Download Chemistry Of Lichens Complete

Chemistry of Lichens
LICHEN:
a simple slow-growing plant which
typically forms a low crust-like, leaflike, or branching growth on rocks,
walls, and trees
Introduction to Lichen Chemistry
• Lichens produce a wide array of both
primary (intracellular) and secondary
(extracellular) compounds
- Primary metabolites include amino
acids, polyols, carotenoids,
polysaccharides, and vitamins
• Some, like the polysaccharide cell wall
compounds lichenan and isolichenan, have
taxonomic significance
Introduction to Lichen Chemistry (cont.)
• Carotenoid compounds have also been
intensely studied for clues to evolutionary
relationships
• 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
Biochemical
Pathways
for Lichen
Compounds
Chemical Synthesis Pathways
• Lichen acids are derived from three
chemical pathways:
- Shikimic acid pathway
• 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
• Common
products
include steroids
and triterpenes
like zeorin
(found in many
Cladonia taxa)
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
Orcinol and β-Orcinol Units
Acetate-Polymalonate Pathway:
Depsides
a orcinol depside
Acetate-Polymalonate Pathway:
Depsidones
a β-orcinol depsidone
Acetate-Polymalonate Pathway:
Usnic acid
An extremely
common
yellow-green
cortical
pigment
Also, one of the medically useful lichen acids
Acetate-Polymalonate Pathway:
Anthraquinones
• Most are K+ redpurple pigments in
the cortex or
apothecia
• Parietin a common
example in the
Teloschistaceae
Variations in Cortical Chemistry (cont.)
• Family
Teloschistaceae - all
have orange
anthraquinone
pigment parietin
• Letharia species all
contain vulpinic acid
(a pulvinic acid
derivative)
Acetate-Polymalonate Pathway:
Xanthones
Most
xanthones are
fluorescent
(UV+)
Identification of Lichen
Compounds
•
•
•
•
Classic Spot Tests
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
- Turns red to purple with anthraquinone
pigments, e.g. parietin
• C , Clorox(saturated aqueous bleach)
- Turns red with m-dihydroxy phenols (with
exceptions), e.g. lecanoric acid
- Turns green with dihydroxy dibenzofurans
Spot Test Reactions
(cont.)
• KC (K followed by C)
- Turns yellow with usnic acid
- Turns red with C- depsides and depsidones
which undergo rapid hydrolysis to yield a mhydroxy phenolic moiety, e.g. alectoronic acid
• PD (5% alcoholic p-phenylenediamine)
- Turns yellow, orange, or red with aromatic
aldehydes, e.g. stictic, norstictic,
fumarprotocetraric acids
Spot Test Example
“K+ yellow to
red” test on
lichen medulla
(salazinic acid)
Lichen Microcrystal Tests
- Introduced in the
1930s and 1940s
by Y. Asahina of
Japan
- More accurate
than spot tests
Microcrystal Tests
(cont.)
• Fragments of lichen are extracted with
acetone on a microscope slide
• A “crystallizing agent” is applied with a
coverslip
- G.E. (glycerin, acetic acid: 3:1 )
- G.A.W. (glycerin, alcohol, water: 1:1:1)
- G.A. o-T (glycerin, alcohol-, o-toluidine: 2:2:1)
• Slide is heated gently then allowed to cool
• Crystal formation (shape, color) is observed
under the compound microscope
TLC Procedure
- Acetone extracts are spotted on a standard
silica-gel plate (UV indicator present) at 1
cm 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)
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
- Identification is accomplished by
comparison with authentic samples and
published data
TLC Example
Possible Roles of Lichen Substances
in Nature
• UV screening for protection of photobiont
cells (especially cortical pigments)
• Protection from predation by arthropods,
snails, and other animals
• Membrane permeability effects to facilitate
release of carbohydrates from photobiont
cells
Human Uses of Lichen Substances
• Use of lichen substances by
humans (both historically and
today) for dyes, medicines,
decoration, food.





Food for humans and other animals
Medicinal problems and uses
Lichens as dyes (past and present)
Lichens in the perfume industry
• Antimicrobial effects to protect thallus from
decomposition




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)



Contact dermatitis sometimes occurs in
lumbermen who become sensitized to constant
exposure to lichens
Called “woodcutters eczema”or “cedar
poisoning”
Problem is caused primarily by an allergic
reaction to various lichen acids (as in Evernia
prunastri)





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




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…