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- Title
The “Benzothiazine” Chromophore of Pheomelanins: A Reassessment.
- Authors
Napolitano, Alessandra; De Lucia, Maria; Panzella, Lucia; d'Ischia, Marco
- Abstract
The characteristic absorption and photochemical properties of pheomelanins are generally attributed to “benzothiazine” structural units derived biogenetically from 5- S-cysteinyldopa. This notion, however, conveys little or no information about the structural chromophores responsible for the photoreactivity of pheomelanins. At pH 7.4, natural and synthetic pheomelanins show a defined maximum around 305 nm, which is not affected by reductive treatment with sodium borohydride, and a monotonic decrease in the absorption in the range 350–550 nm. These features are not compatible with a significant proportion of structural units related to 2 H-1,4-benzothiazine and 2 H-1,4-benzothiazine-3-carboxylic acid, the early borohydride-reducible pheomelanin precursors featuring absorption maxima above 340 nm. Rather, these features would better accommodate a contribution by the nonreducible 3-oxo-3,4-dihydrobenzothiazine ( λmax 299 nm) and benzothiazole ( λmax 303 nm) structural motifs, which are generated in the later stages of pheomelanogenesis in vitro. This conclusion is supported by a detailed liquid chromatography/UV and mass spectrometry monitoring of the species formed in the oxidative conversion of 5- S-cysteinyldopa to pheomelanin, and would point to a critical reassessment of the commonly reported “benzothiazine” chromophore in terms of more specific and substantiated structural units, like those formed during the later stages of pheomelanin synthesis in vitro.
- Subjects
MELANOGENESIS; MELANISM; MELANOSIS; MELANOCYTES; ORGANIC acids; SPECTRUM analysis
- Publication
Photochemistry & Photobiology, 2008, Vol 84, Issue 3, p593
- ISSN
0031-8655
- Publication type
Article
- DOI
10.1111/j.1751-1097.2007.00232.x