The crystal and molecular structure of quercetin: A biologically active and naturally occurring flavonoid

Abstract

The crystal and molecular structure of quercetin, one of the bioflavonoids found in plants, is described. The structure is of interest because of the variety of biological systems affected by it. It is known to be an antitumor agent and to exhibit antiallergenic and anti-inflammatory activity. it is an antioxidant and a cardiostimulant. These biological functions are governed by a number of enzymes and so quercetin's interactions with these enzymes are well studied. Crystals suitable for X-ray diffraction were difficult to obtain. Crystal data are as follows: a = 13.060(5), b = 16.564(7), c = 3.725(2) A ̊ , α = 92.05(4), β = 94.39(3), γ = 120.55(3), V = 689.4(5) A ̊ 3 , z = 2, space group P 1 , D c = 1.63 g cm −3, D m = 1.69(1) g cm −3. Quercetin crystallizes with two waters of crystallization that participate in an extended hydrogen bonding network through the crystal lattice. There are intramolecular hydrogen bonds between two hydroxyl groups and the exocyclic oxygen, O4. Quercetin, in the crystal structure, exists as hydrogen-bonded dimers packing almost perpendicular to c. These dimers form a two-dimensional net, mostly in the ab plane, connected via water molecules. Additionally, the waters provide for hydrogen bonding in the z direction. The exocyclic oxygen appears to be the focal point of cohesive forces in the crystal structure. Its electron-withdrawing effects are stabilized by the high degree of hydrogen bonding. This, in turn, leads the exocyclic phenyl ring to exist in an almost planar conformation with respect to the rest of the molecule. The torsion angle between the phenyl ring and the pyrone ring is only 7°. As expected, from an analysis of the atomic charges, it is clear that quercetin has more polar groups than similar flavonoid compounds. Comparison with these structures is made.