Binding of triclosan to human serum albumin: insight into the molecular toxicity of emerging contaminant.

PURPOSE: The interaction between triclosan (TCS) and human serum albumin (HSA) was investigated in order to obtain the binding mechanism, binding constant, the type of binding force, the binding distance between the donor and acceptor, and the effect of TCS on the conformation change of HSA.
METHODS: A HSA solution was added to the quartz cell and then titrated by successive addition of TCS. The fluorescence quenching spectra and synchronous spectra were recorded with the excitation and emission slits of the passage of band set at 10 and 20 nm. Three-dimensional fluorescence spectra of HSA were recorded before and after the addition of TCS. The capillary electrophoresis was conducted with the pressure injection mode at 0.5 psi for 5 s, separation under 25 kV, and detection at 214 nm.
RESULTS: Fluorescence data indicated the fluorescence quenching of HSA by TCS was static quenching, and the quenching constants (K ( a )) were 1.14 × 10(5), 8.75 × 10(4), 6.67 × 10(4), and 5.00 × 10(4) at 293, 298, 303, and 309 K, respectively. The thermodynamic parameters, enthalpy change (ΔH) and entropy change (ΔS) for the interaction were calculated to be -37.9 kJ mol(-1) and 32.6 J mol(-1) K(-1). The binding distance between TCS and tryptophan residues of HSA was obtained to be 1.81 nm according to Fǒrster nonradioactive energy transfer theory. The UV-Vis absorption spectroscopy, the synchronous fluorescence spectroscopy, three-dimensional fluorescence spectroscopy, and circular dichroism spectroscopy revealed the alterations of HSA secondary structure in the presence of TCS. Finally, the interaction between TCS and HSA was further confirmed by capillary electrophoresis.
CONCLUSIONS: TCS was bound to HSA to form the TCS-HSA complex, with the binding distance of 1.81 nm. Hydrophobic interaction and hydrogen bond were dominated in the binding. TCS could change the secondary conformation of HSA. This work provides an insight into noncovalent interaction between emerging pollutants and protein, helping to elucidate the toxic mechanism of such pollutants.