Theoretical investigations on the antioxidant potential of a non-phenolic compound thymoquinone: a DFT approach.

Jewel Hossen,M Abbas Ali,Sultanur Reza

doi:10.1007/s00894-021-04795-0

Abstract

Thymoquinone (TQ) is a natural compound present in black cumin which possesses potent antioxidant activity without having any phenolic hydroxyl group which is responsible for antioxidant activity. In the present study, computational calculation based on density functional theory (DFT) was executed to assess systematically the antioxidant behavior of this compound by considering geometrical characteristics, highest occupied molecular orbital - lowest unoccupied molecular orbital (HOMO-LUMO), and molecular electrostatic potential (MEP) surface. Thermochemical parameters correlated to the leading antioxidant mechanisms such as hydrogen atom transfer (HAT), single electron transfer-proton transfer (SETPT), and sequential proton loss electron transfer (SPLET) were studied in gas and water media. In addition, the changes of thermochemical parameters such as free energy change (∆G) and enthalpy change (∆H) were computed for hydrogen abstraction (HA) from TQ to hydroxyl radical in gas and water phases to investigate its free radical scavenging potency. The low and comparable values of bond dissociation enthalpy (BDE), proton dissociation enthalpy (PDE), ionization potential (IP), proton affinity (PA), and electron transfer enthalpy (ETE) revealed the antioxidant activity. The ∆G and ∆H also indicated apposite thermodynamic evidence in favor of antiradical capability of TQ. The attack of the free radical occurred preferentially at 3CH position of the molecule.

Highlights

Free radicals form in biological reactions in living cells [1], but some external factors such as radiation, pollutants, physical stress, pesticides, several treatments etc. cause its formation [2]
The antioxidant activity of a compound can be thermodynamically evaluated by some physicochemical descriptors such as bond dissociation enthalpy (BDE), proton dissociation enthalpy (PDE), proton affinity (PA), ionization potential (IP), electron transfer enthalpy (ETE), hardness, softness and electronegativity
Antioxidant activity of TQ, a non-phenolic molecule has been successfully evaluated in gas and water phases using Density Functional Theory (DFT) calculation

Summary

Introduction

Free radicals form in biological reactions in living cells [1], but some external factors such as radiation, pollutants, physical stress, pesticides, several treatments etc. cause its formation [2]. Polyphenols, phenolic acids and flavonoids either synthetic or natural can guard biomolecules against unexpected oxidative damage triggered by the free radicals. They inhibit the oxidative mechanisms that lead to degenerative diseases [5][6]. Thymoquinone (TQ) is an active component of black cumin (Nigella sativa L.) essential oil possessing antineoplastic, anti-inflammatory, neuro- and hepato-protective properties These properties are attributed to the antioxidant potential of TQ, which seems to be unlikely due to its structure [7]. It is a common belief that, only the compounds having phenolic OH groups are responsible for antioxidant activity and/or free radical scavenging power [9][10][11][12]. Computational techniques, especially DFT method, have been commendably applied to determine these thermodynamic parameters of antioxidant molecules [3][20]

Objectives

Results

Conclusion