title: 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.
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-beta-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 pi-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 pi-electron systems and overall spectral broadening.
authors.affiliation: Department of Organic Chemistry and Biochemistry, University of Naples Federico II Complesso Universitario Monte S. Angelo, Via Cintia 4, I-80126 Naples, Italy. alessandro.pezzella@unina.it
authors.forename: Alessandro
authors.initials: A
authors.lastname: Pezzella
authors.forename: Alfonso
authors.initials: A
authors.lastname: Iadonisi
authors.forename: Silvia
authors.initials: S
authors.lastname: Valerio
authors.forename: Lucia
authors.initials: L
authors.lastname: Panzella
authors.forename: Alessandra
authors.initials: A
authors.lastname: Napolitano
authors.forename: Matteo
authors.initials: M
authors.lastname: Adinolfi
authors.forename: Marco
authors.initials: M
authors.lastname: d'Ischia
mesh_terms: D004396:Coloring Agents / Q000737:chemistry*; D007211:Indoles / Q000737:chemistry*; D008543:Melanins / Q000737:chemistry*; D008956:Models, Chemical; D015394:Molecular Structure; D010084:Oxidation-Reduction; D011108:Polymers / Q000737:chemistry*; D012996:Solutions; D013053:Spectrophotometry; D014867:Water / Q000737:chemistry
chemical_list: D004396:Coloring Agents; D007211:Indoles; D008543:Melanins; D011108:Polymers; D012996:Solutions; D014867:Water; C041877:eumelanin; C033871:5,6-dihydroxyindole