Seskwiterpene

Seskwiterpene, commonly referred to as sesquiterpene, is a class of terpenes that are built from three isoprene units, totaling C15H24. They are among the major constituents of essential oils and occur in many plants, fungi, and some marine organisms. Structurally diverse, sesquiterpenes can be acyclic, monocyclic, bicyclic, or tricyclic, and many are modified by oxidation, rearrangement, or formation of lactones or alcohols. Biosynthesis proceeds mainly from farnesyl pyrophosphate (FPP) through sesquiterpene synthases, which catalyze carbocation-driven cyclizations and subsequent rearrangements; the IPP/DMAPP precursors are supplied by either the mevalonate pathway or the plastidial MEP pathway in different organisms.

Notable sesquiterpenes include germacrene D, caryophyllene (beta-caryophyllene), humulene, valencene, zingiberene, patchoulol, and α-bisabolol. These compounds are principal constituents of patchouli oil, sandalwood oil, ginger oil, and citrus oils; their structures and odors contribute to fragrance and flavor profiles.

In nature, sesquiterpenes serve ecological roles such as deterrence of herbivores and pathogens. In industry, they are used in perfumery, flavors, and pharmacology. Some sesquiterpene lactones (for example helenalin, parthenolide) exhibit bioactivity and can act as allergens or irritants, so handling these compounds requires care. Many sesquiterpenes have reported antimicrobial, anti-inflammatory, or anticancer activities in laboratory studies, but results vary and more research is needed.

Sesquiterpenes are categorized by carbon skeletons (e.g., germacrane, eudesmane, cadinane, and humulane families) and by functional groups. They are typically analyzed by gas chromatography–mass spectrometry for essential oil profiling.