Isoflavones and Isoflavone Glycosides: Structural-Electronic Properties and Antioxidant Relations—A Case of DFT Study

Abstract

Isoflavonoids and isoflavonoid glycosides have drawn much attention because of their antioxidant radical-scavenging capacity. Based on computational methods, we now present the antioxidant potential results of genistein (1), biochanin A (2), ambocin (3), and tectorigenin 7-O-[β-D-apiofuranosyl-(1-6)-β-D-glucopyranoside] (4). e optimized structures of the neutral and radical forms have been determined by the DFT-B3LYP method with the 6-311G(d) basis set. From the findings and thermodynamic point of view, the ring B system of isoflavones is considered as an active center in facilitating antioxidant reactions. Antioxidant activities are mostly driven by O-H bond dissociation enthalpy (BDE) following hydrogen atom transfer (HAT) mechanism. Antioxidant ability can be arranged in the following order: compounds (4) > (3) > (2) > (1). Of comprehensive structural analysis, flavonoids with 4′-methylation and 6-methoxylation, especially 7-glycosylation would claim responsibility for antioxidant enhancement.

Analysis of Conformational, Structural, Magnetic, and Electronic Properties Related to Antioxidant Activity: Revisiting Flavan, Anthocyanidin, Flavanone, Flavonol, Isoflavone, Flavone, and Flavan-3-ol

“…The formation of the interaction between the hydroxyl group and C4 could improve the planarity of the structure and also increase the π electrons in ring C. However, it is important to highlight that we should not neglect the influence of ketone oxygen for aromaticity due to the presence of the π electrons associated with this atom. The antioxidant activity of flavonoids was shown to be dependent on the stabilization of the O3H group through unsaturation of ring C. 14 , 46 − 51 Despite the presence of planarity in the molecules, the ZZ term (NICS zz ) is well aligned with the anisotropic (NICS anisotropic ) term of ring B ( Figure 5 ) and the aromaticity only occurs in the ring C of anthocyanidin (NICS zz and NICS isotropic terms). This result corroborates the high antioxidant activity previously reported for an anthocyanidin analogue.…”

“…Therefore, the aromaticity of the structures can contribute to understand the antioxidant mechanism. The number and position of hydroxyl groups have been widely studied regarding the antioxidant activity. ,− For example, for luteolin, it was demonstrated that by removing the O3H group in ring C, the antioxidant activity consequently decreases . Moreover, the importance of ring C unsaturation was pointed out, which allows electron delocalization for stabilization of the aryloxyl radical .…”

Studies on the antioxidant activity of apigenin, luteolin and nevadensin using DFT

“…To correct the zero-point energy (ZPE) and to confirm the presence of ground states lacking imaginary frequencies, vibrational frequencies were calculated at the same level of theory. The integral equation formalism polarizable continuum model (IEF-PCM) has been employed for estimating solvent effects [26,27].…”

“…Mechanistic studies have suggested three pathways for radicals scavenging property: HAT (H atom transfer), SET-PT (single electron transfer-proton transfer), and SPLET (Sequential Proton Loss Electron Transfer) [26][27][28][29]. The first mechanism, HAT, involves an H-atom transfer process, in which a hydrogen atom is removed from a flavonoid antioxidant compound to a free radical (R•), which is illustrated as follows:…”

Current Trends in Computational Quantum Chemistry Studies on Antioxidant Radical Scavenging Activity

“…This threshold is proposed 46 , 101 , 112 because a small energy difference between conformers could indicate an interconversion process: rotatability of OH groups 165 or side chains, 84 bonds deformation due to keto–enol tautomery, 21 shift in E/Z-conformers equilibria, 133 and bending of dihedral angles. 16 , 20 , 28 , 33 , 50 , 67 , 128 , 165 They can all modulate hydrogen bonds, electrons cloud delocalization, and polarizability, as well as radical accessibility to specific sites of an antioxidant. Furthermore, the molar fraction cannot be too low if the compound is desired to pass biological barriers.…”