Chemistry of Evernia prunastri depsides and conversion to orcein dyes

 Lichen Acids Chemical Structure and Conversion to Orsellinic Acid

5-Mar-22 Gevan R Marrs

Background

Each of the three main 3 articles I have that show the 3 reactions converting lichen depsides to orsellinic acid, then on to orsinol and finally orcein dye compounds portray the starting depside as lecanoric acid. Lecanoric acid is mentioned as a common acid found in those lichens well-known for orchil dyes, purples and reds, and a C+R spot test reaction. Since Evernia prunastri does not contain lecanoric acid, but three others, I wanted to compare structures lichen acids of known “orchil productive” lichens with Evernia prunastri lichen acids to see if I can deduce how to facilitate purple dyes from Evernia.

Approach

I surveyed various sources to find the chemical structures of the main dye producing lichen acids, then sought references to specifici lichen species (or genera) that contained those lichen acids. Then I tried to speculate and/or find information to support ways in which Evernia lichen acid(s) can create orcein. The Appendix shows the full collected set of data.

Data

Lecanoric acid – C+R, well-known to create orsellinic acid, then orcinol leading to orcein – pH sensitive red and purple dyes.

Found in: Ochrolecia Rocellichen acid Parmelia (some) Punctelia rudecta Usnea subclavata Parmotrema (some) Umbilicaria (some spp. incl mammulata, americana and deusta) Flavopunctilia soredica

Gyrophoric acid – C+R, well-known to create orcin leading to orcein – pH sensitive red and purple dyes

Found in: Ochrolecia Lasillia Umbilicaria (some spp incl mammulata, americana and deusta) Xanthoparmelia

Lichen Acids in Evernia prunastri

Evernic acid – C-, not shown (that I have found yet) creating orcinol leading to orcein – NOT pH sensitive pinkish to purplish-brown dyes.

Found in: Evernia prunastri

Note that Evernic acid is Lecanoric acid with a methoxy group replacing hydroxy (O-CH₃ replacing O-H) on terminal ring.

Usnic acid – C-,– Brown and some Yellow dyes

Found in: Evernia prunastri Usnea Cladonia Lecanora Ramalina farinacea and R. pollinaria Parmelia Lobelia oregana, pr pulmnaria Flavoparmelia caperata (brown dye) Flavoparmelia soredica (brown dye)

Not obvious to me how this would readily generate Orsellinic acid. I don’t think any of the other listed species (e.g., Usnea spp.) generate reds or purples with AFM, so this doesn’t seem to be likely a source of purplish dyes via AFM for Evernia prunastri.

Physodic acid – KC+R to O -  ? dyes

Found in: Evernia prunastri

I’m really unclear how the carbon chains would be stripped to give a kind of depsidone structure….

Reaction comparisons for different depsides

Classic depsides -> orcin reactions are Lecanoric acid hydrolysis to two Orsellenic acid molecules, these are then decarboxylated to 2 Orcinol molecules. For example, Rather et al 2019 shows:

Conclusions

So it seems to me that:

1.     Lecanoric acid is the prototypical, successful precursor to the classic purple and red orchil dyes.

2.     Gyrophoric acid can have one ring hydrolyzed off the tri-cyclic depside, creating one molecule Orsellinic acid and leaving one molecule Lecanoric, which can then hydrolze to two more Orsellinic acid molecules. So in a sense Gyrophoric acid is like “super-Lecanoric”. Upreti et al 2010 says explicitly that this happens. (Page 126). 

3.     I don’t see why one cannot split off via hydrolysis on Evernic acid one molecule of Orsellinic acid, leaving one with ? (methoxy-Orsellinic acid[1]? Oh, it is called Everninic acid per Upreti 2010). Kind of a “half-hearted” Lecanoric acid, as it only produces one molecule orsellinic acid on first hydrolysis. I’m not sure what can transpire if the everninic acid is decarboxylated. Oh - The Wikipedia entry for Orsellinic acid states: “This (orsellinc acid) is also produced when everninic acid and ramalic acid is boiled with barium hydroxide.” Upreti et al 2010 also states that indeed this happens (p. 126). They show:

By chance a reference was recently posted (Mosbach and Schultz 1971) that states specifically there is an enzyme that does convert everninic acid to orsellinic acid! “

“Orsellinate decarboxylase, catalyzing the decarboxylation of orsellinic acid (2,4-dihydroxy. 6-methyl benzoic acid) to orcinol has been isolated from the lichen Lasallia prustulata. The substrate specificity of the enzyme has been tested: only orsellinic acid, 3-chloro-, 5-chloro. orsellinic acids, homo-orsellinic acid (2,4-dihydroxy-6-ethyl benzoic acid) and everninic acid (4-methoxy-2-hydroxy-6-methyl benzoie acid) were decarboxylated.”

This does not necessariliy mean that the conditions of our AFM is an enzymatic decarboxylation, but it means the reaction can occur, so could go on to produce orcinol and then orcein.

My hypothesis is that the relative difficulty that craft dyers seem to have with Evernia AFM compared to C+R lichens may well be the added difficulty of achieving conversio of everninic acid to orsellic acid.

References

1971 Klaus Mosbach and Joachim Schultz, Studies on Lichen Enzymes Purification and Properties of Orsellinate Decarboxylase Obtained from Lasallia pustulata, Avdelning för Biokemi, Kemicentrum, Lunds Universitet (Received March 18/ July 22, 1971)

2018 Luqman Jameel Rather*, Salman Jameel, Showkat Ali Ganie and Khursheed Ahmad Bhat, Lichen Derived Natural Colorants: History, Extraction, and Applications, Handbook of Renewable Materials for Coloration & Finishing pp. 102-114, Scrivner.

2010 Upreti, D.K., S. Joshi and S. Nayaka. 2010. Chemistry of common dye yielding lichens of India. ENVIS Forestr Bull., 10 (1): 122-133.

Full PDF of this article.

Appendix Database of Lichen chemical precursors and dye colors by AFM (yellow highlights are species accessible to me)

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[1] everninic acid ; Synonyms: everninic acid;orsellinic acid 4-methyl ether ; 

 

[2] Upreti 2010 and others show Orcinol as a hydrolysis product of a depside precursor in lichen. Whether Shaheen reference suggests detection after hydrolysis of a precursor is unknown. Since the lichen genera cited are lichens with either lecanoric or gyrophric acid, it seems they (Shaheen et al 2019) may be describing an intermediate. Orcinol is clearly a precursor to ammonia-fostered production of orchil / arkil / orcein purple dyes.