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Short-Lived Quinonoid Species from 5,6-DihydroxyindoleDimers en Route to Eumelanin Polymers: IntegratedChemical, Pulse Radiolytic, and Quantum MechanicalInvestigation

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dcterms:abstract

The transient species formed by oxidation of three dimers of 5,6-dihydroxyindole (1), a majorbuilding block of the natural biopolymer eumelanin, have been investigated. Pulse radiolytic oxidation of5,5‘,6,6‘-tetrahydroxy-2,4‘-biindolyl (3) and 5,5‘,6,6‘-tetrahydroxy-2,7‘-biindolyl (4) led to semiquinonesabsorbing around 450 nm, which decayed with second-order kinetics (2k = 2.8 × 109 and 1.4 × 109 M-1s-1, respectively) to give the corresponding quinones (500−550 nm). 5,5‘,6, 6‘-Tetrahydroxy-2,2‘-biindolyl(2), on the other hand, furnished a semiquinone (λmax = 480 nm) which disproportionated at a comparablerate (2k = 3 × 109 M-1 s-1) to give a relatively stable quinone (λmax = 570 nm). A quantum mechanicalinvestigation of o-quinone, quinonimine, and quinone methide structures of 2−4 suggested that oxidized2−4 exist mainly as 2-substituted extended quinone methide tautomers. Finally, an oxidation product of 3was isolated for the first time and was formulated as the hydroxylated derivative 5 arising conceivably bythe addition of water to the quinone methide intermediate predicted by theoretical analysis. Overall, theseresults suggest that the oxidation chemistry of biindolyls 2−4 differs significantly from that of the parent 1,whereby caution must be exercised before concepts that apply strictly to the mode of coupling of 1 areextended to higher oligomers.

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