Carrella, Emanuele (2025) New glycated eumelanin-based polymers for applications in organic bioelectronics. [Tesi di dottorato]

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Abstract

Nature-inspired products, such as biopolymers, have gained significant attention as functional materials in organic electronics, offering potential for both technological and biomedical applications. Among biomaterials, melanins are one of the most intriguing materials. The most studied type of melanin is eumelanin, a photoprotective pigment found in mammals, which arises from the oxidative polymerization of 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA). Eumelanin exhibits several valuable properties, such as: a) broad optical absorption across the UV-visible spectrum; b) effective UV dissipation mechanisms; c) photoconductivity in the solid state; and d) water-dependent conductivity. However, despite the growing interest in using synthetic eumelanin for organic electronics and bioelectronics, its practical use has been limited by several challenges, including its complete insolubility in all solvents, which makes it difficult to develop standardized, reproducible synthetic methods, low conductivity, and an incomplete understanding of the structure–property–function relationships. The synthesis of functionalized eumelanin monomers to introduce new, significant properties into eumelanin units was demonstrated to be one of the most effective strategies to overcoming these material issues. In this context, this project aimed to create novel classes of synthetic melanin monomers that are functionalized to improve physical and chemical properties of the final material, such as solubility and conductivity. The main activities of this project can be summarised as follows: a) to synthesize eumelanin-inspired monomers by designing new functionalization strategies; b) to characterize these new materials; c) starting from the latter, to create and characterize eumelanin-inspired materials for use as active layers in organic electronic devices, aiming to improve some device performance. One key part of the project successfully developed a synthetic method to functionalize DHI and DHICA by introducing selenosugars at the C-3 position of O-acetylated DHI and DHICA, using easily accessible glycosyl diselenides as seleno-glycosylating agents. This method allows the incorporation of monosaccharides, disaccharides, and amino sugars, obtaining a class of new pro-soluble melanin monomers. Additionally, this strategy was extended to obtain the first amino acid-functionalized melanin monomer from a symmetrical N-protected selenocystine derivative. Other functionalizations explored were studied during my visiting period in Brazil in collaboration with Prof. Carlos De Oliveira Graeff and involved the incorporation of chalcogens into DHI monomer to investigate the impact of these heavy atoms on melanin's structure. The third part of the project aimed to better understand the already-known 3-thioglycosylated eumelanins to test the interaction of this polymers with natural molecules. In this regard, an especially interesting result of this research was the high affinity exhibited by synthetic thiolactosylated eumelanins towards galectins, a fundamental class of β-galactoside receptors with a profound impact in several key biological processes with therapeutical implications.

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