Binding isotherms of surfactants used in detergent formulations to bovine serum albumin

Protein-surfactant interactions are the focus of extensive research due to their many applications in food technology and detergent industry. In this work, we investigate the interaction between bovine serum albumin (BSA) and five relevant surfactants to the cleaning industry, which differ in head group charge, namely: sodium alkyl ether sulphate (C12-C14 AE3S), Cocoamidopropyl amine-oxide (CapAO), alkyl dimethyl amine oxide (C12C14AO), octaethylene glycol monodecyl ether (C10EO8) and didecyldimethylammonium chloride (DDAC). The results collected with fluorescence emission spectroscopy highlight zwitterionic and nonionic surfactants have the lowest affinity for the protein, as their interaction does not result in protein denaturation. Instead, higher and mutually close binding constants are found for AE3S (anionic) and DDAC (cationic) due to the presence of electrostatic interactions between the surfactant heads and the charged residues of BSA. AE3S leads to irreversible protein unfolding. The case of DDAC is more complex and has been studied through a combination of fluorescence, DLS, PGSE-NMR and zeta-potential measurements. At low concentration DDAC binding neutralizes negatively charged residues present in BSA, causing a reversible flocculation of BSA after the isoelectric point (23 DDAC molecules per protein). Further DDAC adsorption, at 70 DDAC molecules per protein, leads to an excess of positive charge on the protein which restores electrostatic repulsions between BSA-surfactant complexes.