The "benzothiazine" chromophore of pheomelanins: a reassessment.
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 2H-1,4-benzothiazine and 2H-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 (lambdamax 299 nm) and benzothiazole (lambdamax 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.
Napolitano
Alessandra
A
Department of Organic Chemistry and Biochemistry, University of Naples Federico II Complesso Universitario, Monte S. Angelo, Naples, Italy. alesnapo@unina.it
De Lucia
Maria
M
Panzella
Lucia
L
d'Ischia
Marco
M
D002851:Chromatography, High Pressure Liquid; D003548:Cysteinyldopa / Q000737:chemistry*; D008543:Melanins / Q000737:chemistry*; D008547:Melanophores / Q000737:chemistry*; D015394:Molecular Structure; D010084:Oxidation-Reduction; D013056:Spectrophotometry, Ultraviolet; D013843:Thiazines / Q000737:chemistry*
C446962:1,4-benzothiazine; D008543:Melanins; D013843:Thiazines; C018362:pheomelanin; D003548:Cysteinyldopa