11. DMPO-mediated Conversion of Singlet Oxygen to Hydroxyl Radical in the Presence of Phenolic Compounds

Jun-ichi Ueda, Keizo Takeshita and Toshihiko Ozawa

Keywords: DMPO, singlet oxygen, hydroxyl radical, phenolic compounds, UVA

Skin is a defensive barrier and constantly exposed to certain kinds of oxidative stress such as ultraviolet and ionizing irradiation. Singlet oxygen (¹O₂) is produced through the interaction of the ultraviolet-A component (UVA) of sunlight with endogenous photosensitizers such as porphyrins and flavins in the skin. Since phenolic compounds are widely used for the production of pharmaceuticals as well as cosmetic and food flavoring goods, skin is inevitably exposed to phenolic compounds. Therefore, it is considered that a phenolic compound, which is a potent electron donor, may cause the conversion of ¹O₂ to another reactive oxygen species. Then, we intend to elucidate whether oxygen radicals are produced from ¹O₂ in the presence of phenolic compounds.

In this study, ¹O₂ was generated by the irradiation with UVA-visible light ( > 330 nm) of hematoporphyrin (HP), a model compound for endogenous porphyrins in the skin and the assignment of oxygen radicals was performed by the ESR spin-trapping method using a spin trap, 5,5-dimethyl-1-pyrroline N-oxide (DMPO). The reaction of ¹O₂ generated by UVA-visible light ( > 330 nm) irradiation of air-saturated solutions of HP with phenolic compounds in the presence of DMPO gave an electron spin resonance (ESR) spectrum characteristic of the DMPO-hydroxyl radical spin adduct (DMPO/·OH). In contrast, the ESR signal of 5,5-dimethyl-2-pyrrolidone-N-oxyl (DMPOX), an oxidative product of DMPO, was observed in the absence of phenolic compounds. The ESR signal of DMPO/·OH decreased in the presence of ·OH scavengers, ethanol and sodium formate, and disappeared completely in the presence of sodium azide, a quencher of ¹O₂, indicating the ¹O₂-mediated formation of free ·OH during the reaction. A linear correlation between the amounts of DMPO/·OH produced and oxidation potentials of phenolic compounds was observed. When DMPO was replaced with 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DEPMPO), no DEPMPO adduct of oxygen radical species was obtained. These results indicate that ¹O₂ reacts at first with DMPO, and the resulting DMPO-¹O₂ adduct is immediately decomposed/reduced to form ·OH. Phenolic compounds should participate in this reaction as electron donors.