The interaction between 3-thiol-4-(2,4-dichlorobenzylideneamino)-5-methyl-4H-1,2,4-triazole (CBTZ) and bovine serum albumin (BSA) under physiological conditions was investigated by fluorescence,UV-vis absorption and circular dichroism (CD) spectroscopy as well as molecular modeling methods. The result of fluorescence experiment indicates the static quenching as a result of the formation of the CBTZ-BSA complex. The binding constants (Ka) at different temperatures were calculated according to the modified Stern-Volmer equation. The enthalpy change (-H) and entropy change (-S) were determined based on the van′t Hoff equation. Both negative-H and-S indicated that van der Waals and hydrogen-bonding forces were the dominant intermolecular forces to stabilize the CBTZ-BSA complex. Site marker competitive replacement experiments demonstrated that binding of CBTZ to BSA primarily took place in sub-domain IIA (Sudlow's site I). The binding distance (r = 7.2 nm) between CBTZ and the tryptophan residue of BSA was estimated according to the theory of fluorescence resonance energy transfer (FRET). The conformational studies by circular dichroism (CD) and three-dimensional fluorescence spectroscopy showed that the presence of CBTZ induced minor changes of the secondary structure of BSA. Molecular modeling study further confirmed the binding mode obtained experimentally.
FU JiaXinWANG KaiWeiGE YuShuJIANG FengLeiSUN XiaoHongLIU YangLIU Yi
