| Abstract: | The Kinetics of the binding of librium to bovine serum albumin were investigated at an ionic strength of 0.15 M, in the pH-range between 5.5 and 10.5, using the temperature-jump method. Two relaxation times were observed. The first, rapid one is assigned to the equilibrium between reactants and complex, whereas the second, slower relaxation time measures a conformational change of this complex. Both relaxation times are independent of pH up to about 8.0 when they increase sharply to a new plateau at a pH of about 9.3. A possible parallelism between this behavior and the N ? B transition of albumin is discussed. The overall equilibrium constant K, as well as the equilibrium constant between the two conformers, K2, were estimated from our kinetic results. Whereas the change of K with changing pH was within the experimental error, the equilibrium between the two conformers of the complex is somewhat shifted by an increase of the pH. At all values of pH, however, they are present in comparable concentrations. The value of K was also measured spectrophotometrically. The results of the two methods showed reasonable agreement. The relaxation times were affected very little by a change in temperature between 12 and 25°C from which it is concluded that the isomerization has a high negative value of ΔS?. |
