The interaction between a new hypoglycemic agent SU118 and BSA were studied using
absorption and
fluorescence spectrophotometry. Hypochromicity and an isobestic point at 330 nm were observed in the absorption spectra of BSA in the presence of SU118. It was found that the fluorescence intensity of BSA was efficiently quenched when SU118 was added to the BSA solution. These results showed that SU118 could interact with BSA to form a complex in solution. The
fluorescence quenching data could be fitted to the SternVolmer equation and gave a SternVolmer quenching constant of 8.63×104 L/
moL (20 ℃). The dependence of the SternVolmer constants on the temperature indicated that the mechanism of the quenching process was static. The thermodynamic parameters were estimated according to such temperature dependence. The interaction was exothermic with a Van't Hoff
enthalpy of -30.09 kJ/moL. The negative values of the enthalpy and entropy changes indicated that van der Waals force and hydrogen bonding were the predominant intermolecular forces stabilizing the SU118BSA complex. In addition, binding constants were obtained by two methods: 1.32×104 L/moL for absorption
titration and 8.63×104 L/moL for fluorescence titration. They were comparable regarding the difference between the two methods. Finally, the conformational change of BSA due to the addition of SU118 was discussed by synchronous fluorimetry.
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