The effect of sodium dodecyl sulfate on the conformation of bovine serum albumin

The effect of sodium dodecyl sulfate (SDS) on the conformation of bovine serum albumin (BSA) was first determined by cyclic voltammetry, and verified by isothermal titration microcalorimetry, fluorescence spectroscopy, circular dichroism (CD) spectroscopy, and freeze-fracture transmission electron microscopy (FF-TEM). In a very low SDS concentration range, SDS makes the globular protein loose through a specific binding in the structure, and then surface adsorption of SDS around the protein occurs. This makes the cathodic peak current of BSA arising from disulfide bonds increase first and then decrease steeply. When the SDS concentration is larger than 1.0 × 10−3 mol L−1, the electrostatic repulsion between the head groups of SDS could initiate the partial unfolding, and the gradual exposure of disulfide bonds facilitate the redox of BSA, leading to a slow decrease in the peak current. When the SDS concentration reaches the critical aggregation concentration (4.4 × 10−3 mol L−1), the SDS binding and resulting protein denaturation are largely enhanced. As only a small amount of disulfide bonds are surrounded by the micelle-like clusters, the decrease of the cathodic peak current of BSA becomes even slow. Above the critical micellar concentration, the repulsion between the charged micelle-like clusters and the increased hydrophobic interactions of hydrophobic SDS chains with the hydrophobic backbone of BSA results in the complete unfolding of the protein. The cathodic peak current is hardly affected with SDS addition due to the complete surrounding of disulfide bonds by micelle-like clusters. The corrected mixing enthalpy of SDS with BSA was exothermic at all SDS concentration (0–1.1 × 10−2 mol L−1), and could be interpreted in terms of various molecular events: specific binding, cooperative binding, protein unfolding, micelle cluster, and free micelle formation, which confirms the electrochemical behavior. The above results can be further verified by fluorescence spectral feature of BSA and FF-TEM images of SDS–BSA system. CD spectra studies indicate that the interactions of SDS with BSA only cause partial loss of the secondary structure of BSA.