Lichen Acids

a large group of organic compounds occurring in many genera of lichen (for example, Ramalina, Evernia, Cladonia, Anzia).

Several specific lichen acids are usually characteristic of each species of lichen, a fact that is useful in taxonomy. Aqueous lichen extracts have long been used in medicine, since the majority of lichen acids exhibit antibiotic activity. The chemical structure of all lichen acids is characterized by the presence of two polysubstituted phenol or phenol-carboxylic acid residues bonded to each other in a variety of combinations.

A depside structure is characteristic of the antibiotics Sferorodin and lecanoric (I), anacic, and ramalinic acids. The antibiotic physodic acid (II) is the simplest example of a depsidone structure. A dibenzofurane structure is typical of usnic acid (III), a widely occurring, effective antibiotic used in the treatment of skin diseases and tetanus. All of the lichen acid types mentioned are biosynthesized entirely from acetic acid residues, and most exhibit an active antimycobacterial effect. The fourth structural type characteristic of lichen acids (1,4-diphenyl butadiene) includes the poisonous vulpinic, chrysopetraric, and leprarinic acids; it also includes Epanorin and rhizocarpic acid, which contain the amino acid residues of leucine and tyrosine.