Surfactant-induced protein unfolding as studied by small-angle neutron scattering and dynamic light scattering

The structural changes of protein bovine serum albumin ( BSA) during its unfolding on the addition of anionic surfactant sodium dodecyl sulfate ( SDS) have been studied using small-angle neutron scattering ( SANS) and dynamic light scattering (DLS). It is observed that at small surfactant concentrations, individual surfactant molecules bind to the protein, increasing the size of the protein. On the other hand, surfactant molecules at higher concentrations aggregate to form micelle-like clusters along the unfolded polypeptide chains of the protein. SANS data indicates 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 does not show any change, while the number of such micelle-like clusters in protein-surfactant complexes increases with the surfactant concentration. The conformation of the unfolded protein has been determined directly using contrast variation SANS measurements by contrast matching the surfactant to the medium. It is found that the protein acquires a random coil Gaussian conformation on unfolding, with its radius of gyration increasing with an increase in surfactant concentration. The results of DLS measurements are found to be in good agreement with those obtained using SANS.