75% DDAO and 25% SDS where 5 mM total surfactant was required to begin unfolding the protein. We are actively looking into the cause of the increased stability of βLG in surfactant mixtures compared to single surfactant solutions. We hypothesize that the ionic SDS and amphoteric DDAO form mixed micelles resulting in a decrease of free SDS monomers and/or mixed micelles with a surface charge that results in decreased interactions with βLG.