Short-lived quinonoid species from 5,6-dihydroxyindole dimers en route to eumelanin polymers: integrated chemical, pulse radiolytic, and quantum mechanical investigation.
The transient species formed by oxidation of three dimers of 5,6-dihydroxyindole (1), a major building block of the natural biopolymer eumelanin, have been investigated. Pulse radiolytic oxidation of 5,5',6,6'-tetrahydroxy-2,4'-biindolyl (3) and 5,5',6,6'-tetrahydroxy-2,7'-biindolyl (4) led to semiquinones absorbing around 450 nm, which decayed with second-order kinetics (2k=2.8x10(9) and 1.4x10(9) M-1 s-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 (lamdamax=480 nm) which disproportionated at a comparable rate (2k=3x10(9) M-1 s-1) to give a relatively stable quinone (lamdamax=570 nm). A quantum mechanical investigation of o-quinone, quinonimine, and quinone methide structures of 2-4 suggested that oxidized 2-4 exist mainly as 2-substituted extended quinone methide tautomers. Finally, an oxidation product of 3 was isolated for the first time and was formulated as the hydroxylated derivative 5 arising conceivably by the addition of water to the quinone methide intermediate predicted by theoretical analysis. Overall, these results 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 are extended to higher oligomers.
Pezzella
Alessandro
A
Department of Organic Chemistry, University of Naples Federico II, Via Cintia, I-80126 Naples, Italy.
Panzella
Lucia
L
Crescenzi
Orlando
O
Napolitano
Alessandra
A
Navaratman
Suppiah
S
Edge
Ruth
R
Land
Edward J
EJ
Barone
Vincenzo
V
d'Ischia
Marco
M
D019281:Dimerization; D007211:Indoles / Q000737:chemistry*; D008543:Melanins / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D010084:Oxidation-Reduction; D011108:Polymers / Q000737:chemistry*; D011789:Quantum Theory; D011809:Quinones / Q000737:chemistry*; D013053:Spectrophotometry; D013237:Stereoisomerism; D013329:Structure-Activity Relationship; D013997:Time Factors
D007211:Indoles; D008543:Melanins; D011108:Polymers; D011809:Quinones; C041877:eumelanin; C033871:5,6-dihydroxyindole