Elucidating the energetics of the interaction of non-toxic dietary pigment curcumin with human serum albumin: A calorimetric study

• Curcumin binds to HSA with affinity of the order of 105 M−1.
• The binding was favoured by negative enthalpy and positive entropy changes.
• The binding was dominated by non-polyelectrolytic forces.
• Negative heat capacity value along with enthalpy–entropy compensation was observed.

Thermodynamic quantities for the interaction of the anticancer dietary pigment curcumin with human serum albumin were measured by using isothermal titration calorimetry. The equilibrium constant of the complex formation at T = 293.15 K was found to be (5.25 ± 0.05) 105 M−1. The binding was exothermic with TΔS0 = (24.82 ± 0.01) kJ · mol−1, where ΔS0 is the standard molar entropy change and ΔHo = −(7.28 ± 0.04) kJ · mol−1, where ΔHo is the standard molar enthalpy change. The stoichiometry of binding was established to be 1:1. The equilibrium constant decreased with increasing Na+ concentration. The equilibrium constant decreased from (5.25 ± 0.05) · 105 M−1 to (2.88 ± 0.03) · 105 M−1 by increasing the salt concentration from (10 to 50) mM. Both polyelectrolytic and non-polyelectrolytic forces contributed to the standard molar Gibbs free energy change. However the contribution from the latter was dominant and almost invariant at all Na+ concentrations. The negative standard molar heat capacity change along with significant enthalpy–entropy compensation suggests the involvement of multiple weak non-covalent forces in the binding process.

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