Interaction Between n-octyl- -D-thioglucopyranoside and Bovine Serum Albumin

The binding of the nonionic surfactant n-octyl-D-thioglucopyranoside (OTG) to the globular protein bovine serum albumin (BSA) has been investigated by using experimental techniques such as surface tension, steady-state fluorescence and dynamic light scattering. It was observed that the surfactant micellization is delayed in the presence of protein; this was interpreted as a consequence of the fact that part of the surfactant is not available for the formation of micelles, because it is partitioned into the protein hydrophobic sites. This was taken as an evidence of the interaction between surfactant and protein. The fluorescence emission spectra of intrinsic tryptophans revealed that the protein is partially denatured in the presence of high surfactant concentrations. The analysis of the binding features as obtained by two different methods, (i) one based on surface tension measurements, and (ii) another based on the behaviour of the intrinsic BSA fluorescence, indicated that the binding process is non-cooperative at low surfactant concentration, but becomes cooperative when it is high enough. The reduction of the average aggregation number in the presence of protein was interpreted as a sign of the formation of clusters of surfactant adsorbed on the protein surface. A slight conformational change in the protein structure at low surfactant concentration was revealed by resonance energy transfer from tryptophan residues to 8-anilinonaphthalene-1-sulfonate. A treatment of the autocorrelation functions as obtained by dynamic light scattering measurements, based on the application of appropriate fitting techniques, allowed for the discrimination between two kinds of structures in the OTG/BSA system: surfactant-protein complexes, with a “pearl necklace” structure, in equilibrium with the free micelles of OTG.