Single Electron Transfer Followed By Proton Transfer Research Articles

Antioxidant Activity of Quercetin and Its Glucosides from Propolis: A Theoretical Study

Among the multiple components of propolis, flavonoids contribute greatly to the antioxidant activities of propolis. Flavonoids mainly exist in the form of sugar-conjugated derivatives. Quercetin glycosides represent the predominant flavonoid fraction in propolis. In this work, density functional theory (DFT) calculations were applied to analyze the antioxidative properties of quercetin and its glucosides in the gas and in the liquid phase (ethanol, water). Three main antioxidant mechanisms, hydrogen atom transfer (HAT), single electron transfer followed by proton transfer (SET-PT) and sequential proton loss electron transfer (SPLET) were used to analyze the antioxidative capacity of the investigated compounds. Solvent effects dominantly affect SET-PT and SPLET. Thus, the thermodynamically preferred mechanism can be altered. HAT and SPLET are the thermodynamically dominant mechanisms in gas and solvent phases, respectively. Therefore, in the gas phase, the sequence of the antioxidative capacity is similar with the bond dissociation enthalpy values: quercetin > quercetin-5-O-glucoside > quercetin-7-O-glucoside > quercetin-3-O-glucoside > quercetin-3′-O-glucoside > quercetin-4′-O-glucoside. While, in the solvent phases, the sequence is similar with the proton affinity values: quercetin-4′-O-glucoside > quercetin-5-O-glucoside > quercetin > quercetin-3-O-glucoside > quercetin-7-O-glucoside > quercetin-3′-O-glucoside. OH groups in B-ring and C-ring contribute mainly to the antioxidative activities of quercetin and glucosides compared with A-ring.

Free Radical Scavenging Potency of Dihydroxybenzoic Acids

In order to evaluate the free radical scavenging potency of dihydroxybenzoic acids (DHBAs) the Density Functional Theory (DFT) was used. The M05-2X/6-311++G(d,p) and B3LYP-D2/6-311++G(d,p) theoretical models were applied. Three possible antioxidant mechanisms were examined: hydrogen atom transfer (HAT), single-electron transfer followed by proton transfer (SET-PT), and sequential proton loss electron transfer (SPLET) mechanisms. All of these mechanisms have been studied in nonpolar (benzene and pentylethanoate) and polar solvents (water) using an implicit solvation model (SMD). The following thermodynamic quantities related to these mechanisms were calculated: bond dissociation enthalpy (BDE), ionization potential (IP), and proton affinity (PA). The obtained results indicated the HAT mechanism as the most favourable reaction pathway for antioxidative action of DHBAs in benzene. On the other hand, SPLET is indicated as predominant reaction mechanism in polar solvent. The SET-PT mechanism was not favourable reaction path for antioxidative action in any of the solvents under investigation.

Investigation of the antioxidant and radical scavenging activities of some phenolic Schiff bases with different free radicals.

The antioxidant properties of some phenolic Schiff bases in the presence of different reactive particles such as (•)OH, (•)OOH, (CH2=CH-O-O(•)), and (-•)O2 were investigated. The thermodynamic values, ΔH BDE, ΔH IP, and ΔH PA, were used for this purpose. Three possible mechanisms for transfer of hydrogen atom, concerted proton-electron transfer (CPET), single electron transfer followed by proton transfer (SET-PT), and sequential proton loss electron transfer (SPLET) were considered. These mechanisms were tested in solvents of different polarity. On the basis of the obtained results it was shown that SET-PT antioxidant mechanism can be the dominant mechanism when Schiff bases react with radical cation, while SPLET and CPET are competitive mechanisms for radical scavenging of hydroxy radical in all solvents under investigation. Examined Schiff bases react with the peroxy radicals via SPLET mechanism in polar and nonpolar solvents. The superoxide radical anion reacts with these Schiff bases very slowly.

Computational study of the structure–free radical scavenging relationship of procyanidins

Procyanidins (PCs) are effective free radical scavengers, however, their antioxidant ability is variable because they have different degrees of polymerisation, are composed by distinct types of subunits and are very susceptible to changes in conformation. In this work the structure–free radical scavenging relationship of monomers, dimers and trimers of PCs was studied through the hydrogen atom transfer (HAT), sequential proton-loss electron-transfer (SPLET) and single electron transfer followed by proton transfer (SET-PT) mechanisms in aqueous phase, employing the Density Functional Theory (DFT) computational method. The structure–free radical scavenging relationship of PCs showed a very similar behaviour in HAT and SET-PT mechanisms, but very different in the SPLET mechanism. The structural factor that showed more effects on the ability of PCs to scavenge free radicals in aqueous phase was the conformation.

Quantum-chemical study on relationship between structure and antioxidant properties of hepatoprotective compounds occurring in Cynara scolymus and Silybum marianum

Accurate quantum computations based on the density functional theory have been performed to study the relationship between the electronic geometry and antioxidant capacity of chlorogenic acid, silybin and all geometric stereoisomers of cynarine, isolated from plant extract of Cynara scolymus and Silybum marianum. To elucidate their antioxidant activity, the HOMO orbital distribution, adiabatic ionization potential (AIP), spin density in free radicals, homolytic dissociation enthalpies (BDE), and proton dissociation enthalpies (PDE) of the O – H bonds have been calculated. For minimum energy conformations, the antioxidative parameters were quantitatively analyzed at the B3LYP/6-311G(d,p) level of theory. The results have shown that the hydrogen transfer mechanism is more preferable in nonpolar medium than in water. From the results obtained we can conclude that SET-PT (single electron transfer followed by proton transfer) is the most preferred mechanism in water medium. The catechol moiety and planar geometry of trans-stilbene and cis-stilbene moieties in cynarine stereoisomers, chlorogenic acid and their phenoxy radicals strongly contribute to enhancement of the antioxidant activity of these compounds. Trans,trans-cynarine appears to be the best candidate for proton and hydrogen atom donor. It has been predicted that cis,cis, trans,cis and cis,trans stereoisomers of cynarine show antioxidant capacity. Our study shows that all of the investigated compounds reveal strong antioxidant activity.

PM6 study of free radical scavenging mechanisms of flavonoids: why does O–H bond dissociation enthalpy effectively represent free radical scavenging activity?

It is well known that the bond dissociation enthalpy (BDE) of the O-H group is related to the hydrogen atom transfer (HAT) mechanism of free radical scavenging that is preferred in gas-phase and non-polar solvents. The present work shows that the BDE may also be related to radical scavenging processes taking place in polar solvents, i.e., single electron transfer followed by proton transfer (SET-PT) and sequential proton loss electron transfer (SPLET). This is so because the total energy requirements related to the SET-PT [sum of the ionization potential (IP) and proton dissociation enthalpy (PDE)] and the SPLET [sum of the proton affinity (PA) and electron transfer enthalpy (ETE)] are perfectly correlated with the BDE. This could explain why the published data for polyphenolic antioxidant activity measured by various assays are better correlated with the BDE than with other reaction enthalpies involved in radical scavenging mechanisms, i.e., the IP, PDE, PA and ETE. The BDE is fairly well able to rank flavonoids as antioxidants in any medium, but to conclude which radical scavenging mechanism represents the most probable reaction pathway from the thermodynamic point of view, the IP and PA (ETE) should also be considered. This is exemplified in the case of the radical scavenging activity of 25 flavonoids.