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

Abstract

Eumelanin is a brown-black pigment comprising 5,6-dihydroxyindole (DHI) and its 2-carboxy derivative (DHICA), but the detailed structure of eumelanin is unclear. Chemical degradation is a powerful tool for analyzing melanin. H2 O2 oxidation degradation of eumelanin affords pyrrole-2,3,5-tricarboxylic acid (PTCA) and pyrrole-2,3-dicarboxylic acid (PDCA). The ratio of PDCA to PTCA provides information about the eumelanin structure. In this article, we propose simple equations on the basis of previous experimental results on dimer yields for evaluating the yields of PTCA and PDCA from any DHI oligomers. Assuming the chemical disorder model of DHI-melanin, we solve an equation where a theoretical expression for the ratio of PDCA to PTCA is set to the corresponding experimental value to obtain a plausible Poisson distribution of DHI oligomers. The results demonstrate that the main contributors to DHI-melanin are tetramers and pentamers as shown by the mass spectrometry.

Raman spectroscopy quantification of eumelanin subunits in natural unaltered pigments

“…It must be considered, however, that these studies only analyzed eumelanin degradation products and did not include unaltered eumelanin like in this study. It must also be considered that our model predicted DHICA contents in synthetic eumelanins slightly higher than real values (Table ), which may be due to the fact that some DHICA units are decarboxylated during the polymerization to DHI units (Okuda et al., ). Our results thus indicate that the analysis of unaltered eumelanins by Raman spectroscopy can provide new insights into the structure of eumelanin and suggest that the DHI:DHICA ratio may be highly variable among natural eumelanins.…”

“…Eumelanins were prepared by adding 2 ml of a solution of mushroom tyrosinase (Sigma‐Aldrich; 4,270 U/mg) in 0.1 M sodium phosphate buffer (pH 6.8) to a solution of 1 mmol of either DHI, DHICA or a 50:50, 25:75 or 75:25 mixture of DHI and DHICA in the same buffer (Ito, Wakamatsu, Glass, & Simon, ; Okuda, Yoshino, Wakamatsu, Ito, & Sota, ; Ozeki, Wakamatsu, Ito, & Ishiguro, ). Oxidation was induced by vigorous stirring during 4 hr at 25°C, and then stopped by adding 6 M HCl.…”

Structural and optical properties of soluble melanin analogues with enhanced photoluminescence quantum efficiency

“…Another possible explanation for the increase of the carboxylic group when oxygen pressure is used during synthesis is the formation of pyrrole-2,3-dicarboxylic acid (PDCA) and pyrrole-2,3,5-tricarboxylic acid (PTCA) from melanin degradation due to nucleophilic attack of OOH − ions. 34,[36][37][38][39] Nonetheless, it is expected that this process occurs only at high concentrations of H 2 O 2 , as in some bleaching experiments. 34 Thus, we believe that the amount of H 2 O 2 formed during our procedure will not be enough to induce catechol ring fission.…”

Polydopamine and eumelanin molecular structures investigated with ab initio calculations

“…This variety of oligomers and redox forms that constitute PDA and eumelanin is referred to as chemical disorder model. 32 …”

“…18–22 A recent study based on mass spectroscopy results of DHI-melanin also suggested that the majority constituents of the material are tetramers and pentamers of DHI. 32 Consequently, we narrow down our investigation to DHI oligomers up to tetramers. Within the framework of this bottom-up approach and considering the location of the most reactive positions of DHI, we systematically generate and analyze all probable early-polymerized DHI oligomers, ranging from dimers to tetramers, to provide a fundamental explanation of some important structural features of PDA and eumelanin, as well as to propose a set of molecular models that represents the chemically diverse nature of these materials.…”