Light Emission from the Fe(2+)-EGTA-H₂O₂ System: Possible Application for the Determination of Antioxidant Activity of Plant Phenolics.

Oxidative reactions can result in the formation of electronically excited species that undergo radiative decay depending on electronic transition from the excited state to the ground state with subsequent ultra-weak photon emission (UPE). We investigated the UPE from the Fe(2+)-EGTA (ethylene glycol-bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid)-H₂O₂ system with a multitube luminometer (Peltier-cooled photon counter, spectral range 380 to 630 nm). The UPE of 92.6 µmol/L Fe(2+)-185.2 µmol/L EGTA-2.6 mmol/L H₂O₂ reached 4319 ± 755 relative light units during 2 min measurement and was about seven times higher (p < 0.001) than the UPE of incomplete systems (Fe(2+)-H₂O₂, EGTA-H₂O₂) and medium alone. Substitution of Fe(2+) with Cr(2+), Co(2+), Mn(2+) or Cu(2+) as well as of EGTA with EDTA (ethylenediaminetetraacetic acid) or citrate completely abolished UPE. Experiments with ROS scavengers revealed the dependence of UPE on hydroxyl radicals suggesting occurrence of oxidative attack and cleavage of the ether bond in EGTA backbone structure and formation of triplet excited carbonyl groups with subsequent light emission. Plant phenolics (ferulic, chlorogenic and caffec acids) at concentration 87 µmol/L and ascorbate at 0.46 mmol/L inhibited UPE by 90 ± 4%, 90 ± 5%, 97 ± 2% and 92 ± 1%, respectively. Quenching of UPE from Fe(2+)-EGTA-H₂O₂ system can be used for evaluation of antioxidant activity of phytochemicals.