Mechanism of Antiradical Activity of Coumarin-Trihydroxybenzohydrazide Derivatives: A Comprehensive Kinetic DFT Study

Abstract

As part of this study, the mechanisms of the antioxidant activity of previously synthesized coumarin-trihydrobenzohydrazine derivatives were investigated: (E)-2,4-dioxo-3-(1-(2-(2″,3″,4″-trihydroxybenzoyl)hydrazineyl)ethylidene)chroman-7-yl acetate (1) and (E)-2,4-dioxo-3-(1-(2-(3″,4″,5″-trihydroxybenzoyl)hydrazineyl)ethylidene)chroman-7-yl acetate (2). The capacity of the compounds to neutralize HO^• was assessed by EPR spectroscopy. The standard mechanisms of antioxidant action, Hydrogen Atom Transfer (HAT), Sequential Proton Loss followed by Electron Transfer (SPLET), Single-Electron Transfer followed by Proton Transfer (SET-PT), and Radical Adduct/Coupling Formation (RAF/RCF) were examined using the QM-ORSA methodology. It was estimated that the newly synthesized compounds, under physiological conditions, exhibited antiradical activity via SPLET and RCF mechanisms. Based on the estimated overall rate constants (k_overall), it can be concluded that 2 exhibited a greater antiradical capacity. The obtained values indicated a good correlation with the EPR spectroscopy results. Both compounds exhibit approximately 1.5 times more activity in comparison to the precursor compound used in the synthesis (gallic acid).

Keywords: QM-ORSA; antioxidant; coumarin; free radicals; gallic acid.

Figure 1

2D structures of investigated compounds 1 (left) and 2 (right) with atomic numbering.

Figure 2

The DEPMPO–HO^• adduct solution’s EPR spectra in the presence (red line) and absence (black line) of the compounds (a) 1 and (b) 2. The blue dots represent the peaks whose height was measured.

Figure 3

The optimized geometries of the acid-base species 1 (up) and 2 (down) in an aqueous solution determined by using the M06-2X/6-311++G(d,p) level of theory, in combination with the CPCM solvation model. Legend: grey—carbon atom, white—hydrogen atom, red—oxygen atom, blue—nitrogen atom.

Scheme 1

Thermodynamically favored reaction pathways between the most abundant acid-base species of investigated compounds and reactive radical species HO^•. The black color describes the thermodynamically favored reaction mechanisms between neutral species H₄A and HO^•. The red color describes the favored reaction mechanisms between monoanionic species H₃A⁻ and HO^•, while the blue color describes the reaction mechanisms between dianionic species H₂A²⁻ and HO^•.

Figure 4

Illustrative representation of the electron transition from the HOMO orbitals of 1 (left) and 2 (right) to the SOMO (Singly Occupied Molecular Orbital) orbital of the HO^• with the corresponding values of the orbital energies. Legend: grey—carbon atom, white—hydrogen atom, red—oxygen atom, blue—nitrogen atom.

Figure 5

The shape of the SOMO orbitals of the optimized transition state geometries for HAT/PCET reaction pathways between H₄A of 1 (top) and 2 (bottom) compounds and HO^• with characteristic intraatomic distances (Å). Legend: grey—carbon atom, white—hydrogen atom, red—oxygen atom, blue—nitrogen atom.

Figure 6

Energy profile for the (a) HAT/PCET reaction pathway between H₃A^• of compound 1 (4″OH) and HO^•; (b) RCF reaction pathway between H₃A^• of compound 2 (C-3) and HO^• in the singlet (blue) and triplet (red) spin states. Interatomic distances are given in angstroms [Å]. Legend: grey—carbon atom, white—hydrogen atom, red—oxygen atom, blue—nitrogen atom.