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Equilibrium and Kinetic Studies of the Binding of Monoanionic and Dianionic Ligands to Bovine Serum Albumin

1993; Oxford University Press; Volume: 66; Issue: 10 Linguagem: Inglês

10.1246/bcsj.66.2808

1348-0634

Kiyofumi Murakami,

Surfactants and Colloidal Systems

Abstract The binding of monoanionic azo dyes (4-(2-hydroxy-1-naphtylazo)benzenesulfonate (Orange II) and 6-hydroxy-5-phenylazo-2-naphthalenesulfonate (Ponceau 4G)) and dianionic one (6-hydroxy-5-(4-sulfophenylazo)-2-naphthalenesulfonate (Sunset Yellow FCF)) to bovine serum albumin (BSA) at pH = 7.0 and 25 °C has been studied by equilibrium dialysis, spectrophotometry, and a temperature-jump method. The binding parameters for the monoanionic dyes were found to take quite similar values to each other. This shows that both of the dyes bind to BSA in essentially the same manner, irrespective of the difference in the position of the sulfonato group on the ligands. On the other hand, the data for the dianionic dye showed that the secondary sites, observed for the monoanionic dyes, almost completely disappeared, while the number and the binding constant for the primary sites were comparable. Kinetic data concerning the BSA–Sunset Yellow FCF system were found to be consistent with a scheme in which a bimolecular binding is followed by two isomerizations of the complex at the primary site. All of the equilibrium and rate constants for the scheme have been determined. A comparison of the results to those for other ligands has shown that the bimolecular association rate constant successively decreases with an increase in the number of anionic charges on the ligands. In the case of monoanionic dyes, however, the number of observed relaxations showed a tendency to increase with an increase in the number of bound dyes on the secondary sites. On the basis of these results, the detailed binding mechanism is discussed from the view point of the relation between the specificity of ligand-binding and the number of charges on the ligand.