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

Abstract

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/cm²). 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.

KEYWORDS:

• Essential oils
• HPLC-ESR-MS
• phototoxicity
• sensitizing effect
• singlet oxygen

Acknowledgements

We would like to thank Ms Shenli Hew from the Department of Clinical Research Center, Wakayama Medical University, for proofreading and editing the article. We also would like to thank Dr. Yoshie Tanaka for her helpful discussions.

Disclosure statement

There are no conflicts of interests to declare.