Effect of surfactants on casein structure: a spectroscopic study

Area of Science:

• Biochemistry
• Protein Chemistry
• Surfactant Science

Background:

• Intrinsically unstructured proteins (IUPs) like caseins lack a defined 3D structure.
• Surfactants typically denature globular proteins, but their effect on IUPs is less understood.
• Caseins are a family of phosphoproteins crucial in food science and nutrition.

Purpose of the Study:

• To investigate the structural impact of oppositely charged surfactants, CTAB and SDS, on alpha s-, beta-, and kappa-caseins.
• To elucidate the conformational changes and binding models of surfactant-casein complexes.
• To compare the folding efficiency induced by cationic (CTAB) versus anionic (SDS) surfactants.

Main Methods:

• Fluorescence spectroscopy to monitor protein conformational changes.
• Circular dichroism (CD) spectroscopy to assess secondary and tertiary structure alterations.
• Systematic variation of surfactant concentrations to study dose-dependent effects.

Main Results:

• Caseins adopted more ordered conformations in the presence of both CTAB and SDS, contrary to typical protein denaturation.
• CTAB induced a more pronounced folding effect on caseins compared to SDS, as evidenced by spectral changes.
• Differential interactions were observed: SDS showed repulsive electrostatic interactions with negatively charged caseins, while CTAB exhibited attractive interactions.

Conclusions:

• Surfactants can induce ordered structures in intrinsically unstructured proteins like caseins.
• The charge of the surfactant head group significantly influences the type and extent of protein structural changes.
• Two distinct 'necklace and bead' models describe surfactant-casein complexes, depending on the surfactant type and electrostatic interactions.