Spectroscopic and molecular docking studies of the interactions of sunset yellow and allura red with human serum albumin

Binding interactions of human serum albumin (HSA) with sunset yellow (SY) and allura red (AR), two food colorants, were investigated at the molecular level through fluorescence and UV absorption as well as molecular docking. The collective results of the study under the simulated physiological conditions proposed a static type of binding occurring between the two dyes and HSA. When compared with AR (293 K: Ksv = (4.21 ± 0.36) × 10⁴ L·mol^?1; K_b = (0.30 ± 0.23) × 10⁶ L·mol^?1), SY (293 K: Ksv = (6.80 ± 0.10) × 10⁴ L·mol^?1; K_b = (3.11 ± 2.01) × 10⁶ L·mol^?1) had stronger quenching ability and higher affinity for HSA due to less steric hindrance. It can be deduced that the energy transfer from HSA to the two dyes occurred with high probability based on the Förster resonance energy transfer theory (r < 7 nm, 0.5 R₀ < r < 2.0 R₀). The spectral analysis suggested that the formation of the dye-HSA complex resulted in the change in microenvironment around Tyr and Trp residues and in the secondary structure of the protein. According to molecular docking simulation, the two structural analogs almost bound to the same site of HSA, near Sudlow's Site I, but significant difference existed in the number and location of hydrogen bond (H-bond) formed between the dyes and HSA. From the molecular docking along with the thermodynamic parameters (AR: ΔH^o = ?(58.79 ± 15.24) kJ·mol^?1, ΔS^o = ?(115.1 ± 31.10) J·mol^?1·K^?1; SY: ΔH^o = ?(52.24 ± 3.15) kJ·mol^?1, ΔS^o = ?(50.07 ± 11.14) J·mol^?1·K^?1), it could be inferred that H-bond and van der Waals forces were the major binding forces involved in formation of the dye-HSA complexes.