A DFT mechanistic and kinetic study on the reaction of phloroglucinol with ^•OH in different media: Hydrogen atom transfer versus oxidation

Abstract

A theoretical study on the reaction of phloroglucinol with •OH has been performed with the aim of elucidating the geometric, energetic and kinetic properties of the reaction as well as identifying the preferred reaction pathway. Three reaction mechanisms have been considered, namely, direct hydrogen atom abstraction, addition–elimination mechanism in the absence and in the presence of a base catalyst and oxidation mechanism in the absence and in the presence of O2. The study has been performed using the DFT/M06[Formula: see text]2X, DFT/BHHLYP and DFT/MPW1K methods in conjunction with either the 6-31++G(d,p) or the 6-311++G(3df,2p) basis set. The energetic parameters are influenced by the type of function utilized and the media in which the calculation is done. The direct hydrogen abstraction mechanism provides the smallest branching ratio with respect to the •OH addition mechanisms. The PG + •OH reaction under atmospheric conditions saturated with O2 would predominantly form tetrahydroxybenzene; the predominant product within the biological system would largely depend on physiological conditions; under pH [Formula: see text] 7 and with oxygen dissolved within the biological system, the preferred product would be tetrahydroxybenzene; however, if the reaction takes place in some part of the biological system where the pH [Formula: see text] 7, the preferred product would be the phenoxyl radical.

Antioxidant and anticancer properties of plant‐based bioactive flavonoids cardamonin and alpinetin: A theoretical insight from •OOH antiradical and Cu (II) chelation mechanisms

“…The product species also has higher Gibbs free energy than the sum of the Gibbs free energy of the isolated reactant species, so that overall, the reaction can be considered as exergonic in nature; similar trends were observed with the reaction involving phloroglucinol (PG) moiety and • OOH. [ 76 ] However, the PG + • OOH reaction appear to have a lower barrier height than the CAR + • OOH reaction, which suggests that PG + • OOH is faster (and therefore more favourable to radical scavenging) than CAR + • OOH. An assessment of the results for CAR + • OOH and 2, 4, 6‐trihydroxyacetophenone (ACPG) reaction with • OOH, [ 76 ] suggests that ACPG + • OOH has a lower reaction barrier height than CAR + • OOH, which implies that ACPG + • OOH corresponds to a faster reaction than CAR + • OOH; in other words, ACPG could be a better radical scavenger than CAR.…”

“…The bond dissociation enthalpy values (Δ H BDE ) are all positive, with the values for ALP being lower than those for CAR; this means that ALP has greater tendency towards radical scavenging through HAT mechanism than CAR. A comparison of the Δ H BDE values for CAR with those of phloroglucinol and 2,4,6‐trihydroxyacetophenone [ 76 ] suggests that the values for phloroglucinol and 2,4,6‐trihydroxyacetophenone are lower than those for CAR, which could point to the fact that both phloroglucinol and 2,4,6‐trihydroxyacetophenone are better radical scavengers than CAR. In other words, the replacement of the phenolic H on the phloroglucinol moiety by the OCH 3 group results in decreased BDE , which in turn describes lower tendency towards radical scavenging ability for CAR with respect to both phloroglucinol and 2,4,6‐trihydroxyacetophenone.…”

“…The thermodynamic reactivity parameters for CAR and ALP towards [76] suggests that the values for phloroglucinol and 2,4,6-trihydroxyacetophenone are lower than those for CAR, which could point to the fact that both phloroglucinol and 2,4,6-trihydroxyacetophenone are better radical scavengers than CAR. In other words, the replacement of the phenolic H on the phloroglucinol moiety by the OCH 3 group results in decreased BDE, which in turn describes lower tendency towards radical scavenging ability for CAR with respect to both phloroglucinol and 2,4,6-trihydroxyacetophenone.…”

“…• OOH, [76] suggests that ACPG + • OOH has a lower reaction barrier height than CAR + • OOH, which implies that ACPG + • OOH corresponds to a faster reaction than CAR + • OOH; in other words, ACPG could be a better radical scavenger than CAR. PC resulting from the ALP + • OOH reaction has almost the same electronic energy as the sum of the electronic energy for the isolated reactant species.…”

“…Based on previous reaction of polyphenol derivatives with • OOH have directed that the efficiency of the antioxidant molecules to scavenge peroxyl radical is better observed in non‐polar medium [ 28,34,35 ] than in polar medium. For this reason, the reactions involving • OOH that are investigated in this work are carried out in the presence of the benzene solvent, which serves as a representative medium for the non‐polar lipid bilayer medium present within biological systems.…”