Analysis of Lichen Metabolites, a Variety of Approaches

Abstract

Lichens produce secondary metabolites which have been first considered as a chemical support fully involved in lichen taxonomy. As a consequence, analytical methods were developed and applied to these organisms from a long time, some of them being standardized. Then, lichen analysis benefitted from new developments and techniques applied for isolation and identification of secondary metabolites which are exposed and discussed herein. Some ancient techniques for lichen taxonomy are still used as spot tests , which involve application of specific reagents directly on the lichen thallus. TLC is also still extensively used with standardized protocols affording more accurate information on lichens’ metabolic profiles. Identification of lichen compounds from the shape of crystals observed under microscope was facilitated as some major lichen metabolites are extracted in high yields. X-rays are now used in some cases where classical spectroscopic, UV, IR, MS, and NMR techniques do not allow unambiguous assignments. Using such techniques for isolated lichen compounds, some characteristic identification patterns of these substances are presented here. Hyphenated techniques, coupling separation and identification, are more and more used and broaden the analysis facilities of lichen compounds. They enable early dereplication and subsequent focusing on bioactive or original compounds. Other trends in lichenology involve in situ analyses thanks to specific and innovative NMR or MS techniques that yield valuable information directly from the natural complex matrix. Several advantages can be expected from those approaches: no extraction steps, qualitative and quantitative information in a few minutes or hours, and direct analysis of genuine compounds, avoiding chemical artefacts associated with extraction and purification processes (Table 11.2). Moreover, some of these methods pave the way for the development of imaging techniques that might help correlating metabolites to their specific ecological environment. Altogether, such enhancements might enable harnessing of lichens’ unique chemo-diversity.