Isothermal titration calorimetry binding properties of Cibacron Blue F3GA in complex with human serum albumin

Binding interactions between Cibacron Blue-F3GA (CB-F3GA) and human serum albumin (HSA, at physiologically ten-fold lower concentration) was studied by isothermal titration calorimetry (ITC) and in-silico docking computations. ITC experiments revealed two separate binding sites on HSA with different binding affinities for CB-F3GA. The high-affinity binding site (PBS-II) on HSA binds CB-F3GA at nanomolar scale (K_D1  = 118 ± 107 nM) with favorable binding enthalpy (ΔH^o ₁  = - 6.47 ± 0.44 kcal/mol) and entropy (-TΔS^o ₁  = -2.98 kcal/mol) energies. CB-F3GA binds to the low-affinity binding site (PBS-I) at μM scale (K_D2  = 31.20 ± 18.40 μM) with favorable binding enthalpy (ΔH^o ₁  = - 5.03 ± 3.86 × 10^-2 kcal/mol) and entropy (-TΔS^o ₁  = -1.12 kcal/mol) energies. ITC binding data strongly suggest that CB-F3GA binding to PBS-II site increases the formation of dimeric-HSA clusters (N₁  = 2.43 ± 0.50), while binding to PBS-I leads to tetrameric-HSA clusters (N₂  = 4.61 ± 0.90). These results suggest that a higher degree of HSA aggregation upon drug binding may be expected under physiological conditions, a notion that should be further investigated for the delivery and toxicity of drug-HSA interactions.