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.
Highlights
In all life forms, opposing forces provide the energy that flows through networks in an organism, which fuels life [1]
To determine which mechanism is involved in the scavenging of the first radical by Q, the O-H bond dissociation enthalpy (BDE), ionization potential (IP), and proton affinity (PA) of Q were calculated to estimate the likelihood of hydrogen atom transfer (HAT), single electron transfer followed by proton transfer (SET-PT) and sequential proton loss electron transfer (SPLET), respectively
Other pathways, such as sequential proton loss hydrogen atom transfer (SPLHAT), are considered [33]. The features of these pathways have been nicely described by Galano and Alvarez-Idaboy [34]. They pointed out that in the proton coupled electron transfer (PCET) pathways the antioxidant activity largely depends on the electronegativity of the H donor and acceptor, while in the SPLET pathways, the characteristics of the solvent are pivotal in the first step of the antioxidant reaction
Summary
In all life forms, opposing forces provide the energy that flows through networks in an organism, which fuels life [1]. The major source of energy in our body is provided by the redox reactions of glycolysis, lipid oxidation, and the citric acid cycle [2] This redox energy flows through biochemical networks and is the driving force of vital cellular reactions. A substantial part of the energy flow is uncontrolled and produces reactive oxidizing species that can inflict damage to crucial cellular components such as lipids, proteins and DNA. This plays a central role in numerous patho-physiological processes, from ageing to inflammation [1,3]. Their redox modulating activity has been implicated in the beneficial health effect of flavonoid antioxidants [4,5,6]
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