Unfolding of rabbit serum albumin by cationic surfactants: Surface tensiometry, small-angle neutron scattering, intrinsic fluorescence, resonance Rayleigh scattering and circular dichroism studies

Abstract

Unfolding of rabbit serum albumin (RSA) by cationic surfactants cetyltrimethylammonium bromide (CTAB) and tetradecyltrimethylammonium bromide (TTAB) was studied by exploiting surface tensiometry, small-angle neutron scattering (SANS), intrinsic fluorescence, resonance Rayleigh scattering (RRS) (also referred as turbidity at 350/350), and circular dichroism (CD) techniques. Surface tension measurements revealed the formation of highly surface-active complexes occurring as a consequence of RSA-surfactants interactions. SANS measurements show that, in the low surfactant concentration regime (0-10 mM), increase in the dimension of the ellipsoidal protein occurs. Conversely, at higher concentrations (20-80 mM), the surfactant molecules result in the formation of a fractal structure representing a 'necklace model' of micelle-like clusters randomly distributed along the polypeptide chain. The overall size of the complex increases and the fractal dimension decreases on increasing the surfactant concentration. The size of the micelle-like clusters decreases while the number of such clusters and their aggregation number increase with increasing CTAB concentration. Taken all observant together, the fluorescence, RRS, and CD studies were found to be consistent with the SANS measurements. Both CTAB and TTAB were found to behave likewise and the effect of hydrophobicity was clearly visible in the CD, RRS, and intrinsic fluorescence results. The Rayleigh scattering study shows that TTAB was more skilled to solubilize the serum albumin and may be more convenient than CTAB to isolate proteins from inclusion bodies.