Exploring the frontiers of synthetic eumelanin polymers by high‐resolution matrix‐assisted laser/desorption ionization mass spectrometry

Abstract

New trends in material science and nanotechnologies have spurred growing interest in eumelanins black insoluble biopolymers derived by tyrosinase-catalysed oxidation of tyrosine via 5,6-dihydroxyindole (DHI) and its 2-carboxylic acid (DHICA). Efficient antioxidant and photoprotective actions, associated with peculiar optoelectronic properties, are recognised as prominent functions of eumelanin macromolecules within the human and mammalian pigmentary system, making them unique candidates for the realisation of innovative bio-inspired functional soft materials, with structure-based physical-chemical properties. An unprecedented breakthrough into the mechanism of synthetic eumelanin buildup has derived from a detailed investigation of the oxidative polymerization of DHI and its N-methyl derivative (NMDHI) by linear and reflectron matrix-assisted laser/desorption ionization mass spectrometry. Regular collections of oligomers of increasing masses, spanning the entire m/z ranges up to 5000 Da (>30-mer) and 8000 Da (> 50-mer) for the two building blocks, respectively, were disclosed. It is the first time that the in vitro polymerisation of dihydroxyindoles to form synthetic eumelanins is explored up to its high mass limits, giving at the same time information on the polymerisation mode, whether it follows a stepwise pattern (being this the conclusion in our case) or a staking sequencing of small-sized entities. It also highlighted the influence of the N-methyl substituent on the polymerization process; this opens the way to the production of N-functionalized, synthetic eumelanin-inspired soft materials, for possible future technological applications.

Excitonic effects from geometric order and disorder explain broadband optical absorption in eumelanin

“…X-ray diffraction studies 21,22 and scanning tunneling microscopy measurements [23][24][25] showed that the size of eumelanin protomolecules is around 15-20 Å, which suggested that the tetramers and pentamers formed by covalently bonded DHIs are the most possible molecular structures of eumelanin. Matrix-assisted laser desorption/ionization mass spectrometry studies [26][27][28][29][30][31] showed that the mass of eumelanin protomolecules is within or below B600-1,200 amu, and large polymeric structures are not observed, with the exception of the data reported in a recent report 32 . Hence, most of the investigations performed by mass spectrometry supported that the tetramers and pentamers constitute the majority of eumelanin, while slightly larger structures, such as the hexamers and octamers, are also possible.…”

In Situ Infrared Spectroscopic Monitoring and Characterization of the Growth of Polydopamine (PDA) Films

“…The Raper–Mason model was first reported and has been confirmed by numerous measurements . Arzillo and Reale and co‐workers proposed that the polyamine is composed of aggregates of oligomers resulting from the polymerization of 5,6‐dihyhydroyindole (DHI) . Dreyer et al suggested that PDA is not a simple covalent polymer, but a supramolecular aggregate of monomers (mainly 5,6‐dihydroxyindoline and its dione derivative) which are assembled together through the charge transfer, and/or π‐stacking, hydrogen bonding interactions .…”

Degree of polymerization of 5,6‐dihydroxyindole‐derived eumelanin from chemical degradation study

“…Recently, MALDI spectra of DHI‐melanin and N ‐methyl‐DHI‐melanin, obtained by horseradish peroxidase H 2 O 2 treatment, have been measured up to approximately 5000 and 8000 D, respectively (Reale et al., ). The findings therein are consistent with the present hypotheses and results.…”