Quercetin (QUE), rutin (RUT) and catechin (CAT) are main active components of Apocynum venetum L. The binding mechanisms of these active components to human serum albumin (HSA) have been investigated utilizing fluorescence and UV absorption spectra. The results revealed that the fluores-cence quenching arose mainly from static quenching by complex formation when the c_QUE/c_HSA≤3.5, and the proportion of dynamic quenching increased in higher drug concentration; while the quenching mechanism was mainly static in the drug con-centration range studied for RUT. However, CAT cannot form complex with HSA. The binding site number was one for QUE and RUT, and the binding constants were (1.51±0.13)×10⁵ and (0.81±0.08)×10⁵ L•mol^－1, respectively. The intrinsic fluorescence of QUE and RUT conspicuously enhanced in the presence of HSA due to excited-state proton transfer (ESPT) and it further confirmed the complex formation of HSA with QUE and RUT, individually. The UV absorp-tion bands of QUE significantly red-shifted after interacting with HSA, which signified that the phenol group dissociated during the QUE-protein binding process and the binding was driven by electrostatic force. However, the combination of RUT and HSA did not induce obvious red shift of UV absorption bands of RUT, and their binding force originated probably from the hydrogen bonding between RUT and HSA. Based on the second derivative UV absorption spectra, the binding modes of QUE and RUT were discussed.

Key words: active components of Apocynum venetum L., fluorescence spectra, UV absorption spectra, second derivative spectra