Characterization of microenvironments of 2-(4'-hydroxyphenylazo)benzoic acid bound to bovine serum albumin by studying the solvent effects.

The binding and thermodynamic parameters of the two binding forms of 2-(4'-hydroxyphenylazo)benzoic acid to bovine serum albumin were estimated. The number of the binding sites of the azo form was 3.2 and that of the hydrazone form was 0.7. The binding of the hydrazone form was affected only by enthalpy changes, in contrast to that of the azo form which was affected by both enthalpy and entropy changes. To estimate the complex microenvironments of 2-(4'-hydroxyphenylazo)benzoic acid molecules on bovine serum albumin, the solvent effects were studied. The 2-carboxyl group of 2-(4'-hydroxyphenylazo)benzoic acid participates in the azo-hydrazone tautomerism . The 2-carboxylate ion forms an ion pair with triethylamine in ethylene dichloride, chloroform, and benzene, resulting in the appearance of the hydrazone form. The hydrazone formation in the system of 2-(4'-hydroxyphenylazo)benzoic acid-triethylamine (1:1) in chloroform was affected only by enthalpy changes, in the same manner as in the system of 2-(4'-hydroxyphenylazo)benzoic acid-bovine serum albumin. We speculate the presence of the two kinds of ion pairs on the basis of the changes of the azo-hydrazone tautomerism in chloroform, and that the azo form takes the contact ion pair and the hydrazone form takes the solvent-separated ion pair. A new possible model for the interaction of the azo and hydrazone forms and bovine serum albumin is proposed.