A comparison of scytonemin and its carbon analogue in terms of antioxidant properties through free radical mechanisms and conformational analysis: a DFT investigation

Kinetics and Mechanism of the Antioxidant Activities of C. olitorius and V. amygdalina by Spectrophotometric and DFT Methods

The kinetics and mechanism of the antioxidant activities of the methanolic extract of the leaves of two vegetables [Corchorus olitorius (C. olitorius) and Vernonia amygdalina (V. amygdalina)] have been studied using experimental and theoretical approaches. The kinetics (second order and pseudo-first order) of the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activities of the leaf extracts at varying times (30-90 min) were determined using the UV-visible spectrophotometry method at λ_max = 517 nm, whereas the mechanism was studied by density functional theory at two levels of functionals (B3LYP and LC-ωPBE) using bond dissociation enthalpy and adiabatic ionization potential values. Molecular properties such as the highest occupied molecular orbital, lowest unoccupied molecular orbital, electronegativity (χ), electrophilicity (ω), hardness (η), and softness (S) of the predominant phenolic antioxidants were also compared. The second-order kinetics is favored by both plants rather than pseudo-first order; however, V. amygdalina with a second-order rate constant k ₂ of 0.0152 (mM)^-1 min^-1 is faster in scavenging DPPH radicals than C. olitorius with a k ₂ value of 0.0093 (mM)^-1min^-1. Chlorogenic acid and luteolin-7-O-β-glucuronide, which are the most abundant phenolic acid antioxidant in C. olitorius and V. amygdalina, both preferably scavenge the DPPH radical via a hydrogen atom transfer mechanism. This is evident from their lower bond dissociation enthalpy values than the adiabatic ionization potential values. Successful molecular docking of these phenolic compounds indicates that both compounds form favorable interactions with the therapeutic target, xanthine oxidase.

Antioxidant properties of Aloe vera components: a DFT theoretical evaluation

Prediction of the antioxidant activity of three Aloe vera components (aloesone, aloe-emodin, and isoeleutheol) was performed based on density functional theory calculations using the B3LYP hybrid functional and the 6-311++ G** basis set. Calculation of highest occupied molecular orbital (HOMO), lowest occupied molecular orbital (LUMO), and E_gap revealed that aloe-emodin has the lowest E_gap value, indicating good antioxidant activity. Also in terms of electron affinity, softness, electrophilicity, and chemical potential, aloe-emodin is a potent structure with potential high radical scavenging activity. Calculation of the ionisation potential revealed that isoeleutherol likely also possesses a high degree of antiradical scavenging. To study the conjugating system of the radicals, density plots of HOMO, natural bond orbital analyses, and spin density plots were used. According to calculations, the isoeleutherol radical is more delocalised and the most stable radical. Calculated proton affinity values revealed that the most probable antioxidant mechanism is sequential proton loss-electron transfer. Our results were compared with available experimental data. Published experimental data were found to correlate well with our theoretical predictions. These results support the usefulness of theoretical calculations not only for identifying potentially useful structures of studied compounds but also for predicting their relative activity.