5,6-dihydroxyindole tetramers with "anomalous" interunit bonding patterns by oxidative coupling of 5,5',6,6'-tetrahydroxy-2,7'-biindolyl: emerging complexities on the way toward an improved model of eumelanin buildup

J Org Chem. 2007 Nov 23;72(24):9225-30. doi: 10.1021/jo701652y. Epub 2007 Nov 2.

Abstract

Chemical or enzymatic oxidation of 5,6-dihydroxyindole (1) leads to the rapid deposition of a black solid resembling eumelanin pigments by way of a complex oligomerization/polymerization process that proceeds in the early stages via dimers 2-3 and trimers 5-6 characterized by 2,4'- and 2,7'-couplings. Despite extensive efforts, the structures of the higher oligomers, which define the structural architecture and physicochemical properties of the eumelanin particles, have so far defied elucidation. Using a dimer-dimer coupling strategy that has recently allowed the first successful entry to a tetramer of 1, we report now three additional tetramers obtained by oxidation of 5,5',6,6'-tetrahydroxy-2,7'-biindolyl (3) with the peroxidase/H2O2 system. On the basis of extensive 2D NMR and mass spectrometric analysis, the products were identified as 5,5',5'',5''',6,6',6'',6'''-octaacetoxy-7,2':3',3'':2'',7'''-tetraindolyl (acetylated 8, 3%), 5,5',5'',5''',6,6',6'',6' ''-octaacetoxy-2,7':4',4'':7'',2'''-tetraindolyl (acetylated 9, 4%), and 5,5',5'',5''',6,6',6'',6'''-octaacetoxy-2,7':2',3'':2'',7'''-tetraindolyl (acetylated 10, 5%), in which the inner units are linked through unexpected 3,3'-, 4,4'-, and 2,3'-linkages. If verified in further studies, the newly uncovered coupling patterns would entail important consequences for current models of eumelanin structure based on one-dimensional structural chains with extended pi-electron conjugation or pi-stacked flat oligomer aggregates.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Acetylation
  • Dimerization
  • Hydrogen Peroxide / chemistry
  • Indoles / chemistry*
  • Magnetic Resonance Spectroscopy
  • Mass Spectrometry
  • Melanins / chemistry*
  • Models, Chemical
  • Oxidation-Reduction
  • Peroxidase / chemistry
  • Peroxidase / metabolism
  • Polymers / chemistry*
  • Stereoisomerism

Substances

  • 5,5',6,6'-tetrahydroxy-2,7'-biindolyl
  • Indoles
  • Melanins
  • Polymers
  • eumelanin
  • Hydrogen Peroxide
  • Peroxidase
  • 5,6-dihydroxyindole