Conformational diversity of the quercetin molecule: a quantum-chemical view

This paper focuses on the comprehensive conformational analysis of the quercetin molecule with a broad range of the therapeutic and biological actions. All possible conformers of these molecule, corresponding to the local minima on the potential energy hypersurface, have been obtained by the sequential rotation of its five hydroxyl groups and also by the rotation of its (A + C) and B rings relatively each other. Altogether, it was established 48 stable conformers, among which 24 conformers possess planar structure and 24 conformers - nonplanar structure. Their structural, symmetrical, energetical and polar characteristics have been investigated in details. Quantum-mechanical calculations indicate that conformers of the quercetin molecule are polar structures with a dipole moment, which varies within the range from 0.35 to 9.87 Debay for different conformers. Relative Gibbs free energies of these conformers are located within the range from 0.0 to 25.3 kcal·mol^-1 in vacuum under normal conditions. Impact of the continuum with ε = 4 leads to the decreasing of the Gibbs free energies (-0.19-18.15 kcal·mol^-1) and increasing of the dipole moment (0.57-12.48 D). It was shown that conformers of the quercetin molecule differ from each other by the intramolecular specific contacts (two or three), stabilizing all possible conformers of the molecule - H-bonds (both classical ОН…О and so-called unusual С'Н…О and ОН…С') and attractive van-der-Waals contacts О…О. Obtained conformational analysis for the quercetin molecule enables to provide deeper understanding of the 'structure-function' relationship and also to suggest its mechanisms of the therapeutic and biological actions.Communicated by Ramaswamy H. Sarma.