Modeling Fungal Melanin Buildup: Biomimetic Polymerization of 1,8-Dihydroxynaphthalene Mapped by Mass Spectrometry

Interactions of Melanin with Electromagnetic Radiation: From Fundamentals to Applications

Melanin, especially integumentary melanin, interacts in numerous ways with electromagnetic radiation, leading to a set of critical functions, including radiation protection, UV-protection, pigmentary and structural color productions, and thermoregulation. By harnessing these functions, melanin and melanin-like materials can be widely applied to diverse applications with extraordinary performance. Here we provide a unified overview of the melanin family (all melanin and melanin-like materials) and their interactions with the complete electromagnetic radiation spectrum (X-ray, Gamma-ray, UV, visible, near-infrared), which until now has been absent from the literature and is needed to establish a solid fundamental base to facilitate their future investigation and development. We begin by discussing the chemistries and morphologies of both natural and artificial melanin, then the fundamentals of melanin-radiation interactions, and finally the exciting new developments in high-performance melanin-based functional materials that exploit these interactions. This Review provides both a comprehensive overview and a discussion of future perspectives for each subfield of melanin that will help direct the future development of melanin from both fundamental and applied perspectives.

Melanin: insights into structure, analysis, and biological activities for future development

Melanin, a widely distributed pigment found in various organisms, possesses distinct structures that can be classified into five main types: eumelanin (found in animals and plants), pheomelanin (found in animals and plants), allomelanin (found in plants), neuromelanin (found in animals), and pyomelanin (found in fungi and bacteria). In this review, we present an overview of the structure and composition of melanin, as well as the various spectroscopic identification methods that can be used, such as Fourier transform infrared (FTIR) spectroscopy, electron spin resonance (ESR) spectroscopy, and thermogravimetric analysis (TGA). We also provide a summary of the extraction methods of melanin and its diverse biological activities, including antibacterial properties, anti-radiation effects, and photothermal effects. The current state of research on natural melanin and its potential for further development is discussed. In particular, the review provides a comprehensive summary of the analysis methods used to determine melanin species, offering valuable insights and references for future research. Overall, this review aims to provide a thorough understanding of the concept and classification of melanin, its structure, physicochemical properties, and structural identification methods, as well as its various applications in the field of biology.

Insight into the Antioxidant Activity of 1,8-Dihydroxynaphthalene Allomelanin Nanoparticles

Melanins are stable and non-toxic pigments with great potential as chemopreventive agents against oxidative stress for medical and cosmetic applications. Allomelanin is a class of nitrogen-free melanin often found in fungi. The artificial allomelanin obtained by the polymerization of 1,8-dihydroxynaphthalene (DHN), poly-DHN (PDHN), has been recently indicated as a better radical quencher than polydopamine (PDA), a melanin model obtained by the polymerization of dopamine (DA); however, the chemical mechanisms underlying this difference are unclear. Here we investigate, by experimental and theoretical methods, the ability of PDHN nanoparticles (PDHN-NP), in comparison to PDA-NP, to trap alkylperoxyl (ROO•) and hydroperoxyl (HOO•) radicals that are involved in the propagation of peroxidation in real conditions. Our results demonstrate that PDHN-NP present a higher antioxidant efficiency with respect to PDA-NP against ROO• in water at pH 7.4 and against mixed ROO• and HOO• in acetonitrile, showing catalytic cross-termination activity. The antioxidant capacity of PDHN-NP in water is 0.8 mmol/g (ROO• radicals quenched by 1 g of PDHN-NP), with a rate constant of 3 × 105 M-1 s-1 for each reactive moiety. Quantum-mechanical calculations revealed that, thanks to the formation of a H-bond network, the quinones in PDHN-NP have a high affinity for H-atoms, thus justifying the high reactivity of PDHN-NP with HOO• observed experimentally.

Antioxidant Activities of Hydroxylated Naphthalenes: The Role of Aryloxyl Radicals

Herein is delineated a first systematic framework for the definition of structure-antioxidant property relationships in the dihydroxynaphthalene (DHN) series. The results obtained by a combined experimental and theoretical approach revealed that 1,8-DHN is the best performing antioxidant platform, with its unique hydrogen-bonded peri-hydroxylation pattern contributing to a fast H atom transfer process. Moreover, the comparative analysis of the antioxidant properties of DHNs carried out by performing DPPH and FRAP assays and laser flash photolysis experiments, revealed the higher antioxidant power associated with an α-substitution pattern (i. e. in 1,8- and 1,6-DHN) with respect to DHNs exhibiting a β-substitution pattern (i. e. in 2,6- and 2,7-DHN). DFT calculations and isolation and characterization of the main oligomer intermediates formed during the oxidative polymerization of DHNs supported this evidence by providing unprecedented insight into the generation and fate of the intermediate naphthoxyl radicals, which emerged as the main factor governing the antioxidant activity of DHNs.

Hydrophobic Melanin via Post-Synthetic Modification for Controlled Self-Assembly

Allomelanin is a class of nitrogen-free melanin mostly found in fungi and, like all naturally occurring melanins, is hydrophilic. Herein, we develop a facile method to modify synthetic hydrophilic allomelanin to yield hydrophobic derivatives through post-synthetic modifications. Amine-functionalized molecules of various kinds can be conjugated to allomelanin nanoparticles under mild conditions with high loading efficiencies. Hydrophobicity is conferred by introducing amine-terminated alkyl groups with different chain lengths. We demonstrate that the resulting hydrophobic allomelanin nanoparticles undergo air/water interfacial self-assembly in a controlled fashion, which enables the generation of large-scale and uniform structural colors. This work provides an efficient and tunable surface chemistry modification strategy to broaden the scope of synthetic melanin structure and function beyond the known diversity found in nature.

Guaiacol as a natural melanin biosynthesis inhibitor to control northern corn leaf blight

BACKGROUND

The natural 1,8-dihydroxynaphthalene (DHN) melanin biosynthesis inhibitors (MBIs) are one of the promising approaches to the integrated management of plant diseases but have received scarce attention until now. Herein, to explore the natural DHN MBIs used in the control of northern corn leaf blight (NCLB), a library of 53 essential oil compounds was used to screen the MBIs against Exserohilum turcicum, the causal pathogen of NCLB, using tricyclazole as a reference compound.

RESULTS

The results of morphological change in the colony, thermogravimetric analysis, ultraviolet-visible spectroscopy, and transmission electron microscopy confirmed that guaiacol could effectively inhibit the melanin production at 50 μg/mL under in vitro conditions. The in vitro bioassay results indicated that this inhibition effect was concentration-dependent and the minimum inhibition concentration of guaiacol was 50 μg/mL. The in vivo experimental results demonstrated that guaiacol significantly inhibited appressorium formation and penetration on corn leaf sheaths at the concentration of 500 μg/mL. The pot experiment results revealed that there were no differences between guaiacol (500 μg/mL) and tricyclazole (100 μg/mL) in control efficacy. The enzymatic assay suggested that guaiacol might exert the activity through inhibiting DHN polymerization to form melanins, which was distinct from tricyclazole.

CONCLUSIONS

Taken together, this study screened out guaiacol as a natural MBI from 53 essential oil compounds and verified its effectiveness in the control of NCLB at 500 μg/mL. Above all, this research opened an avenue for exploring natural DHN MBIs in the integrated management of plant diseases. © 2022 Society of Chemical Industry.

Dihydroxynaphthalene-Based Allomelanins: A Source of Inspiration for Innovative Technological Materials

Melanins are a wide class of natural pigments biosynthesized by different kinds of living organisms throughout all of the life domains, from bacteria to fungi, plants, and mammals. The biological functions played by these natural pigments are different (i.e., camouflage, radioprotection, thermoregulation) and ascribable to a peculiar set of physical-chemical properties making melanins a unique class of biopolymers. Among these, allomelanins from 1,8-dihydroxynaphthalene (1,8-DHNmel) produced by some Ascomycetes have recently attracted particular interest for their robustness and ability to protect fungi against both hostile (i.e., attack from fungicidal agents) and extreme (i.e., high energy radiations) environments. Starting from this background, in this mini-review we offer a panorama of the recent advances on the oxidative chemistry of 1,8-DHN leading to the formation of allomelanin mimics with tailored structural and functional properties for technological applications.

Anisotropic Synthetic Allomelanin Materials via Solid-State Polymerization of Self-Assembled 1,8-Dihydroxynaphthalene Dimers

Melanosomes in nature have diverse morphologies, including spheres, rods, and platelets. By contrast, shapes of synthetic melanins have been almost entirely limited to spherical nanoparticles with few exceptions produced by complex templated synthetic methods. Here, we report a non-templated method to access synthetic melanins with a variety of architectures including spheres, sheets, and platelets. Three 1,8-dihydroxynaphthalene dimers (4-4', 2-4' and 2-2') were used as self-assembling synthons. These dimers pack to form well-defined structures of varying morphologies depending on the isomer. Specifically, distinctive ellipsoidal platelets can be obtained using 4-4' dimers. Solid-state polymerization of the preorganized dimers generates polymeric synthetic melanins while maintaining the initial particle morphologies. This work provides a new route to anisotropic synthetic melanins, where the building blocks are preorganized into specific shapes, followed by solid-state polymerization.

Natural products in the predatory defence of the filamentous fungal pathogen Aspergillus fumigatus

The kingdom of fungi comprises a large and highly diverse group of organisms that thrive in diverse natural environments. One factor to successfully confront challenges in their natural habitats is the capability to synthesize defensive secondary metabolites. The genetic potential for the production of secondary metabolites in fungi is high and numerous potential secondary metabolite gene clusters have been identified in sequenced fungal genomes. Their production may well be regulated by specific ecological conditions, such as the presence of microbial competitors, symbionts or predators. Here we exemplarily summarize our current knowledge on identified secondary metabolites of the pathogenic fungus Aspergillus fumigatus and their defensive function against (microbial) predators.

Allomelanin: A Biopolymer of Intrinsic Microporosity

Melanin is a ubiquitous natural pigment found in a diverse array of organisms. Allomelanin is a class of nitrogen-free melanin often found in fungi. Herein, we find artificial allomelanin analogues exhibit high intrinsic microporosity and describe an approach for further increasing and tuning that porosity. Notably, the synthetic method involves an oxidative polymerization of 1,8-DHN in water, negating the need for multiple complex templating steps and avoiding expensive or complex chemical precursors. The well-defined morphologies of these nanomaterials were elucidated by a combination of electron microscopy and scattering methods, yielding to high-resolution 3D reconstruction based on small-angle X-ray scattering (SAXS) results. Synthetic allomelanin nanoparticles exhibit high BET areas, up to 860 m²/g, and are capable of ammonia capture up to 17.0 mmol/g at 1 bar. In addition, these nanomaterials can adsorb nerve agent simulants in solution and as a coating on fabrics with high breathability where they prevent breakthrough. We also confirmed that naturally derived fungal melanin can adsorb nerve gas simulants in solution efficiently despite lower porosity than synthetic analogues. Our approach inspires further analysis of yet to be discovered biological materials of this class where melanins with intrinsic microporosity may be linked to evolutionary advantages in relevant organisms and may in turn inspire the design of new high surface area materials.

Unraveling the Structure and Function of Melanin through Synthesis

Melanin is ubiquitous in living organisms across different biological kingdoms of life, making it an important, natural biomaterial. Its presence in nature from microorganisms to higher animals and plants is attributed to the many functions of melanin, including pigmentation, radical scavenging, radiation protection, and thermal regulation. Generally, melanin is classified into five types-eumelanin, pheomelanin, neuromelanin, allomelanin, and pyomelanin-based on the various chemical precursors used in their biosynthesis. Despite its long history of study, the exact chemical makeup of melanin remains unclear, and it moreover has an inherent diversity and complexity of chemical structure, likely including many functions and properties that remain to be identified. Synthetic mimics have begun to play a broader role in unraveling structure and function relationships of natural melanins. In the past decade, polydopamine, which has served as the conventional form of synthetic eumelanin, has dominated the literature on melanin-based materials, while the synthetic analogues of other melanins have received far less attention. In this perspective, we will discuss the synthesis of melanin materials with a special focus beyond polydopamine. We will emphasize efforts to elucidate biosynthetic pathways and structural characterization approaches that can be harnessed to interrogate specific structure-function relationships, including electron paramagnetic resonance (EPR) and solid-state nuclear magnetic resonance (ssNMR) spectroscopy. We believe that this timely Perspective will introduce this class of biopolymer to the broader chemistry community, where we hope to stimulate new opportunities in novel, melanin-based poly-functional synthetic materials.

“Blackness” is an index of redox complexity in melanin polymers

Blackness as an emergent property associated with redox disorder and π-electron complexity in melanin polymers.

Synthetic mycomelanin thin films as emergent bio-inspired interfaces controlling the fate of embryonic stem cells

Mycomelanin thin films from 1,8-dihydroxynaphthalene can serve as a biointerface inducing adhesion and proliferation of ESCs and promoting their differentiation towards endodermal lineages.

Artificial Allomelanin Nanoparticles

Allomelanin is a type of nitrogen-free melanin most commonly found in fungi. Its existence enhances resistance of the organisms to environmental damage and helps fungi survive harsh radiation conditions such as those found on spacecraft and inside contaminated nuclear power plants. We report the preparation and characterization of artificial allomelanin nanoparticles (AMNPs) via oxidative oligomerization of 1,8-dihydroxynaphthalene (1,8-DHN). We describe the resulting morphological and size control of AMNPs and demonstrate that they are radical scavengers. Finally, we show that AMNPs are taken up by neonatal human epidermal keratinocytes and packaged into perinuclear caps where they quench reactive oxygen species generated following UV exposure.

Astrochemistry and Astrobiology: Materials Sciencein Wonderland?

Astrochemistry and astrobiology, the fascinating disciplines that strive to unravel the origin of life, have opened unprecedented and unpredicted vistas into exotic compounds as well as extreme or complex reaction conditions of potential relevance for a broad variety of applications. Representative, and so far little explored sources of inspiration include complex organic systems, such as polycyclic aromatic hydrocarbons (PAHs) and their derivatives; hydrogen cyanide (HCN) and formamide (HCONH2) oligomers and polymers, like aminomalononitrile (AMN)-derived species; and exotic processes, such as solid-state photoreactions on mineral surfaces, phosphorylation by minerals, cold ice irradiation and proton bombardment, and thermal transformations in fumaroles. In addition, meteorites and minerals like forsterite, which dominate dust chemistry in the interstellar medium, may open new avenues for the discovery of innovative catalytic processes and unconventional methodologies. The aim of this review was to offer concise and inspiring, rather than comprehensive, examples of astrochemistry-related materials and systems that may be of relevance in areas such as surface functionalization, nanostructures, and hybrid material design, and for innovative technological solutions. The potential of computational methods to predict new properties from spectroscopic data and to assess plausible reaction pathways on both kinetic and thermodynamic grounds has also been highlighted.

A Robust Fungal Allomelanin Mimic: An Antioxidant and Potent π-Electron Donor with Free-Radical Properties that can be Tuned by Ionic Liquids

Developing effective strategies to increase the chemical stability and to fine-tune the physico-chemical properties of melanin biopolymers by rational control of π-electron conjugation is an important goal in materials science for biomedical and technological applications. Herein we report that poly-1,8-dihydroxynaphthalene (pDHN), a non-nitrogenous, catechol-free fungal melanin mimic, displays a high degree of structural integrity (from MALDI-MS and CP/MAS ¹³ C NMR analysis), a strong radical scavenging capacity (DPPH and FRAP assays), and an unusually intense EPR signal (g=2.0030). Morphological and spectral characterization of pDHN, along with deassembly experiments in ionic liquids, indicated amorphous aggregates of small globular structures with an estimated stacking distance of 3.9 Å and broadband absorption throughout the visible range. These results indicate that DHN-based melanins exhibit a high structural integrity and enhanced antioxidant and free-radical properties of potentially greater biomedical and technological relevance than for typical indole-based eumelanins.

Characterization and Fate of Hydrogen-Bonded Free-Radical Intermediates and Their Coupling Products from the Hydrogen Atom Transfer Agent 1,8-Naphthalenediol

1,8-Naphthalenediol (dihydroxynaphthalene, 1,8-DHN) has been shown to be a potent hydrogen atom transfer (HAT) antioxidant compound because of the strong stabilization of the resulting free radical by intramolecular hydrogen bonding. However, the properties, reactivity, and fate of the 1,8-DHN phenoxyl radical have remained so far uncharted. Herein, we report an integrated experimental and computational characterization of the early intermediates and dimer products that arise by the oxidation of 1,8-DHN. Laser flash photolysis (LFP) studies of HAT from 1,8-DHN to the cumyloxyl and aminoxyl radicals showed the generation of a transient species absorbing at 350, 400, and >600 nm attributable to the 1,8-DHN phenoxyl radical. Peroxidase/H₂O₂ oxidation of 1,8-DHN was found to proceed via an intense blue intermediate (λ_max 654 nm) preceding precipitation of a black melanin-like polymer. By halting the reaction in the early stages, three main dimers featuring 2,2'-, 2,4'-, and 4,4'-bondings could be isolated and characterized in pure form. Density functional theory calculations supported the generation of the 1,8-DHN phenoxyl radical and its subsequent coupling via the 2- and 4-positions giving extended quinone dimers with intense transitions in the visible range, consistent with UV-vis and LFP data. Overall, these results allowed to elucidate the mechanism of oxidative polymerization of 1,8-DHN of possible relevance to melanogenesis in fungi and other processes of environmental and astrochemical relevance.