The effects of pH, temperature, and buffer concentration on the self-assembling behavior, secondary structure, and surface hydrophobicity of donkey and bovine β-casein

The self-assembling behavior, secondary structure, and surface hydrophobicity of purified donkey beta-casein in terms of pH, temperature, and buffer concentration were investigated in comparison with commercial bovine beta-casein. Critical micelle concentration of both beta-caseins decreased with the lowering of pH (pH 8.0-6.0) and the increasing temperatures (25-50 degree celsius). Critical micelle temperature of both beta-caseins increased moving from pH 6.0 to 8.0 and aggregates larger than micelles formed at pH 6.0 that is close to their isoelectric point. Fluorescence spectroscopy analysis demonstrated that the maximum surface hydrophobicity was achieved at pH 6.0. The secondary structure was examined using circular dichroism spectroscopy, highlighting an increase of alpha-helix content and a decrease of unordered structures with the decrease of pH and increase of temperature. This work provides insights on parameters promoting molecular interactions involved in donkey beta-CN self-association, useful to develop nanocarriers for encapsulating bioactive compounds in pharmaceutical and nutraceutical applications.