Limonene is the most abundant monoterpenoid hydrocarbon found in citrus essential oils. It exhibits versatile biological activities, ranging from herbivore deterrence to anticancer effects through mechanisms such as membrane perturbation and induction of xenobiotic metabolism.
Chemical Properties
Limonene (C₁₀H₁₆, MW 136.23 g/mol) exists as a colorless, chiral liquid (bp 176°C, mp -97°C, density 0.84 g/cm³). It contains endocyclic (Δ1) and exocyclic (Δ8) double bonds, enabling Diels-Alder reactivity and allylic oxidation. (R)-limonene predominates in citrus (98%), whereas (S)-limonene is found in pine needles. Racemization occurs at 300°C. It is slightly soluble in water (13.8 mg/mL), highly volatile (vapor pressure 1.55 mmHg), and can polymerize under acid-catalyzed conditions to form limonene dioxide.
Biosynthetic Pathway
Within citrus glands, the MEP pathway produces geranyl pyrophosphate (GPP), which is converted to limonene by limonene synthase (CitLS, EC 4.2.3.20, Mg²⁺-dependent) via a 4-membered ring transition state. Subsequent CYP71B-mediated allylic hydroxylations yield perillyl alcohol, carveol, and carvone. Additionally, limonene-10-hydroxylase generates urinary metabolites.
