Flavonoid baicalein (BAI) has wide biological and pharmacological activities, and investigation on the interaction between BAI and protein plays a key role in understanding its pharmacology. The complexes of BAI with various isomers of human serum albumin (HSA) were characterized by UV absorption and fluorescence spectroscopic approaches. The UV absorption spectra of BAI were significantly changed in weak base condition, signifying that the OH group in the A ring of BAI has been dissociated, while the structure of BAI remained unchanged in the pH range from 4.5 to 2.0. The UV absorption band I of BAI has an obvious red-shift after interaction with various isomers of HSA, indicating that a specific interaction has occurred between BAI and the protein. The fluorescence quenching processes of HSA induced by BAI mainly arose from static quenching, and their binding constants were calculated. The results showed that the binding constants of BAI with HSA decreased with the decrease of the buffer pH values, which probably originated from the changes of the protein structures. The fluorescence emission band of BAI has been significantly enhanced after interaction with different isomers of HSA. From above results, it can be concluded that BAI formed complexes with various isomers of HSA, and the binding site of BAI was located on the sub-domain IIA of HSA, nearing the TRP-214 amino acid residue. Combining the computer molecular modeling, the binding mode between the drug and protein has been discussed.

Key words: baicalein, human serum albumin, isomer, UV absorption, fluorescence