7. Phenolic Compounds Catalyze the Conversion of Singlet Oxygen into Hydroxyl Radical in the Presence of DMPO
Jun-ichi Ueda and Toshihiko Ozawa
Keywords: singlet oxygen, spin trap, ESR, phenolic compounds
The relationship between the activities of phenolic compounds to quench singlet oxygen (1O2) and their oxidation potentials was investigated by electron spin resonance (ESR) spectroscopic and cyclic voltammetric methods. 1O2 was generated by UVA ( > 330 nm)-irradiation of hematoporphyrin (HP) and detected as an oxidative product, 2,2,6,6-tetramethyl-4-piperidone-1-oxyl (TEMPON), of 2,2,6,6-tetramethyl-4-piperidone (TEMPD). Phenolic compounds used here were the following ten compounds: isoeugenol, p-eugenol, guaiacol, phenol, p-methoxyphenol, Trolox C, dopamine, 2,6-dimethoxyphenol, 4-hydroxybenzoic acid, and hydroquinone. The ESR measurements indicated that Trolox C quenched 1O2 more than other phenolic compounds at physiological pH, whereas phenol and 4-hydroxybenzoic acid quenched it only slightly. Since the oxidation potential of Trolox C is lower than those of phenol and 4-hydroxybenzoic acid, these results suggested that quenching activity of phenolic compounds against 1O2 was closely related to their oxidation potentials.
To investigate what kind of reactive oxygen species were generated during the reaction of 1O2 with phenolic compounds, the compounds were added to this 1O2-generating system in the presence of two spin traps 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and the derivative 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DEPMPO), and the spin adducts of reactive oxygen species were measured by ESR. As a consequence, the formation of DMPO adduct of hydroxyl radical (.OH) (DMPO/.OH) was observed, whereas no DEPMPO/.OH was produced. This result suggested that no phenolic compound used here reacted with 1O2 to generate reactive oxygen species.
Furthermore, the relationship obtained between the amounts of DMPO/.OH produced and oxidation potentials of phenolic compounds indicated that phenolic compounds with lower oxidation potentials may efficiently accelerate the conversion of 1O2 to .OH compared to those with higher oxidation potentials, although the formation of DMPO/.OH was dependent on the initial reaction of DMPO with 1O2. On the other hand, in the absence of phenolic compounds, DMPOX was observed, and the signal intensity was dependent on both irradiation time and DMPO concentration.