The interaction between bovine serum albumin (BSA) and such a natural pharmaceutical homologue as coumarin, umbelliferone and aesculetin was investigated using fluorescence spectroscopy (FS) and ultraviolet spectroscopy (UV).The experimental results showed that the homologues inserted into the hydrophobic pockets of BSA, quenching the inner fluorescence of BSA by forming the pharmaceutical-BSA complex.Both static quenching and nonradiative energy transferring were confirmed to result in the fluorescence quenching.It was found that the enlargement of molecular polarity and volume of the pharmaceutics caused the increment of the quenching efficiency, the stereo-distance (r) and the apparent binding constant (K_A) between pharmaceutical molecule and BSA, and the decrement of binding sites (n) of pharmaceutical molecule on BSA.The process of binding homologue molecule on BSA was a spontaneous molecular interaction procedure in which entropy increased and Gibbs free energy decreased.The interaction between the coumarin and BSA was driven mainly by hydrophobic force whereas both hydrophobic force and dipole-dipole force coexisted in umbelliferone-BSA and aesculetin-BSA systems, which indicated that the driving forces of the pharmaceutical-BSA interaction changed with the molecular polarity of pharmaceuticals, too.

Key words: natural pharmaceutical, homologue of coumarin, bovine serum albumin, fluorescence spectroscopy