Interaction of paeonol (2'-hydroxyl-4'-methoxyacetophenone, Pae) as well as its two isomers (2'-hydroxyl-5'-methoxyacetophenone, Hma and 4'-hydroxyl-3'-methoxyacetophenone, Ace) with bovine serum albumin (BSA) in buffer solutions (pH≈7.0) has been determined with isothermal titration calorimetry at 298.15 K. Data process has been based on the supposition that there are several independent classes of binding sites on each BSA molecule for the molecules of each drug. The results obtained by using this supposition combined with Langmuir adsorption model show that there are two classes of such binding sites. The binding constants, changes of enthalpy, entropy and Gibbs free energy were obtained, which shows that the two classes of binding are mainly enthalpy driven processes. On the same class of binding site, the negative value of binding enthalpy decreased in the order of Pae, Hma and Ace. The difference of thermodynamic data was considered to be caused by the different locations of the substituting groups on aromatic phenyl ring of guest molecules. Circular dichroism (CD) spectra show that the three isomers changed the secondary structure of BSA. These results indicate that such a biomacromolecule-drug interaction includes contributions of the binding and the partial change of structure of BSA molecules induced by the three isomers.

Key words: isothermal titration calorimetry, bovine serum albumin (BSA), paeonol, CD spectra