Abstract
The interaction of theophylline with bovine serum albumin (BSA) was studied at pH 6.85, using the equilibrium dialysis method. The investigation was carried out at three different temperatures, 15, 25, and 37°C. The data were analyzed by assuming two types of independent binding sites, and the binding parameters and thermodynamic parameters were determined to be as follows ; n^^-1=0.61, ΔH1=-9.72 kcal/mol, ΔS1=-19.0 (e.u.) (ΔG1=-3.93 kcal/mol at 25°C), n^^-2=1.19, ΔH2=-2.86 kcal/mol, ΔS2=1.6 (e.u.) (ΔG2=-3.34 kcal/mol at 25°C). It was also found that benzoic acid binds to BSA in competition with theophylline. The interaction of benzoic acid with BSA was studied in the same way as that of theophylline and the binding parameters and thermodynamic parameters were determined to be as follows ; n^^-1=0.91, ΔH1=-8.81 kcal/mol, ΔS1=-7.5 (e.u.) (ΔG1=-6.59 kcal/mol at 25°C), n^^-2=2.94, ΔH2=-3.32 kcal/mol, ΔS2=4.7 (e.u.) (ΔG2=-4.71 kcal/mol at 25°C). The general features of the thermodynamic parameters for both theophylline and benzoic acid were similar to those of salicylic acid. Quenching of the fluorescence of tryptophan residues on BSA in the presence of theophylline or benzoic acid was observed. On the basis of all the experimental results, it is considered that theophylline and benzoic acid bind primarily to tryptophan residue on BSA by stacking due to van der Waals forces and secondarily bind to other amino acid residues on BSA through ionic and hydrophobic interactions.
