Antioxidant Activity of Flavonoid Compounds in Lime (Citrus aurantifolia) via Density Functional Theory

Abstract The advance of molecular quantum biophysics has opened the path towards a deeper understanding of the microscopic mechanism behind free radicals activity that are responsible for cell damages and oxidative stress. In this work, we perform an \textit{ab-initio} analysis on the antioxidant activity of three flavonoid compounds e.g. hesperetin, quercetin, and naringenin in lime (\textit{Citrus aurantifolia}). We study the antioxidant capabilities of these compounds to understand the quantum mechanical mechanism in combating free radicals in the framework of Density-Functional Theory (DFT) using B3LYP exchange-correlation and def2/SVP basis sets. We found via vibrational frequency calculations, that the antioxidant behaviors of these molecules predominantly arise from hydrogen atom transfer, single electron transfer followed by proton transfer, and sequential proton loss electron transfer mechanisms. Our calculations also show that quercetin showcased superior antioxidant activity compared to hesperetin and naringenin based on their bond dissociation energy, ionization potential + proton dissociation enthalpy, and proton affinity + electron transfer enthalpy values at the C9-OH group, with values of 295.29 kJ/mol, 81.88 kJ/mol, and 81.88 kJ/mol for quercetin respectively. An in-depth look into the antioxidant properties, using global reactivity parameters also affirmed quercetin's preeminence in antioxidant characteristics over hesperetin and naringenin, excelling in nearly all considered parameters, though hesperetin's chemical potential stood out at -3.51 eV.