Study on the Interaction between Chlortetracycline and Bovine Serum Albumin

The reactionmechanismand the thermodynamic characteristics between an antibiotics drug (chlortetracycline,CTC) and bovine serumalbumin (BSA) were investigated bythe fluorescence spectra and flowmicrocalorimetry in aTrisHCl buffer solution (pH=7.0, made isotonicwith sodium chloride) at 25℃. Itwasmanifested that CTC had a powerful ability to quench the intrinsic fluorescence of BSAmainly via a static quenching but CTC itself had no emission fluorescence for the 300500 nm range in wavelength. A equation which conld be used to determine the binding parameters of small molecule ligand binding to bio-macromolecule had been presented based on the site binding model and florescence quenching. Furthermore, the apparent binding constantKwas found to be 1.20×105L/mol and the binding sitesnto be 1.75 through the equation, and the thermodynamic parameters were determined to be the molar change of enthalpy △rHm=-17.50 kJ/mol, the molar change ofGibbs function△rGm=-30.37 kJ/mol and the molar change of entropy △rSm=43.17 J/mol K by theKvalue and flowmicrocalorimetry for the reaction. It may be suggested that the interaction between chlortetracycline and BSA is strong and the main binding force is hydrophobic interaction according to the thermodynamic parameters. By using a spectra overlap integral between the absorption spectrum of chlortetracycline and the emission spectrum of BSA, the distances (r1=1.67 nm,r2=1.46 nm) of Trp-212 residue to the binding sites of drug were estimated and the transfer efficiency (E1=0.41, E2=0.66) between chlortetracycline and BSAwas also obtained from the theory of forster non-radiation energy transfer theory. Fromthese distances, the region and binding sits of drug in BSA can be determined and an allosteric domain model of CTC-BSA complex was postulated from the above results.