Interaction between bovine serum albumin and equimolarly mixed cationic–anionic surfactants decyltriethylammonium bromide–sodium decyl sulfonate

Abstract The interactions of bovine serum albumin (BSA) with the anionic surfactant sodium decylsulfonate (C 10 SO 3 ), the cationic surfactant decyltriethylammonium bromide (C 10 NE) and equimolarly mixed cationic–anionic surfactants C 10 NE–C 10 SO 3 were investigated by surface tension, viscosity, dynamic light scattering (DLS) and circular dichroism (CD). It was shown that the single ionic surfactant C 10 SO 3 or C 10 NE has obvious interaction with BSA. The presence of C 10 SO 3 or C 10 NE modified BSA structure. However, the equimolarly mixed cationic–anionic surfactants C 10 NE–C 10 SO 3 showed very weak interactions with BSA. The surface tension–log concentration ( γ –log  C ) plot for the aqueous solutions of C 10 NE–C 10 SO 3 /BSA mixtures coincided with that of C 10 NE–C 10 SO 3 solutions. Viscometry showed that there is no significant change in the rheological properties for the C 10 NE–C 10 SO 3 /BSA mixed solutions. DLS showed that BSA monomers and mixed aggregates of C 10 NE–C 10 SO 3 existed in the C 10 NE–C 10 SO 3 /BSA mixed solutions. From CD spectra no obvious modification of BSA structure in the presence of C 10 NE–C 10 SO 3 mixtures was observed. The weak interactions between BSA and C 10 NE–C 10 SO 3 might be explained in terms of the very low critical micelle concentration (cmc) of C 10 NE–C 10 SO 3 mixtures that made the concentration of ionic surfactant monomers much lower than that needed for inducing the modification of BSA structure. In other words, the very strong synergism between oppositely charged cationic and anionic surfactants makes the formation of cationic–anionic surfactant mixed aggregates in the bulk solution a more favorable process than binding to proteins.