Gallic acid: thermodynamics of the homolytic and heterolytic phenolic O—H bonds splitting-off

Abstract

The DFT study of primary antioxidant action of gallic acid and its carboxylic anion is presented in the gas-phase, benzene and water. Corresponding reaction enthalpies for three possible mechanisms was calculated using B3LYP/6-311++G** method. Bond dissociation enthalpy (BDE) and proton dissociation enthalpy (PDE) of 4-OH group was found to be the lowest in gas-phase as well as in both solvents approximated by IEF-PCM model. Ionization potentials (IPs) were higher than BDEs in all cases. Deprotonation of carboxylic group result in increased antioxidant potency as drop in BDE, proton affinities (PAs) and IPs was indicated in all environments.

Theoretical DFT Study of the Antioxidant Activity of Five Plant Food Benzoic Acid Derivatives: Dearomatization and Stability of H Abstraction Radicals

“…Turning now on the results in solution, Table 1 reveals that the hydrogen abstraction ability of these compounds is only slightly affected by the two polar solvents, with deviations representing less than 10% of the dissociation enthalpy in the gas phase. This observation agrees well with previous findings on the antioxidant activity of benzoic acid derivatives [22]. Moreover, GA appears to be the most active in H2O and MetOH solutions, with the lowest BDE of 78.2 and 80.4 kcal/mol respectively in water and methanol attributed to the OH group 3.…”

“…neutral molecules, radicals, cations, and anions, were fully optimized using the meta hybrid M06-2X functional in conjunction with the 6-311++G(d,p) basis set as implemented in Gaussian 09 software [43]. The choice of the M06-2X/6-311++G(d,p) level of theory was motivated by the fact that it has been shown to provide reliable results about the antioxidant activities of many families of compounds [22] [44]. Vibrational frequency calculations confirmed the predicted stationary point as being real minima on the potential energy surface.…”

“…For instance, the homolytic and heterolytic splitting-off of the phenolic -OH groups of gallic acid and its carboxylic anion in gas-phase, benzene and water was elucidated by Skorna et al using the B3LYP functional combined with the 6-311++G (d,p) basis set [22]. These authors found that the deprotonation of the carboxylic groups results in an increase in the antioxidant activity in all environments.…”

Free Radical Scavenging Activity of Five Benzoic Acid Derivatives: A Theoretical M06-2X Study

“…Indeed, BDEs deviations between the gas phase and solution are lower than 6, 11, 6, 7 and 8 kcal/mol for GA, PHBA, PCA, SA and VA respectively. These fluctuations evaluate to less than 10% of the dissociation enthalpy in the gas phase, and agree well with previous findings of the antioxidant activity of benzoic acid derivatives [35]. Moreover, GA appears to be the most active in H2O and MetOH solutions, with the lowest BDE of 78.2 and 80.4 kcal/mol respectively in water and methanol attributed to the OH group 3.…”

“…It is worth noting that all of these compounds are present in plant food where they usually occur in bound form as components of complex structures like lignins and hydrolysable tanins, or attached to cell walls and proteins [33] [34]. Although these molecules have been the subject of separate previous works [35] [36], the present study draws its peculiarity from the fact that it involves all of them in a comparative discussion of their antioxidant activity in gas and polar phases at the M06-2X/6-311++G(d,p) level, and following three different mechanisms of free radical scavenging. The antioxidant activity of each compound is assessed using thermodynamic parameters defined with respect to each mechanism.…”