Spectroscopic studies of the interaction of anti-coagulant rodenticide diphacinone with human serum albumin

The interaction of diphacinone with human serum albumin (HSA) was studied by the methods of fluorescence and Fourier transform infrared (FT-IR) spectroscopy under simulative physiological conditions. Diphacinone can strongly quench the intrinsic fluorescence of HSA by static quenching. The apparent binding constants KT at four different temperatures (291, 300, 310, and 318 K) were obtained according to Scatchard procedure. The Stern-Volmer and Scatchard plots both had an intersection at Cdiphacinone/CHSA≈1.2, which indicated that diphacinone binded to different binding sites on HSA. The analytical results of fluorescence data showed when Cdiphacinone/CHSA was lower than 1.2, the numbers of binding sites were near 1.1, and Cdiphacinone/CHSA was higher than 1.2, the number of binding sites was approximately 1.8. The FT-IR spectra had proved that the secondary structure of HSA changed after interacting with diphacinone in aqueous solution. The thermodynamic parameters were calculated by van't Hoff equation. The enthalpy change (ΔH0) and entropy change (ΔS0) were −14.59 kJ mol−1 and 62.49 J mol−1 K−1, respectively. The results suggested that the hydrophobic interaction might play a main role in the interaction of diphacinone with the HSA. The binding distance (r) between diphacinone and tryptophan in HSA was obtained according to the Förster energy transfer theory. Furthermore, the study of molecular modeling indicated that diphacinone could bind to the site I of HSA and hydrophobic interaction was the major acting force, which was in agreement with the binding mode study.