The presence of binding sites of human serum albumin makes it possible to combine many drugs target. The efficacy of tubeimosideII (TBMSII), isolated from Bolbostemma paniculatum (Maxim), binding to human serum albumin (HSA) is critical for pharmacokinetic behavior of TBMSII. The interactions between TBMSII and HSA under simulative physiological conditions were investigated by the methods of fluorescence spectroscopy. Fluorescence data revealed that the fluorescence quenching of HSA by TBMSII was the result of the formation of the TBMSⅡ-HSA complex. According to the modified Stern-Volmer equation, the binding constants (K_a) between TBMSⅡ and HSA at four different temperatures (293, 298, 303, 308 K) were 1.002×10⁵, 0.701×10⁵, 0.514×10⁵, 0.411×10⁵ L•mol^－1 respectively. The thermodynamic parameters, enthalpy change (ΔH) and entropy change (ΔS) for the reaction were calculated to be －44.829 kJ•mol^－1 and －57.497 J•mol^－1•K^－1 according to vant Hoff equation, indicating that the hydrogen bonds and hydrophobic interactions play a dominant role in the binding of TBMSII to HSA. Site marker competitive experiments indicated that the binding of TBMSⅡ to HSA primarily took place in sub-domain IIA, which was in good agreement with the results of molecular modeling study. The distance r between donor (HSA) and acceptor (TBMSII) was obtained to be 4.95 nm according to Försters non-radioactive energy transfer theory. The conformational investigation showed that the presence of TBMSII decreased the α-helical content of HSA (from 22.7% to 19.58%) and induced the slight unfolding of the polypeptides of protein, which confirmed some micro-environmental and conformational changes of HSA molecules.

Key words: tubeimosideII, human serum albumin, fluorescence quenching, circular dichroism, molecular modeling