158
10
Monoterpenoids often constitute part of the so-called essential oils. Essential oils (see
7 Box 10.2) are defined as non-water-soluble compounds, which usually contain volatile
aroma compounds from plants.
The first step in monoterpene formation is the dephosphorylation and ionization of GPP
to a geranyl carbocation (7 Fig. 10.3). Acyclic monoterpenes are then derived by deprotonation. Those include geraniol (odour of roses, lemon grass and geranium), its isomeric form
linalool (in many herbs such as mint, laurels, cinnamon and rosewood and citrus fruits,
often acting as olfactory cues for pollinators), myrcene (in bay, thyme, parsley, cardamom
and hops) and ß-ocimene (odour of lavender and basil). Many of these compounds can be
found in peltate glands from which they are easily released as volatiles upon contact.
For the cyclization of monoterpenes, the limonene synthase (a TPS) is the most important enzyme. From the geranyl cation, linalyl pyrophosphate (LPP) is formed, which lacks
the trans-double bond that inhibits rotation and thereby cyclization. LPP stays bound to
the enzyme. The removal of the pyrophosphate group leads to the linalyl cation, which is
then formed into an α-terpinyl cation by cyclization. The α-terpinyl cation can undergo
various steps of deprotonation, cyclization and ring closure leading to different cyclic end
products (. Fig. 10.3). Deprotonation stabilizes the compound and leads to structures
Box 10.1 Terpene Synthases (TPS)
Terpene synthases are the class of enzymes with a divalent metal ion cofactor that produce several different terpene end products (C5, C10, C15 and C20 terpenes). One enzyme can hereby
generate multiple products from one substrate, but they can also use different substrates, which
increase the variation (Tholl 2006; Chen et al. 2011; Pazouki and Niinemets 2016). The first step
of the reaction usually involves ionization of the substrate by cleaving off the diphosphate group
and the formation of a carbocation. This can lead to cyclization before the reaction is terminated
via deprotonation.
Whereas in the moss Physcomitrella patens only one TPS gene is present, in higher plants over
100 TPS genes can be found. Examples are the isoprene synthase, the monoterpene synthases
(such as limonene synthase, copalyl diphosphate synthase, pinene synthase, myrcene synthase,
ent-kaurene synthase, linalool synthase, sesquiterpene synthases and diterpene synthases).
Box 10.2 Essential Oils
The term “essential oils” was probably coined in the sixteenth century by alchemists as “Quinta
essential” (fifth element, universal ether). They occur often in the plant families Alliaceae, Apiaceae, Asteraceae, Lamiaceae, Myrtaceae, Poaceae and Rutaceae and are responsible for the scents
of plants especially spices and herbs (Dhifi et al. 2016). This is due to the volatile compounds
within the essential oils that are characterized by a high vapor pressure. They are usually stored
in secretory glands on leaves (e.g. eucalyptus, mint, thyme, rosemary, sage, basil, marjoram, pine,
cypress) and flowers (e.g. orange, lavender, hops, chamomile, clove flower bud), but they are also
be found in rhizomes (e.g. ginger), seeds (e.g. coriander, cardamom, pistachios, nutmeg, pepper),
fruits (e.g. fennel, anise, citrus, apple) and in wood (e.g. cinnamon, sandalwood, rosewood, balsam fir tree, camphor). They are extracted for industrial (e.g. fragrances) and medicinal purposes
by steam distillation, as they are insoluble in water. Essential oils are mixtures of monoterpene
and sesquiterpene group but also phenylpropanoids.
In plants, essential oils are important for communication between plants, to attract
pollinating insects and to repel predators. Due to their insect repellent and insecticidal activity
they are also discussed as biopesticides (Regnault-Roger et al. 2012; Pavela and Benelli 2016).
Chapter 10 · Terpenes and Terpenoids
10
Monoterpenoids often constitute part of the so-called essential oils. Essential oils (see
7 Box 10.2) are defined as non-water-soluble compounds, which usually contain volatile
aroma compounds from plants.
The first step in monoterpene formation is the dephosphorylation and ionization of GPP
to a geranyl carbocation (7 Fig. 10.3). Acyclic monoterpenes are then derived by deprotonation. Those include geraniol (odour of roses, lemon grass and geranium), its isomeric form
linalool (in many herbs such as mint, laurels, cinnamon and rosewood and citrus fruits,
often acting as olfactory cues for pollinators), myrcene (in bay, thyme, parsley, cardamom
and hops) and ß-ocimene (odour of lavender and basil). Many of these compounds can be
found in peltate glands from which they are easily released as volatiles upon contact.
For the cyclization of monoterpenes, the limonene synthase (a TPS) is the most important enzyme. From the geranyl cation, linalyl pyrophosphate (LPP) is formed, which lacks
the trans-double bond that inhibits rotation and thereby cyclization. LPP stays bound to
the enzyme. The removal of the pyrophosphate group leads to the linalyl cation, which is
then formed into an α-terpinyl cation by cyclization. The α-terpinyl cation can undergo
various steps of deprotonation, cyclization and ring closure leading to different cyclic end
products (. Fig. 10.3). Deprotonation stabilizes the compound and leads to structures
Box 10.1 Terpene Synthases (TPS)
Terpene synthases are the class of enzymes with a divalent metal ion cofactor that produce several different terpene end products (C5, C10, C15 and C20 terpenes). One enzyme can hereby
generate multiple products from one substrate, but they can also use different substrates, which
increase the variation (Tholl 2006; Chen et al. 2011; Pazouki and Niinemets 2016). The first step
of the reaction usually involves ionization of the substrate by cleaving off the diphosphate group
and the formation of a carbocation. This can lead to cyclization before the reaction is terminated
via deprotonation.
Whereas in the moss Physcomitrella patens only one TPS gene is present, in higher plants over
100 TPS genes can be found. Examples are the isoprene synthase, the monoterpene synthases
(such as limonene synthase, copalyl diphosphate synthase, pinene synthase, myrcene synthase,
ent-kaurene synthase, linalool synthase, sesquiterpene synthases and diterpene synthases).
Box 10.2 Essential Oils
The term “essential oils” was probably coined in the sixteenth century by alchemists as “Quinta
essential” (fifth element, universal ether). They occur often in the plant families Alliaceae, Apiaceae, Asteraceae, Lamiaceae, Myrtaceae, Poaceae and Rutaceae and are responsible for the scents
of plants especially spices and herbs (Dhifi et al. 2016). This is due to the volatile compounds
within the essential oils that are characterized by a high vapor pressure. They are usually stored
in secretory glands on leaves (e.g. eucalyptus, mint, thyme, rosemary, sage, basil, marjoram, pine,
cypress) and flowers (e.g. orange, lavender, hops, chamomile, clove flower bud), but they are also
be found in rhizomes (e.g. ginger), seeds (e.g. coriander, cardamom, pistachios, nutmeg, pepper),
fruits (e.g. fennel, anise, citrus, apple) and in wood (e.g. cinnamon, sandalwood, rosewood, balsam fir tree, camphor). They are extracted for industrial (e.g. fragrances) and medicinal purposes
by steam distillation, as they are insoluble in water. Essential oils are mixtures of monoterpene
and sesquiterpene group but also phenylpropanoids.
In plants, essential oils are important for communication between plants, to attract
pollinating insects and to repel predators. Due to their insect repellent and insecticidal activity
they are also discussed as biopesticides (Regnault-Roger et al. 2012; Pavela and Benelli 2016).
Chapter 10 · Terpenes and Terpenoids