Polyphenols (synthetically modified or directly provided by human diet) scavenge free radicals by H-atom transfer and may thus decrease noxious effects due to oxidative stress. Free radical scavenging by polyphenols has been widely theoretically studied from the thermodynamic point of view whereas the kinetic point of view has been much less addressed. The present study describes kinetic-based structure–activity relationship for quercetin. This compound is very characteristic of the wide flavonoid subclass of polyphenols. H-atom transfer is a mechanism based on either atom or electron transfer. This is analyzed here by quantum chemical calculations, which support the knowledge acquired from experimental studies. The competition between the different processes is discussed in terms of the nature of the prereaction complexes, the pH, the formation of activated-deprotonated forms, and the atom- and electron-transfer efficiency. The role of the catechol moiety and the 3-OH group of quercetin as scavengers of ...

Free radical scavenging by natural polyphenols: atom versus electron transfer.

Quercetin and the mitochondria: A mechanistic view

Computational strategies for predicting free radical scavengers' protection against oxidative stress: Where are we and what might follow?

Theoretical study on the structural and antioxidant properties of some recently synthesised 2,4,5-trimethoxy chalcones.

Among numerous biological activities, natural polyphenols are antioxidants widely distributed in plants capable of inhibiting lipid peroxidation, which belongs to the most serious degenerative cell processes. Positioning of antioxidants in lipid bilayers can provide an insight to the lipid-peroxidation inhibition at the molecular level. This work aims at determining the location and orientation of quercetin and its most representative (glucuronidated, methylated, and sulfated) metabolites in lipid bilayer via molecular dynamic simulations. We show that quercetin derivatives penetrate the lipid bilayer and that the depths of penetration depend on molecular charge and substitutional variations. In the presence of charged substituents (sulfates and glucuronidates), the molecule is pulled toward the lipid bilayer surface. The orientation also depends on substitution as H-bonds are formed between the polar head groups of the bilayer and the (i) OH groups, (ii) sugar, and (iii) sulfate moieties of the antioxida...

Positioning of antioxidant quercetin and its metabolites in lipid bilayer membranes: implication for their lipid-peroxidation inhibition.

Natural polyphenols are known to exhibit a lot of different biological properties, including antioxidant activity. For some polyphenols these activities are attributed to the presence of a guaiacol moiety. In the present paper we focus on the role of this moiety. For this purpose nine different compounds were enzymatically synthesized from guaiacol. To elucidate the structure−activity relationship of these polyphenols, DFT-(PCM)B3P86/6-311+G(2d,3pd)//(PCM)B3P86/6-31+G(d,p) calculations supported the experimental DPPH free radical scavenging activities. The antioxidant activities were correlated to (i) O−H BDEs (bond dissociation enthalpies), (ii) BDED (BDE of a second H atom abstraction from the phenoxyradicals), (iii) spin density, (iv) HOMO (highest occupied molecular orbital) distribution, (v) IPs (ionization potentials), (vi) ΔG and ΔG# free energies of HAT (H atom transfer), and (vii) HAT rate constants. BDED appeared to be the most important descriptor to understand the free radical scavenging abili...

Free radical scavenging properties of guaiacol oligomers: a combined experimental and quantum study of the guaiacyl-moiety role

Free radical oxygen species act on biological constituents like lipid, cell membrane and deoxyribonucleic acids by changing their biological functions These effects are associated with the appearance of anomalies like allergies, cancer, viral infections, inflammations, etc Antioxidants like flavonoids are abundant in plants and are good scavengers of reactive oxygen species (ROS) Their antioxidant activities are measured through hydrogen atom transfer or charge transfer These mechanisms depend on the structure of the flavonoid or the nature of the free radical In this paper, we study the interaction of one of the most reactive flavonoid quercetin with two ROS: hydroxyl radical ( · OH ) and superoxide radical ( · O 2 - ) using B3LYP/6-31++G*level of theory Correlations between structural changes and antioxidant activity are shown The potential energy surfaces (PES) are calculated in both cases and indicate a transition state (TS) only for the reaction of quercetin with the hydroxyl radical This transition state is only localized at the B3LYP/3-21G level of theory This comparison between the reactants, the TS and the products of the reaction with the · OH radical indicate the nature of the transfer As far as the reaction of quercetin with the superoxide anion radical is concerned, the PES shows the inexistence of one transition state The nature of the charge transfer is thus determined using the natural bond orbital (NBO) and the singlet occupied molecular orbital (SOMO) analysis

DFT study of the reaction of quercetin with ·O2- and ·OH radicals

Background: Acetone is present in the earth ́s atmosphere and extraterrestrially. The knowledge of its chemical history in these environments represents a challenge with important implications for global tropospheric chemistry and astrochemistry. The results of a search for efficient barrierless pathways producing acetone from radicals in the gas phase are described in this paper. The spectroscopic properties of radicals needed for their experimental detection are provided. Methods: The reactants were acetone fragments of low stability and small species containing C, O and H atoms. Two exergonic bimolecular addition reactions involving the radicals CH3, CH3CO, and CH3COCH2, were found to be competitive according to the kinetic rates calculated at different temperatures. An extensive spectroscopic study of the radicals CH3COCH2 and CH3CO, as well as the CH2CHO isomer, was performed. Rovibrational parameters, anharmonic vibrational transitions, and excitations to the low-lying excited states are provided. For this purpose, RCCSD(T)-F12 and MRCI/CASSCF calculations were performed. In addition, since all the species presented non-rigid properties, a variational procedure of reduced dimensionality was employed to explore the far infrared region. Open Peer Review

K. Marakchi

Papers

Free radical scavenging properties of guaiacol oligomers: a combined experimental and quantum study of the guaiacyl-moiety role

DFT study of the reaction of quercetin with ·O2- and ·OH radicals

Barrierless formation processes of acetone in the gas phase [version 2; peer review: 2 approved with reservations]