Unfolding and refolding of bovine serum albumin induced by cetylpyridinium bromide.

The interaction of bovine serum albumin (BSA) with cationic surfactant cetylpyridinium bromide (CPB) in aqueous solution (pH 7.00) was studied quantitatively with ultraviolet (UV)-visible, far-UV, and near-UV circular dichroism, fluorescence, small angle x-ray scattering, and nuclear magnetic resonance measurement. It was found that CPB at low and high concentrations could induce the unfolding and refolding of BSA, respectively. We suggest that in the unfolding process, there existed BSA-CPB complex with the "necklace and bead" structure in which the unfolded BSA wrapped around CPB micelles, and that the hydrophobic interaction between the complexes led to the formation of large aggregates. The aromatic headgroup of CPB interacted with the tryptophan residues of BSA, resulting in the aromatic ring stacking between BSA and CPB. During the refolding process, the BSA molecule was penetrated into the rod micelle of CPB and the hydrophobic moiety of the BSA molecule was exposed outside while its hydrophilic part was hidden inside, thereby disrupting the aromatic ring stacking.