Effect of surfactants on casein structure: a spectroscopic study

Fluorescence and circular dichroism spectroscopy were used to study the effect of two surfactants having oppositely charged head groups - cationic cetyltrimethyl ammonium bromide (CTAB) and anionic sodium dodecyl sulphate (SDS) - on the structure of the intrinsically unstructured proteins alpha s-, beta- and kappa-caseins. Although globular proteins are generally known to denature on interacting with surfactants, the caseins were found to adopt more ordered conformations in presence of both SDS and CTAB. The folding induced by CTAB was more efficient than by SDS, as implied by the behaviour of fluorescence and circular dichroic spectra of the caseins in solutions containing varying concentrations of the surfactants. The differential response of the proteins to the two surfactants may lie in the fact that the negatively charged caseins experience a repulsive electrostatic interaction with the negatively charged head groups of SDS, while their interaction with the positively charged head groups of CTAB is attractive in nature. Our results are consistent with two different types of the 'necklace and bead' model for the structure of surfactant-casein complexes: while groups of SDS molecules converge tail first around exposed hydrophobic surfaces of the caseins to form micelle-like structures, the protein itself wraps around micellar aggregates of CTAB that have cationic head groups in close association with its negatively charged/polar residues.