Abstract: The interaction between ursolic acid (UA) and serum albumin, namely bovine serum albumin (BSA) and human serum albumin (HSA) respectively has been studied by fluorescence spectroscopy. Based on the method of florescence quenching, the binding constants (K) for UA with BSA/HSA were determined to be K_UA-BSA=0.335 7×10⁵ L/mol and K_UA-HSA=0.244 3×10⁵ L/mol, respectively. The binding distance(r)and energy-transfer efficiency(E)between UA and BSA/HSA were also obtained by virtue of the Fōrster theory of non-radiation energy transfer to be as follows: r_UA-BSA=0.73 nm, r_UA-HSA=0.88 nm, E_UA-BSA=0.17, E_UA-HSA=0.18. The effect of UA acting on the conformation of BSA/HSA analyzed by synchronous fluorescence spectroscopy was not significant. The unfolding procedure of BSA/HSA induced by UA was analyzed by fluorescence phase diagram to be a two-state model. In the presence of Ni(Ⅱ), interaction between UA and BSA/HSA was explored to be somewhat different.

Key words: ursolic acid, serum albumin, fluorescence spectroscopy, UV absorption spectroscopy