Theoretical study on the radical scavenging activity of gallic acid

Abstract

The global descriptors of the chemical activity: ionization potential IP, electron affinity EA, chemical potential μ, absolute electronegativity χ, molecular hardness η and softness S, electrophilicity index ω, electro-donating ω^-, electro-accepting ω⁺ powers as well as Ra and Rd indexes for gallic acid (GA) in the gas phase and water medium have been determined. To this aim, the HOMO and LUMO energies were calculated using the DFT method at the B3LYP, M06-2X, LC-ωPBE, BHandLYP, ωB97XD/cc-pVQZ theory levels using C-PCM, IEF-PCM and SMD solvation models, enabling more accurate descriptor calculations than those carried out so far. Quantum-chemical computations were also applied to investigate the GA structure and thermodynamic parameters characterizing its radical scavenging properties. To this aim, the full optimization of the neutral GA and its radical, cationic and anionic forms in vacuum and water medium has been performed, and then the bond dissociation enthalpy BDE, adiabatic ionization potential AIP, proton dissociation enthalpy PDE, proton affinity PA, electron transfer enthalpy ETE, gas phase acidity H_acidity and free Gibbs acidity G_acidity in water have been determined. The calculations revealed that GA in vacuum scavenges free radicals via hydrogen atom transfer (HAT), whereas in water (polar) medium by sequential proton loss electron transfer (SPLET). Analysis of the global activity descriptors of GA indicates that only B3LYP method combined with different solvation models satisfactory reproduces LUMO-HOMO energies and provides the smallest value of the total electron energy of GA. Among the parameters of chemical activity, the indexes Ra and Rd are the most independent of the computational method and the solvation model used. They can be recommended as a reliable source of information on the antioxidant activity of chemical compounds.

Keywords: AIP; Antioxidants; BDE; DFT method; ETE; Gallic acid; Global descriptors; PA; PDE; Solvation models; TMC descriptors.

Fig. 1

Gallic acid, 3,4,5-trihydroxybenzoic acid, CAS-149-91-7.

Fig. 2

Optimized geometries of the selected conformers of GA with the lowest energy values E [Ha]. Conformer I was considered theoretically by Rajan and Muraleedharan [23], while II by Saqib et al. [21]. Conformers III-IV were analyzed by crystallographic means [46,47].

Fig. 3

Ellagic acid, CAS: 476-66-4.

Fig. 4

I. HOMO and LUMO GA frontier orbitals and energy levels calculated using DFT B3LYP/cc-pVQZ level of the theory. II. Contour surface of electrostatic potential of GA.

Fig. 5

Optimized geometries of the GA cation I (x = C), radicals II-V (x = R_n), anions VI – IX (x = A_n) in vacuum and values of the enthalpies H(x) [Ha] determined using the B3LYP/cc-pVQZ level of the theory. The subscript n corresponds to the number of carbons in GA according to Fig. 1. The reported enthalpy values have been used in the calculations of the antioxidant descriptors presented in Table 2.

Fig. 6

Optimized geometries of the GA cation I (x = C), radicals II-V (x = R_n), anions VI-IX (x = A_n) in water and values of the enthalpies H(x) [Ha] determined using the B3LYP/cc-pVQZ level of the theory and C-PCM solvation model. The subscript n corresponds to the number of carbons in GA according to Fig. 1. The reported enthalpy values have been used in the calculations of the antioxidant descriptors presented in Table 2.

Fig. 7

Optimized geometries of the GA cation I (x = C), radicals II-V(x = R_n), anions VI-IX (x = A_n) in water and values of the enthalpies H(x) [Ha] determined at the M06–2X/cc-pVQZ level of the theory in water using the IEF-PCM solvation model. The subscript n corresponds to the number of carbons in GA according to Fig. 1. The reported enthalpy values have been used in the calculations of the antioxidant descriptors presented in Table 3.