Transient quinonimines and 1,4-benzothiazines of pheomelanogenesis: new pulse radiolytic and spectrophotometric evidence.

2.50
Hdl Handle:
http://hdl.handle.net/10541/91345
Title:
Transient quinonimines and 1,4-benzothiazines of pheomelanogenesis: new pulse radiolytic and spectrophotometric evidence.
Authors:
Napolitano, Alessandra; Di Donato, Paola; Prota, Giuseppe; Land, Edward J
Abstract:
Biosynthetic and model in vitro studies have shown that pheomelanins, the distinctive pigments of red human hair, arise by oxidative cyclization of cysteinyldopas mainly 5-S-cysteinyldopa (1) via a critical o-quinonimine intermediate, which rearranges to unstable 1,4-benzothiazines. To get new evidence for these labile species, fast time resolution pulse radiolytic oxidation by dibromide radical anion of a suitable precursor, the dihydro-1,4-benzothiazine-3-carboxylic acid 7 was performed in comparison with that of 1. In the case of 7, dibromide radical anion oxidation leads over a few microseconds (k = 2.1 x 10(9) M(-1) s(-1)) to a phenoxyl radical (lambda(max) 330 nm, epsilon = 6300 M(-1) cm(-1)) which within tens of milliseconds gives rise with second-order kinetics (2k = 2.7 x 10(7) M(-1) s(-1)) to a species exhibiting an absorption maximum at 540 nm (epsilon = 2200 M(-1) cm(-1)). This was formulated as the o-quinonimine 3 arising from disproportionation of the initial radical. The quinonimine chromophore is converted over hundreds of milliseconds (k = 6.0 s(-1)) to a broad maximum at around 330 nm interpreted as due to a 1,4-benzothiazine or a mixture of 1,4-benzothiazines, which as expected are unstable and subsequently decay over a few seconds (k = 0.5 s(-1)). Interestingly, the quinonimine is observed as a labile intermediate also in the alternative reaction route examined, involving cyclization of the o-quinone (lambda(max) 390 nm, epsilon = 6900 M(-1) cm(-1)) arising by disproportionation (2k = 1.7 x 10(8) M(-1) s(-1)) of an o-semiquinone (lambda(max) 320 nm, epsilon = 4700 M(-1) cm(-1)) directly generated by dibromide radical anion oxidation of 1. Structural formulation of the 540 nm species as an o-quinonimine was further supported by rapid scanning diode array spectrophotometric monitoring of the ferricyanide oxidation of a series of model dihydrobenzothiazines.
Affiliation:
The Department of Organic and Biological Chemistry, University of Naples Federico II, Italy.
Citation:
Transient quinonimines and 1,4-benzothiazines of pheomelanogenesis: new pulse radiolytic and spectrophotometric evidence. 1999, 27 (5-6):521-8 Free Radic. Biol. Med.
Journal:
Free Radical Biology & Medicine
Issue Date:
Sep-1999
URI:
http://hdl.handle.net/10541/91345
DOI:
10.1016/S0891-5849(99)00098-2
PubMed ID:
10490271
Type:
Article
Language:
en
ISSN:
0891-5849
Appears in Collections:
All Paterson Institute for Cancer Research

Full metadata record

DC FieldValue Language
dc.contributor.authorNapolitano, Alessandraen
dc.contributor.authorDi Donato, Paolaen
dc.contributor.authorProta, Giuseppeen
dc.contributor.authorLand, Edward Jen
dc.date.accessioned2010-02-08T11:15:30Z-
dc.date.available2010-02-08T11:15:30Z-
dc.date.issued1999-09-
dc.identifier.citationTransient quinonimines and 1,4-benzothiazines of pheomelanogenesis: new pulse radiolytic and spectrophotometric evidence. 1999, 27 (5-6):521-8 Free Radic. Biol. Med.en
dc.identifier.issn0891-5849-
dc.identifier.pmid10490271-
dc.identifier.doi10.1016/S0891-5849(99)00098-2-
dc.identifier.urihttp://hdl.handle.net/10541/91345-
dc.description.abstractBiosynthetic and model in vitro studies have shown that pheomelanins, the distinctive pigments of red human hair, arise by oxidative cyclization of cysteinyldopas mainly 5-S-cysteinyldopa (1) via a critical o-quinonimine intermediate, which rearranges to unstable 1,4-benzothiazines. To get new evidence for these labile species, fast time resolution pulse radiolytic oxidation by dibromide radical anion of a suitable precursor, the dihydro-1,4-benzothiazine-3-carboxylic acid 7 was performed in comparison with that of 1. In the case of 7, dibromide radical anion oxidation leads over a few microseconds (k = 2.1 x 10(9) M(-1) s(-1)) to a phenoxyl radical (lambda(max) 330 nm, epsilon = 6300 M(-1) cm(-1)) which within tens of milliseconds gives rise with second-order kinetics (2k = 2.7 x 10(7) M(-1) s(-1)) to a species exhibiting an absorption maximum at 540 nm (epsilon = 2200 M(-1) cm(-1)). This was formulated as the o-quinonimine 3 arising from disproportionation of the initial radical. The quinonimine chromophore is converted over hundreds of milliseconds (k = 6.0 s(-1)) to a broad maximum at around 330 nm interpreted as due to a 1,4-benzothiazine or a mixture of 1,4-benzothiazines, which as expected are unstable and subsequently decay over a few seconds (k = 0.5 s(-1)). Interestingly, the quinonimine is observed as a labile intermediate also in the alternative reaction route examined, involving cyclization of the o-quinone (lambda(max) 390 nm, epsilon = 6900 M(-1) cm(-1)) arising by disproportionation (2k = 1.7 x 10(8) M(-1) s(-1)) of an o-semiquinone (lambda(max) 320 nm, epsilon = 4700 M(-1) cm(-1)) directly generated by dibromide radical anion oxidation of 1. Structural formulation of the 540 nm species as an o-quinonimine was further supported by rapid scanning diode array spectrophotometric monitoring of the ferricyanide oxidation of a series of model dihydrobenzothiazines.en
dc.language.isoenen
dc.subject.meshDermatitis, Phototoxic-
dc.subject.meshDihydroxyphenylalanine-
dc.subject.meshFerricyanides-
dc.subject.meshFree Radicals-
dc.subject.meshHair Color-
dc.subject.meshHumans-
dc.subject.meshKinetics-
dc.subject.meshMelanins-
dc.subject.meshMolecular Structure-
dc.subject.meshPigments, Biological-
dc.subject.meshPulse Radiolysis-
dc.subject.meshQuinones-
dc.subject.meshSpectrophotometry-
dc.subject.meshThiazines-
dc.titleTransient quinonimines and 1,4-benzothiazines of pheomelanogenesis: new pulse radiolytic and spectrophotometric evidence.en
dc.typeArticleen
dc.contributor.departmentThe Department of Organic and Biological Chemistry, University of Naples Federico II, Italy.en
dc.identifier.journalFree Radical Biology & Medicineen
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