Synthesis and Polymerization Studies of Organic‐Soluble Eumelanins^†

Abstract

The isolation, structure determination and chemical characterization of eumelanins has been plagued by their very low solubility in organic solvents. To gain insights into the structure and reactivity of these unusual and important biologic macromolecules and to pave the way for their use in electronics, we have prepared soluble melanins via the synthesis of monomeric precursors containing lipophilic substituents. Two such monomers derived from 5,6-dihydroxyindole-2-carboxylic acid (DHICA) were prepared, namely the benzyl and octyl ester derivatives. Both benzyl and octyl ester monomers were oxidatively polymerized to yield dark, melanin-like pigments. These polymerization processes were followed by UV-visible, fluorescence and NMR spectroscopy. These studies showed that the polymerizations proceeded by cross-linking at the 4- and 7-positions of the indole nucleus and led to highly heterogeneous polymeric products. Incorporation of a lipophilic benzyl or octyl group resulted in enhanced solubility of the pigments in a wide range of organic solvents. The UV-visible spectra of the organically soluble synthetic melanins were essentially identical to that of natural eumelanin.

Biocatalytic formation of synthetic melanin: The role of vanadium haloperoxidases, L-DOPA and iodide

“…Melanins are polymorphous and multifunctional biopolymers, represented by allomelanins in the plant kingdom and eumelanins, phaeomelanins, neuromelanins and mixed melanins in the animal kingdom [1] and conclusions about its chemical structure and biochemical pathways are still matter of discussion. For example, from the biochemical point of view, L-dopa is the precursor to melanins and cathecolamines but following separate biosynthetic pathways [5].…”

“…L-dopa (3,4-dihydroxyphenyl-L-alanine) a modified amino acid resulting from hydroxylation reactions of phenylalanine and/or tyrosine is a molecule with a crescent recognized central role in living systems since it is involved in processes such as the melanin formation [1], biosynthesis of catecholamine neurotransmitters for Parkinson disease therapy [2] and in the formation of several biomaterials as demonstrated by its presence in tissues as diverse as mussel byssus, marine tubeworms cements and hard mouthparts of some marine invertebrates [3]. The involvement of L-dopa in melanin biosynthesis is well documented.…”

Melanin synthesis under oxygen pressure

“…In addition, many routine advanced techniques dedicated to thin film or solution characterization could be used. In the last 15 years soluble melanin derivatives were synthesized using different functionalized groups …”

Melanin from a physicochemical point of view

“…Recently, with the advent of nanoscale applications, several groups have produced thin eumelanin films that are morphologically relatively homogeneous . Bothma et al reviewed the latest chemical and structural advances in the properties of melanin that will enable more homogeneous molecules, e.g.…”