5,6-Dihydroxyindole-2-carboxylic Acid–TiO₂ Charge Transfer Complexes in the Radical Polymerization of Melanogenic Precursor(s)

Abstract

A combination of biomedical and technological applications is generating, over the past decades, the well-established interest toward melanins and melanogenesis. Several compounds have been explored to promote/catalyze oxidative polymerization of melanogenic precursors, such as 5,6-dihydroxyindole-2-carboxylic acid (DHICA), to melanin-like biopolymers in vitro. TiO2 has shown a photocatalytic activity driving DHICA polymerization and leading to the formation of melanin–TiO2 hybrid nanostructures with unique biocide behavior even under visible light. However, the mechanism of melanin formation in those hybrids is not yet well understood although a ligand to metal charge transfer (LMCT) process involving DHICA and Ti4+ ions was hypothesized. Here, we focus on melanin formation and apply a complementary analysis, by using photoluminescence (PL), UV–vis, electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR) spectroscopy to reveal the mechanism of DHICA polymerization in the presence of a TiO2-sol. This study discloses TiO2 potentialities to drive and template DHICA polymerization to melanin via LMCT-based photo-oxidative process.

Natural melanin pigments and their interfaces with metal ions and oxides: emerging concepts and technologies

“…7). [132] To conclude this Prospective article highlighting the exciting achievements reported in the field of melanin-based materials in recent years, we would like to emphasize the importance of an interdisciplinary approach for melanin research, where materials chemistry, physical chemistry, and materials physics all must be considered to effectively advance both the understanding of fundamental process in electrochemistry, photophysics, and transport physics, and the development of sustainable technologies. We believe this holistic approach will contribute to the advancement of knowledge about the functional properties of melanin-based materials in biologic systems.…”

Tuning Functional Behavior of Humic Acids through Interactions with Stöber Silica Nanoparticles

“…Recent studies proved that the inorganic phase was able to tune bio-polymeric supramolecular structures, boosting their intrinsic properties and tuning their overall biological activity [20][21][22][23]. Silica has been identified as an ideal support for this method, due to its strong hydrophilicity, acknowledged biocompatibility, as well as tunable size, shape, porosity and surface chemistry [21,24].…”

Bioinspired hybrid eumelanin–TiO2antimicrobial nanostructures: the key role of organo–inorganic frameworks in tuning eumelanin's biocide action mechanism through membrane interaction

“…10 Eumelanins are truly multifunctional biopolymers, 11 performing several biological functions such as photoprotection, photosensitization, free radical quenching, metal ion chelation and even intrinsic antimicrobial efficacy and biolm inhibition without any photosensitization. 1,12,13 Notably, recent emerging evidence has disclosed and investigated the potential of eumelanin and eumelanin based materials as valuable antimicrobial agents, [14][15][16][17] but mechanisms and processes behind biocide activity still remain poorly understood. 1 Indeed some papers argue that antimicrobial action of eumelanins relies on ROS generation.…”