A thorough phytochemical study of Stereocaulon evolutum was conducted, for the isolation of structurally related atranorin derivatives. Indeed, pilot experiments suggested that atranorin (1), the main metabolite of this lichen, would interfere with the lifecycle of hepatitis C virus (HCV). Eight compounds, including one reported for the first time (2), were isolated and characterized. Two analogs (5, 6) were also synthesized, to enlarge the panel of atranorin-related structures. Most of these compounds were active against HCV, with a half-maximal inhibitory concentration of about 10 to 70 µM, with depsides more potent than monoaromatic phenols. The most effective inhibitors (1, 5 and 6) were then added at different steps of the HCV lifecycle. Interestingly, atranorin (1), bearing an aldehyde function at C-3, inhibited only viral entry, whereas the synthetic compounds 5 and 6, bearing a hydroxymethyl and a methyl function, respectively, at C-3 interfered with viral replication.

The extreme resiliency of lichens to UV radiations makes them an interesting model to find new photoprotective agents acting as UV-blockers and antioxidant. In this research, using a new in vitro method designed to overcome the shortage of material associated to many studies dealing with natural products, we show that the three major compounds isolated from the lichen Vulpicida pinastri, vulpinic acid, pinastric acid and usnic acid, were UV blocker agents. Antioxidant assays evidenced superoxide anion scavenging activity. Combination of the most promising compounds against UVB and UVB radiations, usnic acid, vulpinic acid and pinastric acid, increased the photoprotective activity. At the same time, they were found not cytotoxic on keratinocyte cell lines and photostable in the UVA and UVB ranges. Thus, lichens represent an attractive source to find good candidate ingredients as photoprotective agents. Additionally, the uncommon scalemic usnic acid mixture in this Vulpicida species was proven through electronic circular dichroism calculation.

The composition of fatty acids (FAs) is known to discriminate families within genera of bacteria, fungi, octocorals and algae. Here, we applied the more sensitive gas chromatographic (GC) analysis to investigate the taxonomic value of using FA composition to discriminate 15 species of lichen, including three chlorolichens (twoCladoniaspecies andStereocaulon scutelligerum), nine tripartite lichens (Stereocaulonspecies) and three cyanolichens (Lichina pygmaea,Collema cristatum,Peltigera membranacea). One macroscopic cyanobacterium (Nostocsp.), corresponding to the photobiont partner ofPeltigeraandCollemacyanolichens, was included for comparison. Five lipid extraction methods were evaluated, usingStereocaulon scutelligerum. Shaking at 80 °C with chloroform/methanol (2:1, v/v) was the most efficient and reproducible method. The total FA composition of the 16 species was compared, using GC coupled with flame ionization detection or mass spectrometry. A statistical hierarchical cluster analysis was used to determine the similarity between the FA profiles, based on total, saturated, unsaturated and branched-chain FAs (BCFAs). The BCFA profile allowed a distinction between theStereocaulonspecies and the cyanolichens, in contrast to all the other types of FAs. A detailed characterization of eight FA subclasses, provided by the GC analysis, suggested a preferential pathway for the biogenesis of unsaturated FAs through desaturases, which was especially favoured in the cyanolichens.

The phytochemical study of Stereocaulon montagneanum harvested in Sumatra (Indonesia) led to the isolation of 11 known compounds including two metabolites not previously described in the genus Stereocaulon, peristictic acid (8) and menegazziaic acid (10). The complete ¹H and ¹³C NMR spectral assignments of stictic acid derivatives are reported with some revisions. Five depsidones belonging to the stictic acid chemosyndrome were superoxide anion scavengers as potent as ascorbic acid and with no toxicity on two human cell lines.