Surfactants are amphiphilic molecules that can stabilize interfaces, notably between hydrophobic and hydrophilic environments. Interactions between surfactants and proteins play an important role in food processing, pharmaceuticals, cleaning agents, protein purification, and the characterization of membrane proteins. Characterizing these interactions and the structure of protein-surfactant complexes is necessary for proper product formation and development as they can affect enzyme activity, emulsion and foam formation, and stability. While ionic (charged) surfactants, such as sodium dodecyl sulfate (SDS), are known to perturb protein tertiary and secondary structure at low concentrations, the effects of nonionic surfactants, such as lauryldimethylamine oxide (LDAO), on protein stability have been less studied. We investigated the impact of LDAO on the structure of β-Lactoglobulin (βLG), a globular whey protein, using a variety of biophysical techniques such as circular dichroism (CD), pyrene fluorescence, pendant drop tensiometry, and small-angle x-ray scattering (SAXS). Low concentrations of LDAO were found to interact weakly with βLG, causing minimal change in tertiary structure. Pyrene fluorescence and pendant drop tensiometry show that the critical micelle concentration of LDAO is unaffected by the presence of βLG. At and above the critical micelle concentration (CMC) of LDAO, protein-surfactant complexes form and we begin to see protein unfolding. This indicates that the presence of LDAO micelles is necessary for denaturation. SAXS and CD suggest that nonionic surfactants such as LDAO follow an unfolding mechanism different from that seen in ionic surfactants. Mixtures of nonionic and ionic surfactants were also investigated and show unique properties which appear to stabilize the protein even at high surfactant concentrations.
Authors: Evan Danielson, Kerri Peterson
Faculty Advisors: Jason Berberich; Jason Boock; Chemical, Paper, and Biomedical Engineering
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