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Phenolic Compounds


Like all the groups reviewed until now, this one does also consist of a large number of molecules of heterogeneous structure. Their common feature is the presence of at least one hydroxyl-substituted aromatic ring system.

Most phenolic compounds belong to the flavonoids. Lignin, the primary substance of wood, is the most common member of this group. The following table outlines the most important groups of plant phenolic compounds.

The Most Important Classes of Phenolic Compounds in Plants

number of C-atoms
basic skeleton
class
6
C6
simple phenols, benzoquinones
7
C6 - C1
phenolic acids
8
C6 - C2
acetophenone, phenylacetic acid
9
C6 - C3
hydroxycinnamic acid, polypropene,
coumarin, isocoumarin
10
C6 - C4
naphtoquinone
13
C6 - C1 - C6
xanthone
14
C6 - C2 - C6
stilbene, anthrachinone
15
C6 - C3 - C6
flavonoids, isoflavonoids
18
(C6 - C3)2
lignans, neolignans
30
(C6 - C3 - C6)2
biflavonoids
n
(C6 - C3)n
(C6)n
(C6 - C3 - C6)n
lignins
catecholmelanine
(condensed tannins)
according to J. B. HARBORNE, 1980

The starting product of the biosynthesis of most phenolic compounds is shikimate. Phenols are acidic due to the dissociability of their -OH group. They are rather reactive compounds and as long as no steric inhibition due to additional side chains occurs, they form hydrogen bonds. Consequently, many flavonoids have intramolecular bonds. Another important feature is their ability to form chelate complexes with metals. Also, they are easily oxidized and, if so, form polymers (dark aggregates). The darkening of cut or dying plant parts is caused by this reaction. They have usually an inhibiting effect on plant growth. Among the phenylpropanol derivatives of lower molecular weight are a number of scents like the coumarins, cinnamic acid, sinapinic acid, the coniferyl alcohols and others. These substances and their derivatives are at the same time intermediates of the biosynthesis of lignin.


Flavonoids: In 1975, the number of identified flavonoids was estimated to be larger than 2000. Some important representatives and their biological significance are listed in the table below.

The Most Important Classes of Flavonoids and their Biological Significance

class
number of known members
biological significance (so far as known)
anthocyanin(s)
250
red and blue pigments
chalcons
60
yellow pigments
aurones
20
yellow pigments
flavones
350
cream-coloured pigments of flowers
flavonols
350
feeding repellents (?) in leaves
dihydrochalcons
10
some taste bitter
proanthocyanidins
50
astringent substances
catechins
40
some have properties
like those of tannins
biflavonoids ?
65
?
isoflavonoids
15
oestrogen effect, toxic for fungi
nach J. B. HARBORNE, 1980

The basic structure of flavonoids is derived from the C15 body of flavone. They differ from other phenolic substances in the degree of oxidation of their central pyran ring. And, very fundamentally, also in their biological properties. While some classes (the flavonones, for example) are colourless, the members of other classes (the anthocyanes, for example) are always coloured and known as pigments of flowers or other plant parts. Anthocyanes are normally red or yellow, their colour is pH-dependent. Blue pigments are achieved by chelate formation with certain metal ions (FeIII or AlIII, for example).

The variability of the flavonoids is largely based on the hydroxylation and/ or methylation pattern of the three ring systems. A correlation between two flavonoids points often to a relationship between the producing plant species. They have therefore proven to be suitable traits for the study of the phylogenetic relations between higher plants. The quinones are another group of phenolic compounds. We have already met some of its members that function as co-factors. Accordingly, they do not actually belong to the secondary plant products but have to be counted among those of the basic metabolism. As has been mentioned before, phenolic compounds occur usually not unbound within plant tissues. They are mostly coupled to other molecules, often to glucosyl residues, but to sulphate- or acetyl-residues, too. One of the reasons may be that they are toxic when in a free state and are detoxified, at least partially, if coupled. Many low molecular weight compounds, for example thymol, are used in medicine as antiseptics due to their toxicity. Different types of bonds between flavonoids (for example anthocyanes) and a glycosyl residue lead to different derivatives that increase the range of flower colours (and colour shades). The glycosylation of flavonoids has an additional, ecologically not less important function. It has been brought into connection with pest protection and protection against other animals. Based on their biological functions, phenolic compounds can be classified as follows:

The Ecological Meaning of Some Phenolic Compounds for Plants

function
group
example(s) and plant species
where the effect was studied
flower pigments
anthocyanes

chalcons

aurones

yellow flavonoids

flavones

cyanidin-3,5-diglucosid in Rosa

coreopsin in Coreopsis tinstoria

aureusin in Anthirrhinum majus

gossypetine-7-glucoside in Gossypium

apigenin-7-glucoside in Bellis perennis

fruit pigments
anthocyanines

isoflavones

chalcons

petunidin glucoside in Atropa belladonna

osajin in Maclura pomifera

ocanin in Kyllingi brevifolia

allelopathic substances
quinones

phenols

phenolcarboxylic acids

hydrocinnamic acid

juglon in Juglans regia

hydroquinone in Arctostaphylos

sialic acid in Quercus falcata

ferulic acid in Adenostoma

protection against pests
quinones

tannines

flavonols

juglon in Carya ovata

gallotannine in Quercus robur

quercitine-glycosids in Gossypium

fungicide
isoflavones

phenolcarboxylic acids

dihydrochalones

luteon in Lupinus

protocatechunic acid in Allium

phloridcine in Malus pumila

phytoalexines
stilbens

phenylanthrenes

isoflavanes

pterocarpanes

phenylpropanoids

fucocoumarins

reservatrol in Arachis hypogaea

orchinol in Orchis militaris

vestitiol in Lotus corniculatus

pisatin in Pisum sativum

coniferyl alcohol in Linum usitiltissimum

psoralen in Petroselinum crispum

according to J. B. HARBORNE, 1980




© Peter v. Sengbusch - b-online@botanik.uni-hamburg.de