Studies related to the chemistry of melanins. Part XIII. Studies on the structure of dopamine-melanin

Abstract

upon Tyne NEI 7RU As a result of methylation, benzoylation, and other chemical experiments, as well as of studies of the conversion into melanin of samples of dopamine specifically labelled with deuterium at each of the a-, p-, 2-, 5-, and 6-positions (separately), a preliminary outline structure is put forward to represent autoxidative dopamine-melanin. This melanin differs considerably from autoxidative dopa-melanin ; in particular, it is suggested that N-C polymeric linkages may be important, and that more uncyclised side chains are present than in dopa-melanin.

Polydopamine—An Organocatalyst Rather than an Innocent Polymer

“…To obtain more evidence for the involvement of both types of functional groups of PDA in the catalysis, we tried to block them by irreversible reactions with strong electrophiles, such as by methylation with methyl iodide (Table 1, entries 17, 18) or acetylation with acetic anhydride (entry 19) and to check the products in their catalytic behavior. Reaction of PDA with methyl iodide was assumed to methylate the amino groups to quaternary ammonium salts and the hydroxyl groups to methyl ethers 26. Acetic anhydride could cause analogous blocking of these groups by amide or ester formation, respectively.…”

“…21–24 Here, the occurrence of carbonyl and Michael‐acceptor moieties in PDA is exploited enabling reactions with amines or thiols as nucleophiles. Furthermore it is known that PDA can partially be hydrolyzed giving rise to the formation of carboxylic acid groups by opening the catechol ring 25–26. Reactions of PDA with dimethyl sulfate or benzoyl chlorides as strong electrophiles were reported attacking nucleophilic sites such as amino or phenolic hydroxyl groups 26.…”

“…Furthermore it is known that PDA can partially be hydrolyzed giving rise to the formation of carboxylic acid groups by opening the catechol ring 25–26. Reactions of PDA with dimethyl sulfate or benzoyl chlorides as strong electrophiles were reported attacking nucleophilic sites such as amino or phenolic hydroxyl groups 26. Finally the catechol/o‐quinone moieties found in PDA provide redox properties, which can be exploited in reduction and deposition of metals, such as Au, Cu, or Ag 15.…”

High Ionic Strength Formation of DOPA‐Melanin Coating for Loading and Release of Cationic Antimicrobial Compounds

“…In comparison to PDA, DM films would be expected to exhibit a higher concentration of free carboxylic acids that can be exploited for a variety of applications. However, historical reports of the alkaline auto-oxidation of DOPA in water or low ionic strength buffer resulted in a melanin-like product which is soluble [27–29] or a solution-stable supramolecular nanoaggregate. [30] Consistent with this behavior, we as well as others have experienced difficulty forming adherent films from DOPA using the standard conditions developed for spontaneous PDA film formation (buffered aerated H 2 O, pH 8-9), especially on negatively charged substrates such as TiO 2 and SiO 2 .…”

Fast Atom Bombardment Mass Spectrometry in the Study of Dopamine Melanogenesis Intermediates

“…The observed decrease of the relative abundance of the m/z 151 ion vs. time (see Fig. 3) indicates that this ion must be considered one of the first intermediates of the enzyme reaction (Binns et al, 1970). Its molecular formula C8H9N02, as obtained by accurate mass measurements, suggests the structure of dopamine-o-quinone.…”