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The antioxidant potential of polyphenols present in culinary herbs

Epidemiological studies have shown that polyphenols, biologically active compounds present in herbs, as well as in fruit and vegetables, have various health beneficial effects. However, due to the fact that polyphenols generally have low bioavailability (nM range), their biological activity can be ascribed to their colon catabolites, which are present in the circulation in higher concentrations (μM range). The majority of structurally complex polyphenols undergo colon metabolism, and produce a common set of simple phenols and hydrobenzoic acids, which are also widely present in the plant kingdom. The antioxidant activity of selected polyphenols (apigenin, luteolin, and quercetin) and their catabolites (caffeic acid, hydrocaffeic acid, protocatechuic acid, homoprotocatechuic acid, 4-methylcatechol, catechol) was studied in silico using the DFT approach. The geometry optimizations and frequency calculations were carried out using the M05-2X functional and the 6-311++G(d, p) basis set, in conjunction with the SMD continuum solvation model. Two radical scavenging mechanisms were studied, the hydrogen atom transfer (HAT) and the sequential proton loss electron transfer (SPLET). The obtained results indicate the selected polyphenols, originally present in herbs (angelica, celery, chives, coriander, common verbena, dill, fennel, fenugreek, oregano, marjoram, rosemary, anise, sage, lemon balm, wild marjoram, spearmint, tarragon, to name a few), and their catabolites as excellent free radical scavengers, with the average BDE values of 336.39 kJ/mol (BDE1) and 310.45 kJ/mol (BDE2), and the average ETE values of 321.75 kJ/mol (ETE1) and 316.31 kJ/mol (ETE2). The Gibbs free energy change for the HAT/SPLET mechanism of the studied compounds with a particular free radical was also calculated. The obtained results indicate these compounds as efficient deactivators of various free radicals, with the average ΔrGBDE1 for scavenging of the HOO∙ radical of -22.26 kJ/mol and the ΔrGBDE2 of -48.54 kJ/mol, and the averageΔrGPA1 of -34.39 kJ/mol and the ΔrGPA2 of -63.55 kJ/mol.

polyphenols, antioxidant activity, density functional theory (DFT)

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