Intramolecular tautomerization of the quercetin molecule due to the proton transfer: QM computational study

Quercetin molecule (3, 3′, 4′, 5, 7-pentahydroxyflavone, C(15)H(10)O(7)) is an important flavonoid compound of natural origin, consisting of two aromatic A and B rings linked through the C ring with endocyclic oxygen atom and five hydroxyl groups attached to the 3, 3′, 4′, 5 and 7 positions. This molecule is found in many foods and plants, and is known to have a wide range of therapeutic properties, like an anti-oxidant, anti-toxic, anti-inflammatory etc. In this study for the first time we have revealed and investigated the pathways of the tautomeric transformations for the most stable conformers of the isolated quercetin molecule (Brovarets’ & Hovorun, 2019) via the intramolecular proton transfer. Energetic, structural, dynamical and polar characteristics of these transitions, in particular relative Gibbs free and electronic energies, characteristics of the intramolecular specific interactions–H-bonds and attractive van der Waals contacts, have been analysed in details. It was demonstrated that the most probable process among all investigated is the proton transfer from the O3H hydroxyl group of the C ring to the C2′ carbon atom of the C2′H group of the B ring along the intramolecular O3H…C2′ H-bond with the further formation of the C2′H(2) group. It was established that the proton transfer from the hydroxyl groups to the carbon atoms of the neighboring CH groups is assisted at the transition states by the strong intramolecular HCH…O H-bond (~28.5 kcal∙mol(-1)). The least probable path of the proton transfer–from the C8H group to the endocyclic O1 oxygen atom–causes the decyclization of the C ring in some cases. It is shortly discussed the biological importance of the obtained results.