Studies of the interaction between Sudan I and bovine serum albumin by spectroscopic methods

The interaction of Sudan I (SD) and bovine serum albumin (BSA) under physiological condition was investigated by the methods of fluorescence, UV–vis absorbance and circular dichroism (CD) spectroscopy. Fluorescence data revealed that the fluorescence quenching of BSA by SD was the result of the formation of BSA–SD complex, and the effective quenching constants (Ka) were 4.457 × 104, 4.113 × 104, 3.642 × 104 and 3.488 × 104 L mol−1 at 292, 298, 304 and 310 K, respectively. The thermodynamic parameters, enthalpy change (ΔH) and entropy change (ΔS) for the reaction were calculated to be −10.76 kJ mol−1 and 53.97 J mol−1 K−1 according to van’t Hoff equation. The results indicated that the hydrophobic force was the dominant intermolecular force in stabilizing the complex. The distance r between donor (BSA) and acceptor (SD) was obtained to be 4.59 nm according to Förster’s non-radioactive energy transfer theory. After the addition of SD, the synchronous fluorescence spectral results showed that the hydrophobicity of amino acid residues increased and the CD spectral results showed that the α-helix content of BSA decreased (from 63.60% to 50.17%). These revealed that the microenvironment and conformation of BSA were changed in the binding reaction.