The Flow of the Redox Energy in Quercetin during Its Antioxidant Activity in Water

Abstract

Most studies on the antioxidant activity of flavonoids like Quercetin (Q) do not consider that it comprises a series of sequential reactions. Therefore, the present study examines how the redox energy flows through the molecule during Q’s antioxidant activity, by combining experimental data with quantum calculations. It appears that several main pathways are possible. Pivotal are subsequently: deprotonation of the 7-OH group; intramolecular hydrogen transfer from the 3-OH group to the 4-Oxygen atom; electron transfer leading to two conformers of the Q radical; deprotonation of the OH groups in the B-ring, leading to three different deprotonated Q radicals; and finally electron transfer of each deprotonated Q radical to form the corresponding quercetin quinones. The quinone in which the carbonyl groups are the most separated has the lowest energy content, and is the most abundant quinone. The pathways are also intertwined. The calculations show that Q can pick up redox energy at various sites of the molecule which explains Q’s ability to scavenge all sorts of reactive oxidizing species. In the described pathways, Q picked up, e.g., two hydroxyl radicals, which can be processed and softened by forming quercetin quinone.

Current Trends in Computational Quantum Chemistry Studies on Antioxidant Radical Scavenging Activity

“…This threshold is proposed 46 , 101 , 112 because a small energy difference between conformers could indicate an interconversion process: rotatability of OH groups 165 or side chains, 84 bonds deformation due to keto–enol tautomery, 21 shift in E/Z-conformers equilibria, 133 and bending of dihedral angles. 16 , 20 , 28 , 33 , 50 , 67 , 128 , 165 They can all modulate hydrogen bonds, electrons cloud delocalization, and polarizability, as well as radical accessibility to specific sites of an antioxidant. Furthermore, the molar fraction cannot be too low if the compound is desired to pass biological barriers.…”

“…Phytochemicals are plant-derived compounds that are plentiful in many herbs and commonly consumed plants such as beetroot, high in betalains; tea, rich in catechins; or grapefruit, abundant in flavanons . This family can be further subdivided into flavonoids, ,,− ,,− ,− phenolic acids, − lignans, aurones, chalcones, − curcuminoids, , anthocyanidins, − stilbenoids; ,− anthraquinones, − glucosinolates, , alkaloids, , coumarins, terpenes and terpenoids, − and others ,,− ,− (Figure ), all of them being extensively studied with computational quantum chemistry methods, as evidenced by the number of recent scientific findings. Albeit, plants are not the only source of antioxidants substances with promising antiradica...…”

Flavonoids Seen through the Energy Perspective

“…This can be used to determine where the energy taken up by the radical scavenging is concealed in a flavonoid molecule. Moreover, it can give insight into how the energy flows through the molecule, and how the energy is modulated during this process [ 36 , 37 ]. One of the major restrictions is that it is still not possible to accurately determine the impact of the solvent on the redox behavior of flavonoids.…”

“…When a closer look is given, one can see through the Yin and Yang and notice its connection to life. The figure is taken from the thesis of Zhengwen Li [ 36 , 37 ].…”

The radicals of quercetin-derived antioxidants in Triton X-100 micelles

“…10 The five hydroxy groups of quercetin theoretically allow five oxygen-centered neutral radicals and ten radical anions; monosubstitution at oxygen reduces these numbers to four and six, and disubstitution to three. Quantum-mechanical calculations abound, [11][12][13][14][15][16] but many of the radicals lie too close in energy for reliable predictions. Instead, we have resorted to an experimental approach based on blocking oxygens through alkylation.…”