Complexity associated with caprylate binding to bovine serum albumin: Dimerization, allostery, and variance between the change in free energy and enthalpy of binding.

Isothermal titration calorimetry (ITC), differential scanning calorimetry (DSC), and pressure perturbation calorimetry (PPC) were used to study different aspects of the diverse interaction between the fatty acid caprylate and bovine serum albumin (BSA). The ITC thermogram was consistent with exothermic binding to a single site on BSA, which was electrostatic but had little or no hydrophobic contribution. ITC revealed that small changes to solution conditions and temperature were associated with apparent enthalpy-entropy compensation, causing large changes in enthalpy (ΔH) during binding, but with little corresponding changes in free energy (ΔG). ITC also detected a slower endothermic interaction at a low mole ratio. Dynamic light scattering suggested that this was due to dimerization or similar self-association. DSC demonstrated that further interactions took place at higher mole ratios. This was consistent with weak binding of caprylate to multiple binding sites which had a considerable impact on the structural conformation of BSA. PPC showed that the conformational change of BSA was accompanied with a reduction in surface hydrophobicity of the protein. PPC also demonstrated that in solution caprylate's hydrocarbon tail is hidden from water as no clathrate-like water is evident, which is consistent with the lack of hydrophobic contribution during binding. Cumulatively, the three calorimetric techniques offer a comprehensive view of caprylate and BSA interactions, highlighting the role of electrostatic interaction in binding accompanied by probably dimerization and considerable structural change associated with weaker binding to BSA.