Targeting Proteins with Toxic Azo Dyes: A Microcalorimetric Characterization of the Interaction of the Food Colorant Amaranth with Serum Proteins

The interaction of amaranth with two homologous serum albumins from human and bovine (HSA and BSA) was studied by microcalorimetry. The binding stoichiometry for the complexation of amaranth to both BSA and HSA was around 1, and the equilibrium constants were (5.79 +/- 0.07) x 10(5) and (1.76 +/- 0.05) x 10(5) M-1 respectively. The binding reaction to HSA at 298.15 K was driven by a large negative enthalpic contribution and a small but positive entropic contribution, while to BSA, it was entirely enthalpy-driven and the entropic contribution was unfavorable. Parsing of the standard molar Gibbs energy revealed that the complexation was dominated by non-polyelectrolytic forces. Temperature-dependent isothermal titration calorimetry studies revealed that the enthalpic contribution increased and the entropic contribution decreased with the rise in the temperature but the Gibbs energy change remained almost unaltered. Differential scanning calorimetry results revealed that the binding reaction stabilized the serum albumins significantly against thermal unfolding.