Multi-technique approach on the effect of surfactant concentrations on the thermal unfolding of rabbit serum albumin: formation and solubilization of the protein aggregates

Abstract

Effect of cationic surfactant, cetyltrimethylammonium bromide (CTAB) addition on the thermal denaturation of rabbit serum albumin (RSA) has been studied by employing small-angle neutron scattering (SANS), circular dichroism (CD), intrinsic fluorescence and ultra violet (UV) spectroscopy. The studies were performed at three different temperatures viz., 30, 50 and 70 degrees C and at two different concentrations of CTAB: the low concentration of CTAB used was 1mM and the higher concentration was 80 mM (for SANS) and 20mM (for CD, fluorescence and UV). A collective effect of high temperature and low concentration of CTAB led to the protein aggregation followed by solubilization of these aggregates at higher concentration of surfactant. At 1mM CTAB and 30 degrees C, the protein-surfactant complex has a prolate ellipsoidal shape with semi-major axis of 88.9A and semi-minor axis of 19.6A which are slightly greater than the values of the native RSA. At 50 degrees C, the size of the semi-major axis increases while at 70 degrees C an increase in the size of both axes was found. The thermal outcome at higher concentration of CTAB (80 mM) was rather different. Higher concentration of CTAB unfolds the protein by the formation of micelle-like aggregates along the polypeptide chains of the protein and the complex was stabilized at higher temperatures, which was not found with lower concentration of CTAB. The CD results were found to be consistent with the SANS results, i.e., decrease in alpha-helicity of RSA was more when less amount of surfactant was present as compared to the system with higher surfactant concentration. In a similar fashion, results of relative fluorescence intensity (RFI) reveal that increase in temperature causes decrease in lambda(max) of native RSA as well as RSA+1mM CTAB, whereas the lambda(max) remains unchanged for RSA+20mM CTAB systems. That means the structure remains compact in presence of 20mM CTAB while the structure becomes loose when low or zero amount of surfactant was present. The UV results indicate that the protein aggregation takes place in presence of low amount of CTAB and these aggregates become soluble at high concentration of CTAB.