The lightweight monoterpenes are usually volatile and many essential oils belong to this category. Plant families that produce essential oils are limited: most belong to Lamiaceae, Rutaceae, Apiaceae, and Zingiberaceae. Their structural variation is extensive due to cyclization reactions by some promiscuous enzymes like the limonene synthase.
Stereochemistry of compound structures contributes much to their flavors. For example, (+)- and (-)-limonene smell like orange (Citrus sinensis, Rutaceae) and lime (Citrus aurantifolia, Rutaceae), respectively, whereas peppermint (Mentha x piperita, Lamiaceae) contains both. Likewise, (+)- and (-)-carvone smell like caraway (Carum carvi, Apiaceae) and spearmint (Mentha spicata, Lamiaceae), respectively. Monoterpenes also include the iridoid structures, characterized by a cyclopentane ring fused with a six-membered heterocycle with oxygen. One example is gentiopicroside, abundant in the roots of Gentianaceae plants (e.g. Gentiana lutea, G. scabra, and Swertia japonica). These roots are still often used as bitter stomachic in traditional herbal medicine in Asian countries.
Sesquiterpenes are derived from farnesyl diphosphate (FPP) and several thousand lactones have been identified. Many are found in Asteraceae, and are less volatile than monoterpenes. Well known example include γ-bisabolene, the flavor of ginger (Zingiber officinale, Zingiberaceae), α-humulene in hops (Humulus lupulus, Cannabaceae), and costunolide, the bitter component of chicory (Cichorium intybus, Asteraceae). Not a few sesquiterpenes exhibit medicinal activity. Artemisinin from Artemisia annua (Asteraceae) is a promising anti-malarial drug, gossypol from cotton seed oil (Gossypium spp., Malvaceae), which is a sesquiterpene dimer, causes male infertility, and trichothecenes are a family of fungal toxins found in grain food infected by Fusarium species.
Design of Mono- and Sesqui-terpene ID numbers ID番号の設計
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