Phytochemistry 1
Lecture 3
Date:10/2/2015
Chemistry of lichens
Done by: Walaa wahdan
Previously we mentioned the primary and secondary metabolisms, and then we mentioned the
acetate – malonate pathway, today we want to start with the most important plants containing our
compound and one of these is lichen.
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LICHEN: a simple slow-growing plant which typically forms a low crust-like, leaf-like, or
branching growth on rocks, walls, and trees.
In fact they are not a real plant, not real algae or fungi; they are between algae and plant, a
mixture of plant, algae and fungi.
They grow either on tree trunks or on rocks, with blue or yellow color. So for this reason
islands are rich in lichen (where we have moist or shadow, we have lichen).

Lichens produce a wide array of both primary (intracellular) and secondary
(extracellular) compounds.
- Primary metabolites include aminoacids, polyols, carotenoids, polysaccharides,
and vitamins.
- Some, like the polysaccharide cell wall compounds lichenan and isolichenan, have
taxonomic significance.
The secondary metabolites are the most important which give the theraputic
value , also the primary metabolites served as source of energy for animals and
humans.
*Secondary metabolites are often called lichen acids
- Produced primarily by the mycobiont, they are deposited externally on the hyphae
of the cortex and/or medulla.
Note
hyphae refer to algae, the cortex and medulla refer to plant
plant and fungi.
so it’s a mixture of
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Phytochemistry 1

Biochemical
Pathways
for Lichen
Compounds
In the biosynthesis pathway: the alga develops to fungus, we have glucose “as we remember we
mentioned that all plants whether higher plants or lower plants which have chlorophyll, can make
photosynthesis”. Also we mentioned that glucose is the first product, we mentioned the acetyl CoA,
and that we had pyruvate from glycolysis, then from pyruvate we can get acetyl CoA, from acetyl CoA
get mavalonic acid that is responsible for the isoprene or terpinoids (monoterpenes, sesquiteroenes,
diterpenes….). In addition we have the Acetate-polymalonate pathway (or acetate-malonate pathway).
 Chemical Synthesis Pathways:
• Lichen acids are derived from three chemical pathways:
- Shikimic acid pathway (not going to discuss)
• pulvinic acid derivatives (yellow pigments)
- Mevalonic acid pathway
• terpenes
- Acetate-polymalonate pathway
• depsides, depsidones, usnic acid, anthraquinones, xanthones, aliphatic acids
(majority of lichen compounds).
** Mevalonic acid pathway:
Include steroids and triterpenes
like zeorin (found in many
Cladonia taxa).
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Always start from acetyl CoA, and its responsible for all terpinoids.
Here the ZEORIN is triterpinoid (from the structure it has two methyl groups at position #4,
and the third methyl group at position #21).
** Shikimic acid pathway:
• Pulvinic acid derivatives are most common (K- yellow pigments)
** Acetate-polymalonate pathway:
• Includes the most common lichen compounds.
• Derivatives of orcinol or ß-orcinol units.
• Includes depsides, depsidones, depsones, anthraquinones, xanthones,
as well as aliphatic fatty acids.
An aromatic compound
** If we look to these aromatic compounds:
A
B
C
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Phytochemistry 1
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In (A) >>> the two hydroxyl groups are meta to each other (m-)
In (B) >>> the two hydroxyl groups are ortho to each other (o-)
In (C) >>> the two hydroxyl groups are para to each other (p-)
** In B, C that we have the two hydroxyl groups in o- and p- position >>> they are from
shikimic acid pathway.
** Only the m- position substitution of hydroxyl groups is derived from the acetate-malonate
pathway.
*
** Here the triphenolic compound known as phloroglucinol , it
derives from the acetate-malonate pathway, because we still have
the hydroxyl groups at m- position.
If we have two methoxy groups like here, it also derives from
acetate-malonate pathway, because the methoxy groups are mto each other. (Other than that >>>> from shikimic pathway)
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If we look to Orsellinic acid:
*The hydroxyl groups in orsellinic acid are m- >>> derivative of acetate-malonate pathway.
*In β-orsellinic acid also the hydroxyl groups are m- (here the methyl group isn’t important,
we have nothing to do with it) >>> derivative of acetate-malonate pathway.
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Depsides :
a orcinol depside
*another compound which is lecanoric acid, a orcinol depside >>> it is a dimer.
Here even we have an acid group, we still have the phenolic groups at m- position, which
mean it derives from acetate-malonate pathway.
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Depsidones:
A β-orcinol depsidone
*stictic acid also a dimer, which has the methoxy groups m- to each other, so it derives from the
acetate-malonate pathway.
- here is one of the famous compound which is Usnic acid, that has the two phenolic groups mto each other >>>> from acetate-polymalonate pathway.
An extremely
common yellow-green
cortical pigment
Also, one of the medically useful lichen acids
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In addition to these compounds we have the Anthraquinones (we will study it later in
details), which responsible for coloration.
• Most are K+ redpurple pigments in the cortex or
Apothecia.
As we know when we have one ring we call it benzene , if we have the other ring (two rings)
then we have naphthalene, and if we have the three aromatic rings then its anthracene (so
anthraquinone derivative of the anthracene) >>>> they are synthesize from acetatemalonate pathway.
*as we see above the two hydroxyl groups in the first and second ring are m- >>> so it is
anthraquinone.
• Parietin a common example in the
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Teloschistaceae.(presented in our lichens)
Has orange anthraquinone pigment.
Variations in Cortical Chemistry (cont.)
 Family Teloschistaceae – all have orange anthraquinone pigment parietin
 Letharia species all contain vulpinic acid (a pulvinic acid derivative)
>>>> The orange color is due to anthraquinon creatine.
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Here we see the two compounds from shikkimic acid pathway:
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Another compound that is from acetate-malonate pathway (m-) , known as Xanthone which
also a colored material
Most xanthones are
fluorescent
(UV+)
In xanthone the first ring is benzene ring, and the second one is pyran ring (its abenzo pyran
derivative). This compound absorbs UV like anthraquinone (highly colored material).
*
We mentioned that we have the Zeorin as an
example of triterpene, the Vulponic acid and
Pulvinic acid as shikimic acid pathway
derivatives presented in the lichens, and the
Orcinol, Xanthone, anthraquinone from
acetate-polymalonate pathway.
- Identification of Lichen Compounds:


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Classic Spot Tests (by adding simple reagent)
Microcrystal Tests
Paper and Thin Layer Chromatography
High Performance Liquid Chromatography (HPLC)
Spot Test Reactions:
 K (10% aqueous, KOH:
- Turns yellow then red with most o-hydroxy aromatic aldehydes, e.g. salazinic acid
(derivative from shikimic acid pathway)
- Turns red to purple with anthraquinone pigments, e.g. parietin
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
C , Clorox(saturated aqueous bleach), used as bleaching:
- Turns red with m-dihydroxy phenols (with exceptions), e.g. lecanoric acid.
- Turns green with dihydroxy dibenzofurans.
 KC (K followed by C), the most important test :
- Turns yellow with usnic acid.
- Turns red with C- depsides and depsidones which undergo rapid hydrolysis to yield
a m-hydroxy phenolic moiety, e.g. alectoronic acid.
** Anthraquinone is K+ red to purple color.
 PD (5% alcoholic p-phenylenediamine):
- Turns yellow, orange, or red with aromatic aldehydes, e.g. stictic, norstictic,
fumarprotocetraric acids.
- Lichen Microcrystal Tests
- Introduced in the1930s and 1940s by Y. Asahina of Japan
(Japanese scientists are the famous worker on lichen, because they have the environment
which suitable for the growth of lichens (beaches)), large quantities of lichens were found in
England, and Japan
- More accurate than spot tests. In fact spot test is not differentiation test because by this
test we detect the presence of compounds (not lichens itself)>>> so we can't ensure that we
have lichens (many plants could contain the same compounds), put under the microscope
we see the key element which is unique for each plant (can ensure that we have lichens).
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TLC Procedure:
- Acetone extracts are spotted on a standard silica-gel plate (UV indicator present) at 1cm
intervals (usually 18 spots per plate)
- Plate is developed in a standard solvent (3 of the most common):
• SOLVENT A - toluene : dioxane : acetic acid (180:45:5)
• SOLVENT B - hexane : methyl tert.-butyl ether: formic acid (140:72:18)
• SOLVENT C - toluene : acetic acid (170:30)
** Acetic acid, formic acid, and toluene >>> known as mobile phase. Here the mobile
phases are polar solvent (must be polar, because our compounds are polar (anthraquinone,
xanthones…) in order to carry the spots.
TLC Procedure (cont.):
- Spots are marked with a pencil on the plate under UV light (noting any UV reactions).
- Visualization is done by treating the plate with sulfuric acid and heating for 10 min at
120°C. ( all phenolic compounds with heating give different colors)
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- Identification is accomplished by comparison with authentic samples and published data.
- TLC Example:
** Other method of identification is HPLC, put we are not going to mention it.
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Our compounds are highly conjugated compounds.
- Human Uses of Lichen Substances:
• Use of lichen substances by humans (both historically and today) for
- Dyes: previously before we had the synthetic dyes, they used the lichens due to xanthones and
anthraquinones sometimes for cloth or leathers dyeing.
- Medicines.
- Decoration.
- Food: its rich in carbohydrates and primary metabolites which are source of energy.
- Uses of Lichens:
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Food for humans and other animals
Medicinal problems and uses
Lichens as dyes (past and present)
Lichens in the perfume industry
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 Antimicrobial effects to protect thallus from decomposition>>> thallus found in this algae
grow in order to be protected from decomposition. lichens shows an antimicrobial effect, it
inhibit the growth of gram positive bacteria mainly Usnic acid.
- Medical Uses:
 Historically, some lichens were thought to have medicinal properties and were given names
indicating this (“doctrine of signatures”)
 –Lobaria pulmonaria and Parmelia sulcata(cure for lung disease)
 –Peltigera canina(cure for rabies)
 –Letharia vulpina(used as a poison against wolves)
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Medical Uses (cont.)

Usnic acid has been shown to inhibit the growth of gram+ bacteria

Active ingredient in some topical ointments sold in Europe and Russia (“USNO, BINAN”)
 Some lichen fatty acids and carbohydrates have anti-tumor effects in study animals. None
have been developed for humans yet.
–Most of this work has been done in Japan
- Lichen” in Dermatology:
It’s a skin disease which is not related to the plant lichen (no relation between them)
 A skin disease known as “lichen”is, fortunately, not caused by or related to lichenized
fungi
 –The term, however, confuses the literature and often the public
 –No pathogen is actually present and causes are unknown
 –Various versions of the disease are known as lichen planus, lichen sclerosus, lichen
puctatusetc…
Good luck 
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