Food Hydrocolloids, 2018, vol 82pp. 19-28
With well-known nutritional properties, casein contributes to about 80% of protein content in milk and has been classified as highly intrinsically disordered protein (IDP). In this paper, the sulfate dodecyl sodium (SDS)-induced conformational changes of bovine casein were studied by multi-techniques. Isothermal titration calorimetry (ITC) and differential scanning calorimetry (DSC) were used to obtain the stoichiometry of conformational changes and the thermal stability of the formed complexes. Spectral results indicated that casein presented a higher helical content but loss of tertiary structure above critical micelle concentration of SDS, namely, the so-called molten globule like state. The thermal self-association of casein could be prevented by SDS according to far-UV CD even at 70 °C. The 1H NMR spectrum of casein showed that the resonance around 1.0 ppm, the region of α-hydrogen, shifted to the higher field, and the aromatic region around 5.5–8.0 ppm shifted to the lower field, while the NOESY spectra of casein exhibited few chemical shifts with binding of SDS. Combining the results of dynamic light scattering (DLS), scanning electron microscope (SEM) and small angle x-ray scattering (SAXS), one obtains that casein micelles presented an elliptical shape of ∼800 nm in diameter and upon binding with SDS, the casein micelles disassociated into more compact globular particles of 10 nm in diameter with a core-shell structure composed by SDS molecules and casein proteins. The present work, not only provides molecular insights into the mechanism of SDS-induced stability of a model IDP, casein, but also helps understand the role of surfactants on the structure–function relationship of bovine casein in the food industry.