178 Handbook of herbs and spices
compounds together is the presence of a 5-carbon building block, which is referred
to as the isoprene unit. Table 11.1 illustrates the classes of isoprenoids found in
plants. Compositional changes occur in essential oils due to the (i) effect of extrinsic
conditions (ii) effect of interspecific and infrastructure differences (iii) effect of
ontogeny (iv) effect of processing parameters and (v) effect of adulteration (Chi-
kuenshu and Lawrence 1997).
Monoterpenes
In monoterpenes, it is usually possible to detect the presence of two of these isoprene
units and in sesquiterpenes, three. Figure 11.1 depicts the structure of an isoprene
unit. Monoterepenes can be divided into three sub groups (i) acyclic, no ring systems
(ii) monocyclic, one ring and (iii) bicyclic, two rings (Fig. 11.2). Further proliferation
occurs through addition (oxidation) or removal (reduction) of double bonds, and by
addition of oxygen to form alcohols (-OH), ketones (-CO), aldehydes (-CHO) and
esters (-OCO-).
Sesquiterpenes
Sesquiterpenes, because they possess five more carbons than the monoterpenes, have
far greater potential for structural and stereo chemical diversity. Sesquiterpenes form
the largest class of terpenoids and are found in plants, liverworts, mosses, fungi and
algae. They commonly occur with the monoterpenoids in essential oils. They are less
volatile and have less direct organoleptic properties, than monoterpenes. They are an
essential part of most volatile oils, subtly influencing odour (Waterman 1993, Bramley
1997).
11.2.2 Phenylpropenes
The skeleton of phenylpropenes invariably consists of a 6-carbon aromatic ring with
3-carbon side chain attached. The side chain always contains a double bond but only
Table 11.1 Main classes of isoprenoids found in plants
Carbon Name Parent isoprenoid Sub-class
atoms
10 Monoterpenoids GPP Iridoids
15 Sesquiterpenoids FPP Abscissic acid, sesquiterpene lactones
20 Diterpenoids GGPP Gibberellins
25 Sesterpenoids GFPP None
30 Triterpenoids Squalene Phytosterols, saponins, cardenolides
40 Tetraterpenoids Phytoene None
740 Polyprenols, rubbers GGPP+ (C 5 ) n None
GPP – Geranyl pyrophosphate, FPP – Farnesyl pyrophosphate, GGPP – Geranyl geranyl pyrophosphate,
GFPP – Geranyl farnesyl pyrophosphate.
Fig. 11.1 An isoprene unit.