Probing the binding of the flavonoid, quercetin to human serum albumin by circular dichroism, electronic absorption spectroscopy and molecular modelling methods.

The plant derived flavonoid compound quercetin, possesses wide range of biological activities in the human body by interacting with nucleic acids, enzymes and other proteins. As has recently been shown this molecule of polyphenolic type extensively binds to human serum albumin (HSA), the most abundant carrier protein in the blood. Electronic absorption, circular dichroism (CD) spectroscopy and molecular modelling methods were used to characterize optical properties of the quercetin-HSA complex, and to gain information on the binding mechanism at molecular level. The red shift and hypochromism of the longest-wavelength absorption band of quercetin relative to the spectral properties in ethanol suggests that one or more phenolic OH groups of the bound ligand is ionized and that the exocyclic phenyl ring is not coplanar with the benzopyrone moiety. It was found that quercetin shows extrinsic optical activity on interaction with HSA. The induced CD spectra were utilized to calculate the association constant at 37 degrees (1.46+/-0.21 x 10(4)M(-1)) and to probe the ligand binding site. Results of the CD displacement experiments performed with palmitic acid and salicylate were interpreted together with the findings of molecular modelling calculation performed on the quercetin-HSA complex. Computational mapping of possible binding sites of quercetin revealed the molecule to be bound in the large hydrophobic cavity of subdomain IIA. The protein microenvironment of this site was found to be rich in polar (basic) amino acid residues which are able to help to stabilize the negatively charged ligand bound in non-planar conformation. Additionally, the position of quercetin within the binding pocket allows simultaneous binding of other ligands such as warfarin, or sodium salycilate.