Quantum Mechanical Predictions of the Antioxidant Capability of Moracin C Isomers

Abstract

The antioxidant capability of moracin C and iso-moracin C isomers against the OOH free radical was studied by applying density functional theory (DFT) and choosing the M05-2X exchange-correlation functional coupled with the all electron basis set, 6-311++G(d,p), for computations. Different reaction mechanisms [hydrogen atom transfer (HAT), single electron transfer (SET), and radical adduct formation (RAF)] were taken into account when considering water- and lipid-like environments. Rate constants were obtained by applying the conventional transition state theory (TST). The results show that, in water, scavenging activity mainly occurs through a radical addition mechanism for both isomers, while, in the lipid-like environment, the radical addition process is favored for iso-moracin C, while, redox- and non-redox-type reactions can equally occur for moracin C. The values of pKa relative to the deprotonation paths at physiological pH were predicted in aqueous solution.

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

“… 182 This is the case for some RAF reactions, 82 but it is particularly common for SET, where thermochemical and kinetic data may show opposing trends; in some cases, the mechanism may be associated with positive Gibbs energies and still be relevant, as has been demonstrated in a number of previously reported studies. 36 , 46 , 73 , 82 , 154 Highly exergonic SET reactions involving donors with very low IP (such as monoanionic or polyanionic species) may, on the other hand, be found in the inverted region of the Marcus parabola, where that reaction barriers increase as Δ G ° decreases what is often to be found for Gibbs free energies much lower than negative of reorganization energy. 248 − 250 Despite this is an unexpected behavior, it underlines that particles with extremely low IP are unlikely to be efficient free radical scavengers.…”

“… 116 However, because these residues are also weakly acidic, multiple species can coexist in a water environment at the same time. Their molar fraction is governed both internally by their chemical structure 82 and externally by the pH of an environment.…”

“… 253 − 255 Despite its simplicity, it has been shown to reproduce fine the experimentally measured data on free radicals scavenging kinetics. 82 , 84 , 96 , 129 It is computed usually in the framework of 1 M standard state 255 using the Eyring equation ( eq 28 ) where k B and h are the Boltzmann and Planck constants, respectively. Δ G ⧧ is the free energy of activation, calculated as the difference in energies between transition state and reactants, while R and T denote the gas constant and temperature, respectively.…”

On the Scavenging Ability of Scutellarein against the OOH Radical in Water and Lipid-like Environments: A Theoretical Study

“…•− produced in the human body through Fenton reactions. The main reaction mechanisms by which the antioxidant system (indicated here as H 4 A) can inhibit ROS were established in a series of previous studies [6,8,19,20] and can take place through redox-related pathways:…”

“…The various mechanisms of action for the different diseases reported above are still not discovered, but the antioxidant activity is closely linked to the scavenging capacity against oxygen-containing radicals (ROS) like OH, OOH, O 2 •− produced in the human body through Fenton reactions. The main reaction mechanisms by which the antioxidant system (indicated here as H 4 A) can inhibit ROS were established in a series of previous studies [ 6 , 8 , 19 , 20 ] and can take place through redox-related pathways: Hydrogen Atom Transfer (HAT) H n A + R • → H n−1 A • + RH Single Electron Transfer (SET) H n A + R • → H n A +• + R − …”