Viggiano, Sara (2024) Natural and bio-inspired phenolic polymers for the design of innovative functional materials: tailoring properties by ad hoc synthesis and chemical manipulation. [Tesi di dottorato]

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Abstract

This PhD work was directed to the exploitation of phenolic polymers in the field of materials science. Natural phenolic polymers, such as lignins, tannins and melanin pigments, are the focus of increasing interest for applications in a broad variety of sectors as versatile, stable and multifunctional materials. Lignin and tannins are plant-derived phenolic polymers, particularly abundant also in agri-food wastes and by-products, endowed with potent antioxidant and antimicrobial properties, which have prompted their exploitation in tissue regeneration, stimuli-responsive materials, wound healing, drug delivery, and food packaging. The same applies to eumelanins, the characteristic black-to-brown pigments responsible for the color of skin, hair and eyes in mammals, originating from the oxidation of 5,6-dihydroxyindoles. Particular attention has been devoted also to synthetic, nature-inspired phenolic polymers, displaying in some cases several advantages over the corresponding natural analogues. In this context, this research work has been directed to the recovery or preparation, and when required functionalization, of natural or bio-inspired phenolic polymers for the design of functional materials to be used in the biomedical, cosmetic and food sectors. The main outcomes of this PhD research activity can be summarized as follows: o The exploration of the oxidation chemistry of 5,6-dihydroxyindole (DHI) under solid-state conditions led to the isolation of hitherto unknown DHI dimers, namely the 4,4’- and 4,7’- biindolyls; moreover, using proteins as the solid matrices, the 4,4’ and 4,7’ mode of coupling was identified as a potential reaction route occurring in in vivo melanogenesis. o A green procedure combining ball milling with the use of deep eutectic solvents was applied to recover lignin from chestnut shells (CSL), which was exploited as reducing agent toward Ag+ ions for the mechanochemical synthesis of silver nanoparticles; AgNPs@CSL were incorporated into poly(lactic acid) electrospun scaffolds providing functional materials to be used for tissue regeneration applications, thanks to the remarkable antioxidant, cytocompatibility and antimicrobial properties. o An alternative synthetic, biomimetic model of natural lignin was obtained by oxidation of coniferyl alcohol under biomimetic conditions. o Conventional maceration was identified as the best extraction technique for the recovery of phenolic compounds from dry pine needles; propelargonidin/prodelphinidin tannins were identified as the main phenolic components of the extract, emerging as a valid alternative as a green corrosion inhibitor on aluminum alloy. o The solubility, photostability, and ability to inhibit photo-induced lipid peroxidation of hydrolysable and condensed tannins were systematically investigated in view of possible dermo-cosmetic applications. o The access to antioxidant selenoderivatives of condensed tannins was preliminary investigated on catechin as a model compound, by using a protocol involving potassium selenocyanate/cerium(IV) ammonium nitrate. o A pomegranate peel and seed extract rich in ellagitannins and ellagic acid was efficiently combined with whey proteins to implement antioxidant and antimicrobial coatings for food packaging applications. o Highly performing adhesive and underwater resistant materials were prepared starting from soy proteins and natural easily accessible phenolic compounds (caffeic, gallic and chlorogenic acid); the addition of agarose led to the formation of hydrogels endowed with cytocompatibility, hemocompatibility, antibacterial activity and, in some cases, also wound healing activity for biomedical applications. o With the final aim of searching compounds able to inhibit the activity of the enzyme tyrosinase and hence prevent or delay skin hyperpigmentation processes for application in the cosmetic field, or enzymatic browning processes in fruits and vegetables for application in the food sector, extracts from low cost agri-food by-products rich in phenolic compounds were prepared; extracts from chestnut shells, Morus alba branches, and pecan nut shells proved to be the most effective inhibitors against tyrosinase, while extracts from strawberry spirit production by-products, fermented pomegranate by-products and again chestnut shells exhibited an evident retarding effect on the enzymatic browning process in apple smoothies. o Selected gingerol and resorcinol-based cosmetic ingredients were tested for their ability to prevent skin hyperpigmentation, demonstrating to be effective inhibitors of melanin precursor photooxidation and to able to exert a photoprotective role against the damages induced by solar light. Finally, part of the PhD research activity was performed at the CICECO-Aveiro Institute of Materials of the University of Aveiro (Portugal), under the supervision of Professor Mara G. Freire, and at the European School of Chemistry, Polymers and Materials of the University of Strasbourg (France), under the supervision of Professor Vincent Ritleng. The following research activities were developed, respectively: o The enzyme laccase was investigated as an alternative to the conventional 5,6-dihydroxindol-2-carboxylic acid (DHICA) oxidizing systems. Although still preliminary, the results obtained highlighted the possibility to obtain a pigment with apparently more efficient antioxidant properties with respect to the one obtained using the horseradish peroxidase/hydrogen peroxide oxidizing system. o Exploiting their ability to interact with several surfaces and compounds, hydrolysable and condensed wood tannins were used for the coating of polyurethane foams and the subsequent functionalization with palladium; the obtained materials acted as heterogeneous and recyclable catalysts in different reactions (phenylacetylene to styrene semi-hydrogenation, nitrobenzene reduction, and Suzuki coupling) over several consecutive cycles.

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