CASE-DFT Structure Elucidation of Proton-Deficient Chlorodepsidones from the Indonesian Lichen Teloschistes flavicans and Structure Revision of Flavicansone

Biodiscovery efforts in Indonesia have aimed to explore the understudied chemical diversity of its rich lichen flora, seeking to find new products endowed with significant biological properties. The chemical screening of a Teloschistes flavicans extract led to selection of this species for further investigation. LC/MS and 1H NMR-based dereplication pinpointed six chlorodepsidones from the thallus of a sample of this lichen. This led to the streamlined isolation and the subsequent structure elucidation of the three new compounds norflavicansone 1, flavicansone 2, and isocaloploicin 3, along with the known chlorodepsidones 4-6, stictic acid 7, aurantiamide acetate 8, and parietin 9. The challenging structure elucidation of these proton-deficient metabolites benefited from a state-of-the-art workflow involving a synergistic combination of Computer-Assisted Structure Elucidation (CASE) and Density Functional Theory (DFT) calculations of the top-ranked candidates. This investigation also led to the revision of flavicansone's structure, previously described from this species. The three new molecules that are being reported here are remarkable in that they represent hybrid depsidones in which one of the aromatic rings is derived from orsellinic acid and the other is derived from β-orcinol, a rare structural feature for lichen depsidones.

Discovery and excavation of lichen bioactive natural products

Lichen natural products are a tremendous source of new bioactive chemical entities for drug discovery. The ability to survive in harsh conditions can be directly correlated with the production of some unique lichen metabolites. Despite the potential applications, these unique metabolites have been underutilized by pharmaceutical and agrochemical industries due to their slow growth, low biomass availability, and technical challenges involved in their artificial cultivation. At the same time, DNA sequence data have revealed that the number of encoded biosynthetic gene clusters in a lichen is much higher than in natural products, and the majority of them are silent or poorly expressed. To meet these challenges, the one strain many compounds (OSMAC) strategy, as a comprehensive and powerful tool, has been developed to stimulate the activation of silent or cryptic biosynthetic gene clusters and exploit interesting lichen compounds for industrial applications. Furthermore, the development of molecular network techniques, modern bioinformatics, and genetic tools is opening up a new opportunity for the mining, modification, and production of lichen metabolites, rather than merely using traditional separation and purification techniques to obtain small amounts of chemical compounds. Heterologous expressed lichen-derived biosynthetic gene clusters in a cultivatable host offer a promising means for a sustainable supply of specialized metabolites. In this review, we summarized the known lichen bioactive metabolites and highlighted the application of OSMAC, molecular network, and genome mining-based strategies in lichen-forming fungi for the discovery of new cryptic lichen compounds.

Lichen Depsidones with Biological Interest

Depsidones are some of the most abundant secondary metabolites produced by lichens. These compounds have aroused great pharmacological interest due to their activities as antioxidants, antimicrobial, and cytotoxic agents. Hence, this paper aims to provide up-to-date knowledge including an overview of the potential biological interest of lichen depsidones. So far, the most studied depsidones are fumarprotocetraric acid, lobaric acid, norstictic acid, physodic acid, salazinic acid, and stictic acid. Their pharmacological activities have been mainly investigated in in vitro studies and, to a lesser extent, in in vivo studies. No clinical trials have been performed yet. Depsidones are promising cytotoxic agents that act against different cell lines of animal and human origin. Moreover, these compounds have shown antimicrobial activity against both Gram-positive and Gram-negative bacteria and fungi, mainly Candida spp. Furthermore, depsidones have antioxidant properties as revealed in oxidative stress in vitro and in vivo models. Future research should be focused on further investigating the mechanism of action of depsidones and in evaluating new potential actions as well as other depsidones that have not been studied yet from a pharmacological perspective. Likewise, more in vivo studies are prerequisite, and clinical trials for the most promising depsidones are encouraged.

Parmosidone K, a new meta-depsidone from the lichen Parmotrema tsavoense

Further phytochemical investigation on P. tsavoense led to one new meta-depsidone, parmosidone K together with one known compound, barbatic acid. Their structures were determined by 1D and 2D NMR analysis, high resolution mass spectroscopy, and comparison their NMR data with those reported in literatures. Parmosidone K was evaluated for α-glucosidase inhibition and revealed the powerful activity with IC₅₀ value of 3.12 µM.