Characterization of the myricetin-human serum albumin complex by spectroscopic and molecular modeling approaches

The interaction mechanism of flavonol myricetin (3,5,7,3',4',5'-hexahydroxyflavone) and human serum albumin (HSA) has been characterized by fluorescence, electronic absorption, and Fourier transform infrared (FT-IR) spectroscopic approaches and the molecular modeling method. The structural characteristics of myricetin and HSA were probed, and their binding affinities were determined under different pH conditions. The results showed that the binding abilities of the drug to protein decreased under lower pH conditions (pH 3.5 and 2.0) due to the alterations of the protein secondary and tertiary structures. The second derivative absorption spectra of myricetin after interacting with the protein showed that the drug existed as an anion form in the binding pocket. The fluorescence emission intensities of the normal and excited-state proton transfer (ESTP) tautomer of myricetin significantly enhanced in the presence of HSA with conspicuous shifts of the emission bands when excited with a wavelength of 370 nm, while the intensity ratios of the normal to ESTP tautomers rose rapidly with the increase of the HSA concentrations under different pH environments. This illustrated that the fluorescence emission of the normal tautomer (S1-S0, non-proton-transferred) predominated due to the interaction of drug and surrounding polar and ionic side chains of amino acid residues in the binding cavity. The similar spectroscopic properties of myricetin-HSA complex at pH 7.4 and 3.5 showed that the drug was located in subdomain IIA of the protein in the vicinity of the single Trp 214 because of the unfolding of the protein domain III in its F state. From the molecular modeling results, the drug-protein complex was stabilized by electrostatic force and hydrogen bonding with the amino acid residue in the binding pocket, which was consistent with the experimental results.