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Reaction kinetics have been theoretically examined to ascertain the potency of quercetin (Q) and flavonoid catecholic metabolites 1–5 in the inactivation of HOO•, CH3OO•, and O2•− under physiological conditions. In lipidic media, the k overall TST / Eck rate constants for the proton-coupled electron transfer (PCET) mechanism indicate the catecholic moiety of Q and 1–5 as the most important in HOO• and CH3OO• scavenging. 5-(3,4-Dihydroxyphenyl)-γ-valerolactone (1) and alphitonin (5) are the most potent scavengers of HOO• and CH3OO•, respectively. The k overall Mf rate constants, representing actual behavior in aqueous media, reveal Q as more potent in the inactivation of HOO• and CH3OO• via single electron transfer (SET). SET from 3-O− phenoxide anion of Q, a structural motif absent in 1–5, represents the most contributing reaction path to overall activity. All studied polyphenolics have a potency of O2•− inactivation via a concerted two-proton–coupled electron transfer (2PCET) mechanism. The obtained results indicate that metabolites with notable radical scavenging potency, and more bioavailability than ingested flavonoids, may contribute to human health-promoting effects ascribed to parent molecules. [ABSTRACT FROM AUTHOR]

To ascertain quercetin's and rooperol's potency of H-atom donation to CH3OO• and HOO•, thermodynamics, kinetics and tunnelling, three forms of chemical reaction control, were theoretically examined. In lipid media, H-atom donation from quercetin's catecholic OH groups via the proton-coupled electron transfer (PCET) mechanism, is more relevant than from C-ring enolic moiety. Amongst rooperol's two catecholic moieties, H-atom donation from A-ring OH groups is favored. Allylic hydrogens of rooperol are poorly abstractable via the hydrogen atom transfer (HAT) mechanism. Kinetic analysis including tunnelling enables a more reliable prediction of the H-atom donation potency of quercetin and rooperol, avoiding the pitfalls of a solely thermodynamic approach. Obtained results contradict the increasing number of misleading statements about the high impact of C–H bond breaking on polyphenols' antioxidant potency. In an aqueous environment at pH = 7.4, the 3-O− phenoxide anion of quercetin and rooperol's 4′-O− phenoxide anion are preferred sites for CH3OO• and HOO• inactivation via the single electron transfer (SET) mechanism. [ABSTRACT FROM AUTHOR]

Ferulic acid (FA) is used in skin formulations for protection against the damaging actions of the reactive oxygen species (ROS) produced by UVA radiation. Possible underlying protective mechanisms are not fully elucidated. By considering the kinetics of proton-coupled electron transfer (PCET) and radical-radical coupling (RRC) mechanisms, it appears that direct scavenging could be operative, providing that a high local concentration of FA is present at the place of •OH generation. The resulting FA phenoxyl radical, after the scavenging of a second •OH and keto-enol tautomerization of the intermediate, produces 5-hydroxyferulic acid (5OHFA). Inhibition of the lipoxygenase (LOX) enzyme, one of the enzymes that catalyse free radical production, by FA and 5OHFA were analysed. Results of molecular docking calculations indicate favourable binding interactions of FA and 5OHFA with the LOX active site. The exergonicity of chelation reactions of the catalytic Fe2+ ion with FA and 5OHFA indicate the potency of these chelators to prevent the formation of •OH radicals via Fenton-like reactions. The inhibition of the prooxidant LOX enzyme could be more relevant mechanism of skin protection against UVA induced oxidative stress than iron chelation and assumed direct scavenging of ROS. [ABSTRACT FROM AUTHOR]

Reaction kinetics have been theoretically examined to ascertain the potency of quercetin ( Q ) and flavonoid catecholic metabolites 1 - 5 in the inactivation of HOO • , CH 3 OO • , and O 2 •- under physiological conditions. In lipidic media, the koverallTST/Eck rate constants for the proton-coupled electron transfer (PCET) mechanism indicate the catecholic moiety of Q and 1 - 5 as the most important in HOO • and CH 3 OO • scavenging. 5-(3,4-Dihydroxyphenyl)-γ-valerolactone ( 1 ) and alphitonin ( 5 ) are the most potent scavengers of HOO • and CH 3 OO • , respectively. The koverallMf rate constants, representing actual behavior in aqueous media, reveal Q as more potent in the inactivation of HOO • and CH 3 OO • via single electron transfer (SET). SET from 3-O - phenoxide anion of Q , a structural motif absent in 1 - 5 , represents the most contributing reaction path to overall activity. All studied polyphenolics have a potency of O 2 •- inactivation via a concerted two-proton-coupled electron transfer (2PCET) mechanism. The obtained results indicate that metabolites with notable radical scavenging potency, and more bioavailability than ingested flavonoids, may contribute to human health-promoting effects ascribed to parent molecules.