Abstract
The quercetin molecule is known to be an effective pharmaceutical compound of a plant origin. Its chemical structure represents two aromatic A and B rings linked through the C ring containing oxygen and five OH hydroxyl groups attached to the 3, 3′, 4′, 5, and 7 positions. In this study, a novel conformational mobility of the quercetin molecule was explored due to the turnings of the O3′H and O4′H hydroxyl groups, belonging to the B ring, around the exocyclic C-O bonds. It was established that the presence of only three degrees of freedom of the conformational mobility of the O3′H and O4′H hydroxyl groups is connected with their concerted behavior, which is controlled by the non-planar (in the case of the interconverting planar conformers) or locally non-planar (in other cases) TSsO3′H/O4′H transition states, in which O3′H and O4′H hydroxyl groups are oriented by the hydrogen atoms towards each other. We also explored the number of the physico-chemical and electron-topological characteristics of all intramolecular-specific contacts—hydrogen bonds and attractive van der Waals contacts at the conformers and also at the transition states. Long-terms perspectives for the investigations of the structural bases of the biological activity of this legendary molecule have been shortly described.
Highlights
A previously suggested idea has been completely confirmed by careful QM investigation—we have identified for the first time the aforementioned pathways of the conformational variability of the quercetin molecule and documented their structural properties, including symmetrical, polar, energetical, and kinetic characteristics, which are quite important for the understanding of the structural grounds of the biological activity of the quercetin molecule
We logically continued to investigate the conformational mobility [16,17,18] of the quercetin molecule and extend this approach to the rotations of the hydroxyl groups in the 30 and 40 positions, which are carefully presented in Tables 1–3 and Figures 1–3
In this study, which is a logical continuation of our previous works on this topic [16,17,18,19,20,21], new pathways of the transformations of the conformers of the quercetin molecule into each other were found, which occurred due to the torsional mobility of the O30 H and O40 H hydroxyl groups
Summary
The quercetin molecule (3, 30 , 40 , 5, 7—pentahydroxyflavone, C15 H10 O7 ) is an important flavonoid compound, which is found in many foods and plants, in particular in honey [1], and is known to act as a natural drug molecule with a wide range of treatment properties—antioxidant, anti-toxic, etc.—and is involved in drug delivery from the site of administration to the therapeutic target [2,3,4,5,6,7,8,9].The structure of the quercetin contains two aromatic A and B rings linked through the C ring containing oxygen and five OH hydroxyl groups attached to the 3, 30 , 40 , 5 and 7 positions (see Scheme 1) [10,11,12,13,14,15].In a previous study [16], by using the quantum-mechanical (QM) calculations at the MP2/6-311++G(2df,pd)//B3LYP/6-311++G(d,p) level of QM theory together with Bader’s “QuantumTheory of Atoms in Molecules”, for the first time, all possible conformers were established, corresponding to local minima on the potential energy hypersurface of the isolated quercetin molecule.Appl. The structure of the quercetin contains two aromatic A and B rings linked through the C ring containing oxygen and five OH hydroxyl groups attached to the 3, 30 , 40 , 5 and 7 positions (see Scheme 1) [10,11,12,13,14,15]. In a previous study [16], by using the quantum-mechanical (QM) calculations at the MP2/6-311++G(2df,pd)//B3LYP/6-311++G(d,p) level of QM theory together with Bader’s “Quantum. Theory of Atoms in Molecules”, for the first time, all possible conformers were established, corresponding to local minima on the potential energy hypersurface of the isolated quercetin molecule.
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