Characterization of the interactions of human serum albumin with carmine and amaranth using multi-spectroscopic techniques and molecular docking

The binding of human serum albumin (HSA) with two food pigments, carmine and amaranth, were investigated at the molecular level using fluorescence, ultraviolet absorption and molecular docking techniques. Under simulated physiological conditions, the study proposes that the type of binding between the two dyes and HSA was static. Compared with carmine (293 K: KSV = (0.36 ± 0.02) × 105 L mol−1; Kb = (1.28 ± 1.21) × 107 L mol−1), amaranth (293 K: KSV = (1.62 ± 0.06) × 105 L mol−1; Kb = (4.07 ± 2.83) × 107 L mol−1) had a stronger quenching ability and higher affinity for HSA due to its more symmetrical stereochemical structure and less steric hindrance. From Förster’s nonradiative energy transfer theory (r < 7 nm, 0.5 R0 < r < 2.0 R0), it was inferred that energy transfer from HSA to the two dyes was highly probable. Spectral analysis (synchronous fluorescence, three-dimensional fluorescence and ultraviolet spectroscopy) showed that the binding of carmine or amaranth to HSA caused changes in the microenvironment around the Tyr and Trp residues and the secondary structure of HSA. A molecular docking simulation suggested that the two isomers bound to almost the same location of HSA, close to Sudlow’s site I, but there was a significant difference in the orientation of the two molecules. Combining the molecular docking and the thermodynamic parameters (Carmine: ΔH = − (50.03 ± 13.37) kJ mol−1, ΔS = − (52.01 ± 29.79) J mol−1 K−1; Amaranth: ΔH = − (72.23 ± 8.46) kJ mol−1, ΔS = − (109.75 ± 17.11) J mol−1 K−1), it can be inferred that hydrogen bonds and van der Waals forces play a dominant role in the formation of the dye–HSA complex.