Comparison of the binding of the dyes Sudan II and Sudan IV to bovine hemoglobin

Sudan dyes are widely used in industry, and sometimes illegally used as food additives despite their potential toxicity. In this work, the interactions of Sudan II and Sudan IV with bovine hemoglobin (BHb) were investigated by fluorescence, synchronous fluorescence, resonance light scattering (RLS), UV–vis absorption, circular dichroism (CD), and molecular modeling techniques. Binding of Sudan dyes to BHb could cause static quenching of the fluorescence, indicating changes in the microenvironment of tryptophan and tyrosine residues. The binding constants estimated for Sudan II and IV were 1.84×104 L mol−1 and 2.54×104 L mol−1, respectively, at 293 K (20 °C). Each protein molecule bound one Sudan molecule approximately. Sudan II and IV were held at the hydrophobic cavity of BHb mainly by hydrophobic interaction. The decrease of α-helix and the increase of β-sheet seen in the CD spectra revealed a conformational alteration of the protein. From all the results, we conclude that Sudan IV has a stronger impact on the structure and function of BHb than that of Sudan II.