Characterization of Oxygenated Heterocyclic Compounds and in vitro Antioxidant Activity of Pomelo Essential Oil

Citrus essential oils are widely used for aromatherapy and the alternative treatment of chronic diseases. Beyond the aroma substances, they are known to contain bioactive nonvolatile components; however, little knowledge has been gained about nonvolatiles in the essential oil of pomelo (Citrus grandis Osbeck), the largest citrus fruit. The purpose of this study was to analyze the nonvolatile oxygenated heterocyclic compounds (OHCs) of pomelo essential oils and evaluate their in vitro antioxidant activities for further development.

Cold-pressed essential oil (CPEO) and distilled essential oil (DEO) were obtained from the peel of the Liangping pomelo cultivar. High-performance liquid chromatography (HPLC) coupled with a photodiode array and fluorescence detection method was developed to identify and quantify the OHCs of the two essential oils. Ferric reducing antioxidant power and 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl 3-oxide (PTIO) radical scavenging assays were used to determine the antioxidative capabilities.

Thirteen OHCs were identified in CPEO. Coumarins such as meranzin (2.0 mmol L^-1) and furanocoumarins such as isoimperatorin (1.3 mmol L^-1) composed the majority of nonvolatiles in CPEO. These OHCs were characterized by high proportion (58%) of side chain epoxides. Five OHCs, namely, auraptenol, 6',7'-dihydroxybergamottin (6',7'-DHB), imperatorin, isoimperatorin and 8-geranyloxypsoralen were first identified in pomelo CPEO. Eight OHCs were detected at trace amounts in pomelo DEO. Antioxidant assays showed that CPEO was multiple times more potent than DEO regarding the total reducing power and radical scavenging capacity. Clearance of PTIO, a stable reactive oxygen species, followed slow kinetics.

Coumarins and furanocoumarins, two families of OHCs, constituted most of the nonvolatile components in CPEO. The nonvolatiles contributed significantly to the in vitro antioxidant activity of CPEO. Pomelo CPEO showed good prospects as a potential long-lasting natural antioxidant.