Lichens as a Promising Source of Unique and Functional Small Molecules for Human Health and Well-Being

Abstract

Drug design and discovery processes are very important for human health because of the prevalence of many diseases for which there is no known medical cure. Much scientific effort backed by the financial support of governments has gone into discovering and designing novel functional molecules. It is widely accepted that natural products are the lead and key components to explore when it comes to extracting functional compounds from them. This is because designing new molecules based on known natural substances and testing of naturally produced substances seems to be the most rational way of searching for potentially functional and promising compounds. Moreover, recent ethnopharmacological surveys demonstrate that many patients and physicians prefer and trust the natural remedies of complementary and alternative medicine. Moreover, it is well known that many over-the-counter drugs are natural compounds or inspired from the skeletal structure of these functional compounds. Similarly, cosmetics companies have increasingly given greater importance to using natural products in the production of cosmetics and care products that can be shown to be safe, organic, natural, or clean, not least because of the high profit margin of using such materials but also because the end consumer trusts these cosmetics and gladly pays higher prices for them. The attention and efforts of researchers have progressively been drawn toward finding new functional molecules from natural sources or designing new ones based on already known molecules. However, there are some serious limitations due to the contents of their complex compound: for example, the costly and/or time-consuming processes needed to detect pharmacologically active or cosmetically valuable compounds found in highly structured organisms, such as plants, animals, and marine organisms. Nevertheless, lichen species are now believed to be advantageous for researchers because of their exclusive biological and chemical structures. As one of the most ancient and successfully adapted organisms, lichens are defined by their symbiotic association between mycobionts (fungi) and photobionts (photoautotrophic, algal partners). Although photoautotrophic partners produce organic materials by using photosynthetic systems, mycobionts provide a safe and humid environment for them to synthesize secondary metabolites that generally play a protective role in this sophisticated symbiotic structure. The close metabolic relationship between these partners and the centuries-long process of adaptation to the extreme conditions under which lichens live are the main reasons they produce more than 1000 unique secondary metabolites. Such metabolites are known to have strong protective properties against various negative physical and biological influences, and are considered a major source of pharmacoactive or cosmetically valuable compounds by scientists involved in drug and cosmetics discovery research. Lichen-derived small molecules are believed to already exist as pulvinic acid, anthraquinones, xanthones, depisides, depsidones, pseudodepsidones, dibenzofurans, and shikimic acid derivatives, and potentially have a wide variety of pharmacological activities, including antiinflammatory, antipyretic, analgesic, antiviral, antimicrobial, antiprotozoal, antiproliferative, antitumor, photoprotective, enzyme-inhibitory, and antiangiogenic properties. Furthermore, unlike other complex natural product sources, there is no limitation to isolating these functional small molecules from lichens because every lichen species includes secondary metabolites, so much so that lichens could be taxonomically categorized according to the contents of their secondary compounds. Consequently, the simplicity of the chemical constitutuents of lichens allows researchers to isolate secondary metabolites by using simple methods, such as thin-layer chromatography and microcrystallography, although the biological structure of lichens on its own provides unique and functional small molecules. The purpose of this chapter is to draw the attention of scientists to lichen-derived small molecules and their importance for human health and quality of life. The chapter reviews basic information about lichens, secondary metabolites, chemical structures, synthesizing and isolation processes, pharmacologically and cosmetically active mechanisms, and future perspectives in the light of recent works investigating the relationship between lichen-derived substances and their potential use to cure or prevent human diseases and improve the quality of human life.