Light emission from the Fe²⁺ -EDTA-ascorbic acid-H₂ O₂ system strongly enhanced by plant phenolic acids

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 Fe²⁺ -EDTA (ethylenediaminetetraacetic acid)-AA (ascorbic acid)-H₂ O₂ (hydrogen peroxide) system with a multitube luminometer (Peltier-cooled photon counter, spectral range 380-630 nm). The UPE, of 92.6 μmol/L Fe²⁺ , 185.2 μmol/L EDTA, 472 μmol/L AA, 2.6 mmol/L H₂ O₂ , reached 1217 ± 118 relative light units during 2 min measurement and was about two times higher (P < 0.001) than the UPE of incomplete systems (Fe²⁺ -AA-H₂ O₂ , Fe²⁺ -EDTA-H₂ O₂ , AA-H₂ O₂ ) and medium alone. Substitution of Fe²⁺ with Cr²⁺ , Co²⁺ , Mn²⁺ or Cu²⁺ as well as of EDTA with EGTA (ethylene glycol-bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid) or citrate powerfully inhibited UPE. Experiments with scavengers of reactive oxygen species (dimethyl sulfoxide, mannitol, sodium azide, superoxide dismutase) revealed the dependence of UPE only on hydroxyl radicals. Dimethyl sulfoxide at the concentration of 0.74 mmol/L inhibited UPE by 79 ± 4%. Plant phenolics (ferulic, chlorogenic and caffec acids) at the concentration of 870 μmol/L strongly enhanced UPE by 5-, 13.9- and 46.8-times (P < 0.001), respectively. It is suggested that augmentation of UPE from Fe²⁺ -EDTA-AA-H₂ O₂ system can be applied for detection of these phytochemicals.