Although most of the Ascomycetes present DHN-melanin, some reports suggest that A. nidulans does not produce this type of melanin. In this study, we analyzed the pigment extracted from highly melanized strains (MEL1 and MEL2) of Aspergillus nidulans to determine the type of melanin present in this fungus. Our results showed that the pigment produced by MEL1 and MEL2 mutants possesses physical and chemical properties and UV- and IR-spectra very similar to synthetic DOPA-melanin. The characterization of this pigment in terms of its degradation products indicated the presence of indolic units, which were also found in synthetic DOPA-melanin. The analyses of the elemental composition showed that the pigment extracted from these mutants has a high percentage of nitrogen and, therefore, it cannot be DHN-melanin, which presents only trace of nitrogen. This observation was confirmed in the test with tricyclazole because this inhibitor of DHN-melanin biosynthesis did not suppress pigment production in the MEL1 and MEL2 strains. On the other hand, in a medium containing tropolone, an inhibitor of DOPA-melanin biosynthesis, the dark pigmentation of the colonies was not observed indicating that this compound inhibited melanin production in these strains. Taken together, the results obtained in this study indicate that melanin produced by these mutants is DOPA type, representing the first report on characterization of this type of melanin in A. nidulans.

Production of the microbial pigments is one of the emerging fields of research due to a growing interest of the industry for safer products, easily degradable and eco-friendly. Fungi constitute a valuable source of pigments because they are capable of producing high yields of the substance in the cheap culture medium, making the bioprocess economically viable on the industrial scale. Some fungal species produce a dark-brown pigment, known as melanin, by oxidative polymerization of phenolic compounds, such as glutaminyl-3,4-dihydroxybenzene (GDHB) or catechol or 1,8-dihydroxynaphthalene (DHN) or 3,4-dihydroxyphenylalanine (DOPA). This pigment has been reported to act as "fungal armor" due to its ability to protect fungi from adverse conditions, neutralizing oxidants generated in response to stress. Apart from the scavenging activity, melanin exhibits other biological activities, including thermoregulatory, radio-and photoprotective, antimicrobial, antiviral, cytotoxic, anti-inflammatory, and immunomodulatory. Studies have shown that the media composition and cultivation conditions affect the pigment production in fungi and the manipulation of these parameters can result in an increase in pigment yield for large-scale pigment production. This chapter presents a comprehensive discussion of the research on fungal melanin, including the recently discovered biological activities and the potential use of this pigment for various biotechnological applications in the fields of biomedicine, dermocosmetics, materials science, and nanotechnology.

Background: Fusarium solani f. sp. piperis is a phytopathogen that causes one of the most destructive diseases in black pepper crops, resulting in significant economic and crop production losses. Consequently, the control of this fungal disease is a matter of current and relevant interest in agriculture. Objective: The objective was to synthesize eugenol derivatives with antifungal activity. Methods: In this study, using bimolecular nucleophilic substitution and click chemistry approaches, four new and three known eugenol derivatives were obtained. The eugenol derivatives were characterized and their antifungal and cytotoxic effects were evaluated. Results: Eugenol derivative 4 (2-(4-allyl-2-methoxyphenoxy)-3-chloronaphthalene-1,4-dione) was the most active against F. solani f. sp. piperis and showed acceptable cytotoxicity. Compound 4 was two-fold more effective than tebuconazole in an antifungal assay and presented similar cytotoxicity in macrophages. The in silico study of β-glucosidase suggests a potential interaction of 4 with amino acid residues by a cation-π interaction with residue Arg177 followed by a hydrogen bond with Glu596, indicating an important role in the interactions with 4, justifying the antifungal action of this compound. In addition, the cytotoxicity after metabolism was evaluated as a mimic assay with the S9 fraction in HepG2 cells. Compound 4 demonstrated maintenance of cytotoxicity, showing IC50 values of 11.18 ± 0.5 and 9.04 ± 0.2 μg mL-1 without and with the S9 fraction, respectively. In contrast, eugenol (257.9 ± 0.4 and 133.5 ± 0.8 μg mL-1), tebuconazole (34.94 ± 0.2 and 26.76 ± 0.17 μg mL-1) and especially carbendazim (251.0 ± 0.30 and 34.7 ± 0.10 μg mL-1) showed greater cytotoxicity after hepatic biotransformation. Conclusion: The results suggest that 4 is a potential candidate for use in the design of new and effective compounds that could control this pathogen.