| (a) Hydrogen atom transfer (HAT) |
| | Corresponding method of assay | Mechanistic description |
| TRAP (Total Radical Trapping Antioxidant Parameter) assay
ORAC (Oxygen Radical Absorbance Capacity) assay
Beta carotene/crocin bleaching method
Inhibition of induced low-density lipoprotein peroxidation assay
Chemiluminescence quenching, due to luminol-derived radicals scavenging by antioxidants | ArOH + X•⟶ ArO• + XH
An antioxidant (e.g., phenolic compound ArOH) directly interacts with a free radical (X•), yielding a phenolic radical species derived from the antioxidant molecule ArO•, and a neutral species XH. The antioxidant facility to follow HAT mechanism is correlated with low bond-dissociation enthalpy [117]. The presence of dihydroxy functionality imparts good hydrogen donation abilities, correlatable with low bond-dissociation enthalpy values [118]. |
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| (b) Single electron transfer (SET) |
| | Corresponding method of assay | Mechanistic description |
| DMPD (N,N-dimethyl-p-phenylenediamine) method
FRAP (ferric reducing antioxidant power) assay
CUPRAC (cupric reducing antioxidant capacity) method
PFRAP (potassium ferricyanide reducing power) method | ArOH + X•⟶ ArOH•+ + X−
SET assays rely on the capacity of an antioxidant ArOH to reduce the radical species X• by electron donation, which is accompanied by the color change of the radical solution. Low adiabatic ionization potentials are correlated with good electron transfer abilities [117]. Extended delocalization and electron conjugation result in low ionization potentials [118]. Also, pH increase (deprotonation) favors electron transfer. |
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| (c) Mixed HAT and SET |
| | Corresponding method | Mechanistic description |
| | DPPH (2,2-diphenyl-1-picrylhydrazyl) scavenging method | Hydrogen atom transfer and sequential proton-loss electron transfer (SPLET), also designated proton-coupled electron transfer (PCET) [119, 120], were both confirmed as being thermodynamically favorable. | | A SPLET mechanism involving the antioxidant ArOH and the radical ROO• was represented as [121] | | ArOH ⟶ ArO− + H+ | | ArO− + ROO•⟶ ArO• + ROO− | | ROO− + H+⟶ ROOH | | or coupling the second and third steps as [122] | | TEAC (Trolox Equivalent Antioxidant Capacity) method | ArOH ⟶ ArO− + H+ | | ArO− + X• + H+⟶ ArO• + XH | | During the first step the phenolic antioxidant dissociates into its corresponding anion ArO− and a proton, and subsequently the ions which resulted in the first step react with the free radical, yielding a radical form of the phenolic antioxidant ArO• and a neutral molecule XH [122]. | | Proton transfer can also occur following electron transfer, as in single electron transfer-proton transfer mechanism (SET-PT) [122]: | | ArOH + X•⟶ ArOH•+ + X− | | ArOH•+⟶ ArO• + H+ | | During the first step a phenolic antioxidant reacts with the free radical X•, yielding a cationic radical ArOH•+ derived from the phenolic compound and the anionic form of the radical X−. This first step has been reported as thermodynamically significant step. In the second step the cationic radical form of the antioxidant ArOH•+ decomposes into a phenolic radical ArO• and a proton [122]. |
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| (d) Chelation power of antioxidants |
| | Corresponding method | Mechanistic description |
| | Tetramethylmurexide (TMM) assay | Free Cu(II) or Zn(II) which is not complexed by phenolics (e.g., tannins) is bound to tetramethylmurexide (TMM). The complexation with TMM is assessed at 482 nm for Cu(II) and at 462 nm for Zn(II) [123]. | | Ferrozine assay | Free Fe(II) that is not complexed by phenolics (e.g., tannins) is bound to ferrozine. The complexation of divalent iron with ferrozine is assessed at 562 nm [123]. |
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| (e) Oxidation of lipids |
| | Corresponding method | Mechanistic description |
| | Peroxide value assessment | Lipid autoxidation results in generation of hydroperoxides, determined iodometrically or colorimetrically [119]. | | Conjugated diene assay | Fatty acids autoxidation yields conjugated dienes, assessed by UV absorbance at 234 nm [119]. | | Anisidine assay | Secondary lipid oxidation yields p-anisidine-reactive aldehydes (alkenals, alkadienals, and malondialdehyde), the resulted Schiff base being determined at 350 nm [119]. | | Thiobarbituric acid reactive substances | Malondialdehyde and unsaturated aldehydes (alkenals and alkadienals) react with thiobarbituric acid; the reaction product is determined photocolorimetrically at 532 nm [119]. |
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