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

[Graphics] 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 dimerdimer 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 7-electron conjugation or,pi-stacked flat oligomer aggregates.