Assessing the interaction of Hecameg® with Bovine Serum Albumin and its effect on protein conformation: A spectroscopic study

Interaction of the nonionic surfactant Hecameg® with the plasma protein Bovine Serum Albumin (BSA), and its effect on protein conformation, has been studied using spectroscopic techniques such as steady-state and time-resolved fluorescence and circular dichroism. A weak interaction of the surfactant with BSA is reflected by changes in the intrinsic fluorescence of BSA in either steady-state or time-resolved measurements. The fluorescence intensity data allowed us to determine the corresponding binding curve, which suggests a sequential binding mechanism, in which the surfactant first occupies the hydrophobic sites of the inner protein cavity and then, condenses onto the surface hydrophobic sites of BSA via a cooperative mechanism. Additional fluorescence data obtained by synchronous, three-dimensional and anisotropy experiments show that the surfactant mainly interacts with the tryptophan residues of BSA, which seem to experience motional restriction as a result of this interaction. Time-resolved fluorescence data, which were analyzed using the modified Stern-Volmer equation, also support the above mechanism. Finally, far-UV circular dichroism studies indicated that the secondary structure of the protein remains almost unaltered even for BSA to surfactant molar ratio as high as 1 to 100.