Marianne K. Thomsen

Regeneration of phenolic antioxidants from phenoxyl radicals: An ESR and electrochemical study of antioxidant hierarchy

Radicals from the flavonoids quercetin, (+)-catechin, (+/-)-taxifolin and luteolin, and from all-rac-alpha-tocopherol have been generated electrochemically by one-electron oxidation in deaerated dimethylformamide (DMF), and characterised by electron spin resonance spectroscopy (ESR) after spin-trapping by 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). Simulations of the ESR spectrum based on estimated coupling constants of the spin-trapped quercetin radical, confirmed that this antioxidant radical is oxygen-centered. The complex mixture of radicals, quinoid intermediates and stable two-electron oxidation products, were for each antioxidant allowed to react with each of the four other antioxidants, and the progression of reaction followed by ESR after addition of DMPO, and the product solution further analysed by HPLC. All-rac-alpha-tocopherol was found to be most efficient in regenerating each of the other antioxidants from their oxidation products with a regeneration index (defined as moles regenerated of the oxidised phenolic antioxidant divided with moles of all-rac-alpha-tocopherol consumed) of 0.90+/-0.16 for quercetin, 0.48+/-0.11 for (+)-catechin, 0.48+/-0.06 for (+/-)-taxifolin and 0.50+/-0.10 for luteolin in equimolar 1.00 mM solution. Quercetin was found to have the highest regeneration index among the flavonoids: 0.88+/-0.13 for (+/-)-catechin, 0.41+/-0.03 for (+/-)-taxifolin and 0.41+/-0.02 for luteolin. The antioxidant hierarchy based on the reduction potentials determined by cyclic voltammetry under similar conditions (0.93 V for all-rac-alpha-tocopherol, 1.07 V for quercetin, 1.15 V for luteolin, 1.16V for (+)-catechin and 1.20 V for (+/-)-taxifolin) is compared with the observed over-all regeneration (34% for quercetin, 34% for (+)-catechin, 52% for (+/-)-taxifolin and 43% for luteolin by all-rac-alpha-tocopherol).

Formation of radicals during heating lysine and glucose in solution with an intermediate water activity.

Abstract Heating glucose with lysine under alkaline conditions (pH 7.0–10.0) was found to take place with consumption of oxygen together with formation of brown-colored compounds. Highly reactive intermediary radicals were detected when lysine and glucose were heated at intermediate water activity at pH 7.0 and 8.0. The detection was based on initial trapping of highly reactive radicals by ethanol followed by spin trapping of 1-hydroxyethylradicals with α-(4-pyridyl N-oxide)-N-tert-butylnitrone (POBN) and Electron Spin Resonance (ESR) spectroscopy. The generation of reactive intermediary radicals from the Maillard reactions was favored by enhancing alkaline conditions (pH 8.0) and stimulated by presence of the transition metal ion Fe2+. The stability of the nitrone spin traps, N-tert-butyl-α-phenylnitrone and POBN was examined in buffered aqueous solutions within the pH range 1–12, and found to be less temperature dependent at acidic pH compared to alkaline conditions. A low rate (kobs) of hydrolysis of POBN was found at the used experimental conditions of 70°C and pH 7.0 and 8.0, which made this spin trap method suitable for the detection of radicals in the Maillard reaction system.