The interaction of bovine serum albumin with surfactants studied by light scattering

The interaction between bovine serum albumin (BSA) and several surfactants has been investigated by light scattering. Anionic (sodium dodecyl sulfate, SDS), cationic (dodecyl trimethylammonium bromide, DTAB), and nonionic (polyoxyethylene 8 lauryl ether, C12E8) surfactants, all containing a C-12 alkyl chain, were used to study the effect of different headgroups on the complex formation. The hydrodynamic radii of the complexes obtained by dynamic light scattering indicate that cooperative binding of DTAB occurs at higher surfactant concentrations than in comparative solutions of SDS and C12E8. The effect of chain length is shown for the cationic surfactants DTAB and cetyl trimethylammonium bromide (CTAB, C-16 alkyl chain). The higher surface activity of CTAB results in complex formation at a lower surfactant concentration compared to DTAB. The hydrodynamic radii of the BSA-SDS and BSA-DTAB complexes at saturation were determined as similar to 5.9 nm and similar to 4.8 nm, respectively. The hydrodynamic radius of the reduced BSA-SDS complex is somewhat smaller than the corresponding native BSA-SDS complex. Static light scattering (SLS) measurements were performed on BSA-SDS systems to determine the number of BSA molecules in the complex. Prior to SLS measurements the BSA-SDS solutions were dialyzed against a large volume of SDS solution in order to determine the refractive index increment delta n/delta c(BSA) at constant chemical potential. It was observed that a very long dialysis time (several weeks) was needed to reach equilibrium. Measurements on solutions that had not reached equilibrium resulted in improbably high values of the number of BSA molecules in the complex.