The complexity of condensed tannin binding to bovine serum albumin - An isothermal titration calorimetry study

Isothermal titration calorimetry was applied to study the binding of purified proanthocyanidin oligomers to bovine serum albumin (BSA). The molecular weight of the proanthocyanidin oligomer had a major impact on its binding to BSA. The calculated change in enthalpy (Delta H) and association constant (K-a) became greater as the oligomer size increased then plateaued at the heptameric oligomer. These results support a model for precipitation of proteins by proanthocyanidin where increased oligomer size enhanced the opportunity for cross linkages between proteins ultimately forming sediment-able complexes. The authors suggest tannin binding to proteins is opportunistic and involves multiple sites, each with a different K-a and Delta H of binding. The Delta H of binding comprises both an endothermic hydrophobic interaction and exothermic hydrogen bond component. This suggests the calculated entropy value (Delta S) for tannin-protein interactions is subject to a systematic error and should be interpreted with caution. (C) 2015 Elsevier Ltd. All rights reserved.