Grayanic acid is an organic compound found in certain lichens, particularly Cladonia grayi, where it serves as a secondary metabolite with notable taxonomic importance.

Grayanic acid crystallises as colourless, needle-like structures, melts at approximately 186–189 °C (367–372 °F), and displays distinctive fluorescence under ultraviolet light, aiding in its detection and study.

Genetic research has identified a key biosynthetic gene cluster responsible for its formation, highlighting biochemical pathways and enzymes that convert precursor compounds into grayanic acid and related metabolites such as sphaerophorin.

Beyond its chemical characteristics, grayanic acid has proven invaluable in refining lichen taxonomy, as variations in its presence and concentration underpin subtle species distinctions.

By comparing grayanic acid profiles across different populations and geographic regions, researchers have gained insights into evolutionary relationships, species distribution patterns, and the ecological roles that these fungal–algal partnerships play in diverse environments.

Evans described its needle-like crystals, which often formed radiating clusters under specific conditions, and noted a melting point near 185 °C (365 °F), consistent with Asahina's findings.

[2] In 1963, Shoji Shibata and Hsiich-Ching Chiang revised the molecular formula to C23H26O7 and refined the melting point to 186–189 °C, aligning it with subsequent modern analyses.

[6] The molecular structure of grayanic acid consists of a depside skeleton with two benzene rings connected by both ester (-CO-O-) and ether (-O-) linkages, forming a depsidone.

[6] While the initial structural assignment was based primarily on spectroscopic evidence, some uncertainty remained regarding the precise positions of the alkyl groups.

When the fungus is grown in culture, grayanic acid forms visible extracellular deposits on aerial fungal filaments (hyphae).

[7] Infrared spectroscopy identifies structural features such as a chelated carboxyl group at 1650 cm⁻¹, a lactonic linkage at 1750 cm⁻¹, and benzenoid rings with bands at 1570 and 1610 cm⁻¹.

In acetone, benzene ring protons exhibit chemical shifts at 6.13, 6.66, and 6.80 ppm, matching the pattern of related compounds like sphaerophorin.

This chromatographic behaviour aids in identifying grayanic acid in complex lichen extracts, especially in chemotaxonomic studies distinguishing species like Neophyllis melacarpa and N. pachyphylla by their metabolite profiles.

During bicarbonate solution tests, it forms an oily layer between ether and aqueous phases, in addition to its standard solubility properties.

[11] Grayanic acid is also a major secondary metabolite in Jarmania tristis, a byssoid lichen endemic to Tasmania's cool temperate rainforests.

This variation appears geographically influenced, with West Indian specimens showing different proportions of these compounds compared to North American ones.

Production starts days after transferring the fungus from liquid to solid growth medium and increases as aerial fungal filaments develop.

Under optimal conditions, the cultured fungus can achieve production rates comparable to those of some non-lichen fungi producing similar compounds.

The fungus's ability to synthesise grayanic acid in pure culture shows that the compound, while characteristic of the intact lichen, does not require the algal partner.

Initially used with taste tests to separate species, detailed studies in the 1970s revealed more nuanced relationships between chemical composition and morphology.

The final stages of the synthesis involved careful manipulation of protecting groups to yield grayanic acid, which was identical in all respects to the natural product isolated from lichens.

[6] The biosynthesis of grayanic acid involves fungal polyketide synthases and subsequent modifications, following a pathway similar to other lichen depsidones.

Structural similarities and chemical transformation studies led Shibata and Chiang to propose sphaerophorin as a biosynthetic precursor to grayanic acid.

This finding confirmed that the algal partner does not influence the chemotype, establishing the fungal component as the sole regulator of secondary metabolite production.

This dynamic biosynthetic network produces related compounds, such as stenosporonic and divaronic acids, which exhibit variations in their carbon side-chain lengths across populations.

These compounds, described in detail by Shibata and Chiang, share structural similarities with grayanic acid, including benzenoid and ester group arrangements.

These compounds are lower homologs in the same chemical series as grayanic acid, sharing its basic structure but differing in carbon side-chain lengths.