From melanogenesis to melanin technologies

Melanins, a wide class of natural pigments biosynthesised by different kind of living organisms (from bacteria to fungi, plants and mammals), stand today as a unique source of inspiration for the design and implementation of soft biocompatible multifunctional materials. Interest in melanins stems from a peculiar set of physicochemical properties, including the broadband absorption in the UV-visible range, the intrinsic free radical character, the water dependent hybrid ionic–electronic conductor behaviour, that, together with the high robustness and biocompatibility, have stimulated numerous applications as functional biomaterial. Herein, Professor Manini will discuss about how a deep knowledge of the chemical reactivity of the monomer precursors of each of the types of melanin can help in: a) disclosing the main structural features of the biopolymer, also allowing the correlation of these latter with the most outstanding properties via a structure-property relationship approach; b) designing tailored structural modifications that can improve the processing of the resulting biomaterial and confer specific properties for selected applications.

Professor Paola Manini, University of Napoli Federico II, Italy

Professor Paola Manini, University of Napoli Federico II, Italy

Paola Manini is an Associate Professor of Organic Chemistry at the Department of Chemical Sciences of the University of Napoli Federico II. She received her master’s degree in Chemistry cum laude alongside a special commendation from the University of Napoli Federico II in 1997. In 2002, she obtained a PhD in Chemical Sciences at the University of Napoli Federico II and took up a research position at the university in 2006. Since 1998, Paola Manini is member of the Italian Chemical Society. Her research is broadly centred on the organic chemistry of natural products and focuses on: The scientific activity of Paola Manini stems from the organic chemistry of natural products and is mainly focused on the following topics: the organic electronics to functional materials of melanins; the synthesis and application of nature-inspired luminescent materials; and studying the origin of life through chemical reactions under prebiotics conditions.

Melanin pigments have unusual physicochemical properties that make them attractive for developing bioinspired advanced materials. These properties include sunscreening, radical scavenging, metal ion chelation, and electrical conductivity. Because the molecular structures in melanin are obscure despite more than a century of study, models of how these properties emerge from interacting subunits are still lacking. Although many biological pigments are small molecules, melanins are natural nanomaterials, which form granules with characteristic length scales of hundreds of nanometers. Consequently, there is great interest in understanding how molecular-level interactions give way to emergent and collective phenomena as a function of the supramolecular and hierarchical structures present in melanin nanoparticles. In top-down and bottom-up approaches, femtosecond laser spectroscopy is used to investigate sunscreening in synthetic eumelanin and molecular mimics. Past spectroscopic studies on putative eumelanin building blocks have emphasised excited-state proton transfer. Now, access to oxidised subunits such as derivatives of indole-5,6-quinone is revealing new ultrafast deactivation pathways that involve the efficient passage through conical intersections. In top-down work, femtosecond pump-probe experiments on synthetic melanins establish connections between radical photogeneration and ultrafast excited state decay. Experimental results on synthetic melanins and natural organic matter samples will be presented, which suggest how coarse-grained models that emphasise mesoscale interactions can offer new insights into the common steady-state and transient optical properties of melanins and related natural and lab-made carbon-rich nanomaterials.

Professor Bern Kohler, Ohio State University, USA

Professor Bern Kohler, Ohio State University, USA

Bern Kohler is Professor and Ohio Eminent Scholar in the Department of Chemistry and Biochemistry at The Ohio State University. He and his co-workers study the excited states of nucleic acids and melanin pigments using ultrafast laser spectroscopy. Further research projects include the study of organic and inorganic nanomaterials of interest for solar energy conversion and photocatalysis. Professor Kohler received a BS degree in Chemistry from Stanford University (1985), a PhD in Physical Chemistry from MIT (1990), and he completed postdoctoral research at ETH Zürich and the University of California, San Diego. Professor Kohler is a fellow of the AAAS and the winner of the 2017 Inter-American Photochemical Society Award in Photochemistry. He is a former President of the Telluride Science Research Centre. Professor Kohler is currently an Associate Editor for the journal Photochemistry and Photobiology. 

Eumelanin’s structure is usually described as a stacked oligomer system. Oligomer composition is thought to be around 4-5 monomer units based upon 5,6-dihydroxyindole and 5,6-dihydroxyindole-2-carboxylic acid and their derivatives. These 'sheets' are usually understood to stack together via pi-pi interactions [1]. This is consistent with reports that show a characteristic length scale of ~3.4 – 3.9 Å [2-6], results that are usually obtained under vacuum. Given eumelanin’s well known hygroscopic response and the impact that hydration has on its solid-state properties [1], this structural length scale is re-investigated afresh using wide angle x-ray scattering as a function of hydration. Even though the macroscopic samples swell with hydration, Dr Mostert and his team find that the interplanar distance shrinks, down from 4.03 Å to 3.19 Å for synthetic, 3.66 Å to 3.41 Å for a Cu chelated synthetic, and 3.77 Å to 3.47 Å for S. officinalis eumelanins. These results are inconsistent with the standard pi-pi model. The results suggest, for the first time regarding a biomaterial in general, applying the 'Pancake bonding' (PB) stacking framework to explain the eumelanin data [7]. This framework also reconciles a long-term inconsistency between electron paramagnetic resonance and muon spin relaxation (muSR) data for eumelanin. Additional temperature dependent muSR data on variously hydrated synthetic eumelanin is presented to confirm/disconfirm this PB framework. Outstanding issues and experimental opportunities will be highlighted as well as implications for other eumelanin properties.

[1] A. B. Mostert, Polymers, 2021, 13, 1670; [2] A. A. R. Watt et al., Soft Matter, 2009, 5, 3754–3760; [3] J. Cheng et al., Pigm. Cell Res., 1994, 7, 255–262; [4] J. Cheng et al., Pigm. Cell Res., 1994, 7, 263–273; [5] C.-T. Chen et al., ACS Nano, 2013, 7, 1524–1532; [6] L. Migliaccio et al., Front. Chem., 2019, 7, 162; [7] P.A. Abramov et al., PCCP (Comm.), 2023, 25, 16212–16216.

Dr Bernard Mostert, Swansea University, UK

Dr Bernard Mostert, Swansea University, UK

Dr Bernard Mostert is a materials researcher working on conductive biomaterials and bioelectronic devices. He received his PhD at the University of Queensland and has held postdoctoral positions at Lancaster University and the University of Queensland. Recently, he finished a prestigious Marie Skłodowska-Curie fellowship at Swansea University and is currently working in the Centre of Integrative Semiconductor Materials on semiconductor materials and devices. His specialty is understanding the charge transport mechanisms of biomaterials and devices as they are affected by water content, with a special focus on melanin. Melanins have been the inspiration for his more than decade of experience pioneering hydration control measurements in areas including electrical, neutron scattering and magnetic resonance measurements.

Professor Blancafort will present the computational work on melanin of his team, that has evolved along two lines: the study of relevant intermediates in collaboration with the Lumb and Kohler groups, where they have studied the photophysics of the key indolequinone intermediate; and the generation of molecular melanin models, which includes the study of DHI oligomer libraries, the use of machine learning to model oligomer properties, and a kinetic model for the first step of DHI oxidative coupling oligomerisation.

Professor Lluís Blancafort, University of Girona, Spain

Professor Lluís Blancafort, University of Girona, Spain

Lluís Blancafort is a computational chemist whose main interest is the photophysics and photochemistry of molecules of biological and technological relevance. His work on melanin comprises the study of relevant intermediates, carried out in collaboration with experimental groups, and the generation of theoretical melanin models. His expertise covers quantum chemical methods, molecular dynamics and machine learning. Other areas of interest are photocatalysis and aggregation induced emission.

Polycyclic aromatic hydrocarbons and heteroaromatic derivatives are an interesting class of compounds for their redox and photophysical properties and because they can be considered as building blocks of a variety of carbon nanostructured systems. Melanin is a class of biomacromolecules, with several functions in the organisms as, for example, pigmentation and protective layer, radical scavenging and radiation protection; it consists of building blocks of 5,6-dihydroxyindole. Melanin's structures are known to interact each other and self-assemble leading to, for example, rod-shaped nanostructures, as well as they can chelate several ions. This high ability to self-assemble could represent a challenging way to build up functional interfaces. Thus, such nanostructured biopolymers appears to be a versatile molecular systems that are particularly intriguing to develop novel and environmentally compatible materials for exploiting new optoelectronic devices. In this framework, the investigation of reactivity of the building blocks2,3 of melanins and that of the materials itself, under different conditions as the presence of a variety of electrolytes or solvents, allows to understand the factors driving the structure and characteristics of melanins. In particular, the studies of the resulting electronic, ionic transport and other physical chemistry properties of the melanins, together with their modulation, are of paramount importance and these can be conveniently performed by utilising a selection of electrochemical techniques that allow to gather information both on the properties of the bulk material and the electronic behaviour of a film with spatial and temporal resolution.

Professor Massimo Marcaccio, Università di Bologna, Italy

Professor Massimo Marcaccio, Università di Bologna, Italy

Massimo Marcaccio got his master's degree in Chemistry and PhD in Chemical Sciences at the University of Bologna. After a post-doc period he returned to Italy and carried out research activity at the Department of Chemistry of the University of Bologna, where he was appointed Assistant Professor in 1999, Associate Professor in 2014 and promoted to Professor in 2019. His scientific activity concerns the investigation of physical chemical properties of molecular, supramolecular species (eg planar and curved polyaromatic hydrocarbons, fullerenes, metal coordination complexes, carbon nanotubes) and biological systems (proteins and enzymes), in solution and/or immobilised on the electrode surface. Such studies utilise a range of electrochemical, spectroelectrochemical, electrochemiluminescence techniques and scanning probe microscopy. He is author of more than 150 papers.

Practical and industrial use of eumelanin in electrochemical applications is hampered by challenges like limited solubility in aqueous solvents and high production costs, lowering its feasibility in large-scale processes. To combat these constraints, we explored using eumelanin sourced from the black soldier fly (Hermetia illucens, BSF) as an alternative material due to its increased sustainability, being otherwise waste during chitosan extraction processes from the black soldier fly. Mr Al-Shamery's investigations showed the potential of BSF-melanin as renewable biopolymer-electrode material for the cost-effective, safe, and facile assembly of Zn coin cells using ionic liquid electrolytes. Enhanced capacity and mixed conductivity were observed when compared to analogous devices employing synthetic eumelanin with inversed ratios of diffusion and capacitive contributions. Mr Al-Shamery and his team further endeavoured to replicate and optimise the melanin attributes through the BSF-melanin inspired chemical derivatisation of synthetic eumelanin. As the uniformity of the film emerged as a critical factor for achieving superior electrochemical performance, we synthesised and characterised melanin derivatives that incorporate functional groups with varying positions and polarities to modify solubility and metal-ion chelation capabilities. Density functional theory calculations were employed to track changes in the band gaps resulting from the introduction of new moieties. The synthesised derivatives were also evaluated in Zn coin cells using both ionic liquid and aqueous electrolytes. The investigations showed pivotal roles played by the functional group polarity and their positioning in film processibility and redox centre accessibility within eumelanin-based energy storage systems. The implications stemming from the distinctions observed between the different melanin-types will be discussed.

Mr Noah Al-Shamery, Nanyang Technological University and University of Warwick, UK

Mr Noah Al-Shamery, Nanyang Technological University and University of Warwick, UK

Noah Al-Shamery is a Joint PhD student at MSE NTU under Professor Pooi See Lee and at the University of Warwick under Professor Patrick R Unwin. As a chemist by training, his graduate research covers the electrochemistry of eumelanin and melanin derivatives and has been awarded most recently with the E-MRS 2023 Fall Meeting Young Researcher Award. His work has been published in journals by the ACS and the RSC. On the teaching side of things, he has given university classes ranging from statistical thermodynamics and kinetics to spectroscopy and inorganic chemistry. He is team leader for the European Young Chemists’ Network (EYCN) for science communication, core team member of the young Physical Chemists (yPC) of the German Bunsen Society, and former spokesperson for the German Young Chemists Network (JCF) in Oldenburg and Bonn. Furthermore, he has network ties to the SNIC Student Chapter and the International Young Chemists’ Network (IYCN).