Multifunctional Amphicharged Surfactants: Unraveling pH and Thermal Modulation of Their Interactions with Bovine Serum Albumin

Proteins on interaction with surfactants are known to undergo structural and functional alterations. These protein-surfactant interactions are crucial for their applications in the cosmetic and pharmaceutical industries. Despite having various studies involving the interactions of albumins with surfactants, most of them are done at a standard pH (pH 7) and temperature (298 K). Peptide-based surfactants can be designed to have multiple amino and carboxylic acid groups, which would allow them to exist as different ionic species at different pH. This paper provides insights in to the interactions between the protein and different ionic forms of the surfactants which exist at different environmental conditions. Three different surfactants, S1, S2, and S3, are designed with one, two, or four pairs of amino and carboxylic acid functions in the headgroup and a fixed 12-carbon aliphatic tail group. The interactions between these multifunctional surfactants and bovine serum albumin (BSA) at different pH and at different temperatures are studied using fluorescence, UV-visible, and CD spectroscopy. All three surfactants resulted in quenching of the fluorescence of BSA. The strength of the BSA-surfactant complexes along with the binding parameters were calculated, and the mechanism of quenching was investigated. The nature of interactions between the surfactants and BSA could be elucidated by calculating the thermodynamic parameters. The results obtained from CD and UV-visible studies also support the conclusions drawn from fluorescence spectroscopy.