Disentangling Eumelanin “Black Chromophore”: Visible Absorption Changes As Signatures of Oxidation State- and Aggregation-Dependent Dynamic Interactions in a Model Water-Soluble 5,6-Dihydroxyindole Polymer
- 6 October 2009
- journal article
- research article
- Published by American Chemical Society (ACS) in Journal of the American Chemical Society
- Vol. 131 (42), 15270-15275
- https://doi.org/10.1021/ja905162s
Abstract
A fundamental unsettled issue concerning eumelanins, the functional biopolymers of human skin and hair, is why they are black. The experimental difficulty lies in the virtual insolubility of these pigments, causing marked scattering effects and hindering characterization of the intrinsic absorption properties of the heterogeneous species produced by oxidative polymerization of 5,6-dihydroxyindole (DHI) and related monomer precursors. The synthesis of spectrally robust, water-soluble DHI polymers is therefore an important goal in the prospects of disentangling intrinsic absorption properties of eumelanin components by circumventing scattering effects. Reported herein is the first water-soluble DHI polymer produced by oxidation of ad hoc designed 5,6-dihydroxy-3-indolyl-1-thio-β-D-galactopyranoside (1). The dark brown polymer exhibited a distinct band at 314 nm and a broad visible absorption, resembling that of natural eumelanins. Main isolable oligomer intermediates including 2,7′- and 2,4′-biindolyls 2 and 3, attest the close resemblance to the mode of coupling of the parent DHI. Sodium borohydride reduction caused decoloration and a marked absorbance decrease in the visible region around 550 nm, but did not affect the UV band at 314 nm. Measurements of absorbance variations with dilution indicated a linear response at 314 nm, but a significant deviation from linearity in the visible region, with the largest decrease around 500 nm. It is argued that eumelanin black color is not only intrinsically defined by the overlap of π-electron conjugated chromophores within the individual polymer components, as commonly believed, but also by oxidation state- and aggregation-dependent interchromophoric interactions causing perturbations of the heterogeneous ensemble of π-electron systems and overall spectral broadening.Keywords
This publication has 37 references indexed in Scilit:
- New melanic pigments in the human brain that accumulate in aging and block environmental toxic metalsProceedings of the National Academy of Sciences, 2008
- The physical and chemical properties of eumelaninPigment Cell Research, 2006
- 5,6-Dihydroxyindoles and Indole-5,6-dionesAdvances in Heterocyclic Chemistry, 2005
- Melanin photoprotection in the human retinal pigment epithelium and its correlation with light-induced cell apoptosisProceedings of the National Academy of Sciences, 2005
- Metal–ion interactions and the structural organization of Sepia eumelaninPigment Cell Research, 2005
- The Chemical Structure of MelaninPigment Cell Research, 2004
- Isolation and Biophysical Studies of Natural Eumelanins: Applications of Imaging Technologies and Ultrafast SpectroscopyPigment Cell Research, 2003
- A Chemist's View of MelanogenesisPigment Cell Research, 2003
- Tyrosinase Autoactivation and the Chemistry of ortho-Quinone AminesAccounts of Chemical Research, 2003
- Ultrastructural Organization of Eumelanin from Sepia officinalis Measured by Atomic Force MicroscopyBiochemistry, 2001