Pannarin is an organic compound of the depsidone class, first isolated in 1941 from Japanese lichens of the genus Pannaria. It has the molecular formula C18H15ClO6 and forms clear needle-like crystals with a melting point of 216–217 °C (421–423 °F). Its chemical structure was initially proposed on the basis of degradative studies that broke the molecule down into simpler fragments, but this early assignment proved incorrect and was revised in the 1970s using a combination of spectroscopic analysis and chemical synthesis. The revised structure was confirmed by X-ray crystallography in 1991. Laboratory studies have reported that pannarin inhibits growth and induces cell death in certain human cancer cell lines, and shows synergistic antimicrobial effects with some antibiotics against drug-resistant Staphylococcus aureus. It also displays complex behaviour under ultraviolet light, both promoting and limiting damage to biological molecules depending on conditions, and can reduce symptoms of tobacco mosaic virus infection in experimental treatments of tobacco leaves. In nature, pannarin occurs in various lichen species across several genera, including Pannaria, Psoroma, Megalospora and, more rarely, Buellia and Lepraria. Its presence is used as a taxonomic character in lichen systematics.
History Pannarin was first isolated as a chlorine-containing depsidone from Japanese specimens of the lichen Pannaria lanuginosa and related species by Itiro Yosioka in 1941. He assigned a tentative structure to pannarin in 1941 through degradative studies that broke the molecule down into simpler components. In his experiments pannarin was oxidised to an acid, which was then heated to give smaller fragments, including a phenol derivative whose structure he confirmed by synthesis; on this basis he proposed the structure 2-chloro-6-hydroxy-3-methoxy-1,4,8-trimethyl-11-oxo-11H-dibenzo[b,e][1,4]dioxepin-7-carbaldehyde. This structure placed the diaryl ether linkage next to the hydroxy group in ring B, an arrangement otherwise known at the time only from the depsidone variolaric acid. It was later shown to be incorrect. In 1974, Jackman, Sargent, and Elix published a preliminary structural assignment for pannarin, followed by a full account in 1975 that is generally regarded as giving the correct structure. Huneck and Lamb published work on the related compound argopsin, which they showed could be produced by chlorinating pannarin. Blaser and Stoeckli-Evans later noted that the crystal structure of a vicanicin derivative provided indirect support for the structures of both argopsin and pannarin. The molecular structure was ultimately confirmed through X-ray crystallography in 1991. Early taxonomic studies sometimes misidentified other lichen compounds as pannarin. For instance, during chemotaxonomic surveys of South American Pseudocyphellaria species in the 1970s, compounds initially thought to be pannarin were later identified as the related depsidones granulatin and chlorogranulatin.
Properties Pannarin is a member of the class of chemical compounds called depsidones. Its IUPAC name is 8-chloro-9-hydroxy-3-methoxy-1,4,7-trimethyl-6-oxobenzo[b][1,4]benzodioxepine-10-carbaldehyde. In the ultraviolet spectrum it shows two absorption maxima at 212 and 234 nanometres (nm). Its infrared spectrum has characteristic bands around 1720 cm−1 (carbonyl group), 1600 cm−1 (aromatic ring vibrations) and a broad band near 3500 cm−1 (hydroxyl group). Pannarin's molecular formula is C18H15ClO6; it has a molecular mass of 362.75 grams per mole. In its purified crystalline form, it occurs as clear needles, with a melting point of 216–217 °C (421–423 °F). In microcrystallisation experiments, pannarin yields clusters of colourless needles that later change into aggregates of yellowish lamellae. X-ray crystallographic analysis shows that pannarin crystallises in the monoclinic crystal system, space group P21/c, with four molecules per unit cell (Z = 4). The molecule has a folded conformation, with its two phenyl rings inclined at about 123.6° to each other. The central seven-membered dioxepin-11-one ring adopts a boat conformation, and there is an intramolecular hydrogen bond between the formyl group and the adjacent hydroxyl group.
Bioactivity In laboratory experiments, pannarin has been reported to inhibit the growth of human prostate carcinoma DU-145 cells in vitro. In melanoma cell lines, pannarin showed activity at concentrations of 12.54–25 μM, where it induced apoptosis through mechanisms that included DNA fragmentation and increased caspase-3 activity; at higher concentrations (50 μM), it instead triggered necrotic cell death. The authors linked these effects at least in part to the compound's ability to generate reactive oxygen species within cells. Pannarin demonstrates moderate synergistic antimicrobial action against methicillin-resistant Staphylococcus aureus when combined with gentamicin, but shows antagonism with levofloxacin. Because pannarin did not induce membrane permeabilisation in S. aureus, the authors concluded that its action was likely directed at intracellular targets. In vitro tests on promastigote forms of three Leishmania strains showed that pannarin and its chlorinated derivative 1'-chloropannarin inhibited parasite growth at 50 μg/mL, whereas usnic acid was active at 25 μg/mL; in the same study only usnic acid was evaluated in a mouse model of cutaneous leishmaniasis. When irradiated with ultraviolet light, pannarin can both damage and protect biological systems. Under ultraviolet light exposure, it can break down red blood cells through free radical mechanisms, particularly in the presence of oxygen and at higher temperatures. However, pannarin also demonstrates significant protective effects against UV damage – it can shield proteins from both UVA and UVB radiation damage more effectively than related compounds like 1'-chloropannarin and atranorin. The authors suggested that this dual behaviour reflects pannarin's ability to both absorb UV radiation and interact with reactive oxygen species. In controlled experiments, when sprayed on tobacco leaves before tobacco mosaic virus infection, pannarin significantly reduced the symptoms of viral infection by decreasing both the number of necrotic lesions (by 93%) and their size. This protective effect appeared to work through a mechanism independent of the plant's usual salicylic acid or jasmonic acid defence pathways.
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