Mechanism of Interaction between Bovine Serum Albumin and Sodium Dodecyl Sulfate

The binding isotherms of the interaction between bovine serum albumin (BSA) and sodium dodecyl sulfate (SDS) were obtained using electromotive force measurements. Changes in the microenvironmental polarity of aromatic amino acid residues during the interaction were studied using fourth-derivative ultraviolet spectroscopy and fluorescence spectroscopy. The average number (nu) of SDS molecules bound to BSA increased with increasing SDS concentration. The polarity of tryptophan (Trp) residues decreased gradually and then remained almost constant. The polarity of tyrosine residues increased significantly and then decreased a little. The polarity of phenylalanine residues increased very slightly. The results show that SDS molecules bind to BSA in the vicinity of Trp-213 when nu gradually increases from 0 to 14. BSA unfolds from domain IIA, induced by SDS aggregates formed near Trp-213. The nu value then increases rapidly as a result of positive cooperative binding. When the nu value reaches about 302, saturation binding is achieved and the BSA conformation remains almost unchanged.