1. Chemistry of Lichens

2. LICHEN:
a simple slow-growing plant which typically forms a low crust-like, leaf-like, or branching growth on rocks, walls, and trees

4. 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

5. 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

6. Biochemical Pathways for Lichen Compounds

7. 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)

8. Mevalonic Acid Pathway
• Common
products
include steroids and triterpenes like zeorin
(found in many Cladonia taxa)

9. Shikimic Acid Pathway
• Pulvinic acid derivatives are most common (K- yellow pigments)

10. 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

12. Orcinol and β-Orcinol Units

13. Acetate-Polymalonate Pathway: Depsides
a orcinol depside

14. Acetate-Polymalonate Pathway: Depsidones
a β-orcinol depsidone

15. Acetate-Polymalonate Pathway: Usnic acid
An extremely common
yellow-green cortical
pigment
Also, one of the medically useful lichen acids

16. Acetate-Polymalonate Pathway:
Anthraquinones
• Most are K+ red- purple pigments in the cortex or
apothecia
• Parietin a common example in the
Teloschistaceae

18. Variations in Cortical Chemistry (cont.)
• Family
Teloschistaceae - all have orange
anthraquinone pigment parietin
• Letharia species all contain vulpinic acid (a pulvinic acid
derivative)

20. Acetate-Polymalonate Pathway:
Xanthones
Most
xanthones are fluorescent (UV+)

21. Identification of Lichen Compounds
• Classic Spot Tests
• Microcrystal Tests
• Paper and Thin Layer Chromatography
• High Performance Liquid
Chromatography (HPLC)

22. 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

23. 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 m- hydroxy 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

24. Spot Test Example “K+ yellow to
red” test on lichen medulla (salazinic acid)

25. Lichen Microcrystal Tests
- Introduced in the
1930s and 1940s by Y. Asahina of Japan
- More accurate than spot tests

26. 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

27. 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)

28. 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

29. TLC Example

30. 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

31. Human Uses of Lichen Substances
• Use of lichen substances by humans (both historically and today) for dyes, medicines, decoration, food.

32. Uses of Lichens: Past and Present

33. Uses of Lichens 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

34. 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)

35. Medical Problems
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)

36. 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

37. Lichen”in Dermatology
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…

38. “Lichen planus”