Protic equilibria as the key factor of quercetin emission in solution. Relevance to biochemical and analytical studies.

A detailed spectrofluorimetric study on quercetin in aqueous solution proves that its anionic forms are responsible for a strong fluorescence enhancement observed at pH > 6. Anion fluorescence is also observed in organic solvents with strong hydrogen bond acceptor properties. The results provide a new interpretation of biophysical and analytical literature data where the fluorescence of the anionic forms of quercetin has never been explicitly taken into account. In particular, comparison with published binding studies strongly suggests that quercetin deprotonation and consequent fluorescence enhancement takes place when the flavonoid binds to several biological macromolecules. This observation can be very helpful in the understanding at a molecular level of the interaction between quercetin and the biomolecule.