Characterization of radicals arising from oxidation of commercially-important essential oils

Inappropriate use of essential oils may entail risks to human health due to mutational events, carcinogenic effects, genetic damages and sensitizing effect caused by generation of reactive oxygen species. In order to detect radicals that are expected to form during their oxidation, we measured the electron spin resonance (ESR) spectra of a standard reaction mixture (I) containing 25 μM flavin mononucleotide, 0.018% several essential oils (or 0.015% geraniol), 1.9 M acetonitrile, 20 mM phosphate buffer (pH 7.4), 0.1 M α-(4-pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN) and 1.0 mM FeSO4(NH4)2SO4 irradiated with 436 nm visible light (7.8 J/cm(2)). The ESR peak heights of the standard reaction mixture (I) of the essential oils increased in the following order: tea tree > palmarosa >geranium > clary sage > petitgrain > lavender > bergamot > frankincense > ravintsara > ylang ylang > lemongrass > niaouli > eucalyptus globulus > peppermint. The ESR peak height of the standard reaction mixture (I) of geraniol, a main component of palmarosa, was comparable to the one of palmarosa (97 ± 19% of palmarosa). Furthermore, high performance liquid chromatography (HPLC)-ESR analyses of the standard reaction mixture (I) of palmarosa and geraniol gave the same peaks. The results suggest that the radicals formed in the standard reaction mixture (I) of palmarosa are derived from geraniol. HPLC-ESR-mass spectrometry analyses detected m/z 294 ions, 4-POBN/5-hydroxy-3-methyl-3-pentenyl radical adducts and m/z 320 ions, 4-POBN/C7O2H9 radical adducts in the standard reaction (I) of geraniol. The 5-hydroxy-3-methyl-3-pentenyl and C7O2H9 radicals may be implicated in the sensitizing effect of palmarosa.