Quercetin and DNA in solution: analysis of the dynamics of their interaction with a linear dichroism study.

Flow-Linear Dichroism (LD) spectra on quercetin-DNA solutions (buffer-ethanol 30%) showed evidence that the flavonol can intercalate the biopolymer. There is no electrostatic component in the formation of the quercetin-DNA complex. The DNA concentration and the planarity of the chromophore are limiting factors in the interaction. There are no induced LD signals for concentrations of the biopolymer less than 7.8 x 10(-3) M phosphate. The interaction is most probably of a hydrophobic nature between the most hydrophobic segment of the quercetin (benzopyran-4-one) and the intercalation site, which allows the chromophore to penetrate the DNA helix and to arrange its planar structure more or less parallel to the adjacent planes of the nitrogenous bases. A comparison between the planar and hydrophobic flavonol quercetin, and the non-planar and hydrophilic flavanone dihydroquercetin, showed that the interaction of the latter with DNA was strongly limited. The notable biological activity of the quercetin compared to the 'weaker' activity of the dihydroquercetin could also be derived from the different planarity (and probably hydrophobicity) of the two flavonoids. The very low concentration of the quercetin-DNA complex was efficiently shown by the high sensitivity of the LD technique, whereas it could not be resolved by isotropic UV-Vis and induced circular dichroism spectra. The hypothesis of a frame shift mutagenicity activity of quercetin (Science 1977; 197: 577-578) is highly improbable. In fact, the affinity of quercetin for DNA, which emerges from this study, is very low compared with that of a typical intercalator agent (Q. Rev. Biophys. I 1992; 25: 51-170).