The pheomelanin chromophore revisited.

Pheomelanin photochemistry is commonly regarded as reflecting the behavior of benzothiazine chromophores biogenetically derived from 5-S-cysteinyldopa. This notion, which has become a central dogma in pheomelanin research despite the lack of direct experimental support, has in part been built upon the identification many decades ago in pheomelanin-containing tissues of a peculiar group of low molecular weight compounds, the trichochromes, featuring the peculiar 2,2'-bi(2H-1,4-benzothiazine) skeleton. Although commonly regarded as primary determinants of the red colorations of hair in the true “pheomelanic” phenotype, the origin and even the actual occurrence of trichochromes in human hair and tissues has remained a subject of debate. In an attempt to address this issue, the chromophoric features of synthetic pheomelanins and of a red hair pheomelanin were compared with those of some putative precursors prepared under biomimetic conditions. All pheomelanin samples in phosphate buffer at pH 7.4 showed a definite maximum around 300 nm and a monotonic decrease of the absorption in the range 350-500 nm with inflections around 259 and 360 nm. The chromophoric changes accompanying the formation of synthetic pheomelanin by peroxidase/H2O2 oxidation of CD in the presence or in absence of zinc ions were monitored with concomitant LC/UV and MS analysis. At the monomer stage benzothiazine intermediates were detected which changed into benzothiazine dimers and trichochrome related species. At the later stage, in the absence of chromatographically defined species, the oxidation mixture showed a distinct absorption around 300 nm which was evidently due to chromophoric units belonging to the pigment backbone. These features were superimposable to those of 3-oxo-3,4-dihydro-1,4-benzothiazine and benzothiazole species suggesting that these previously overlooked structural units play in fact a role in determining the pigment chromophore. The possible impact of these observations on the phtotochemical and photoprotective properties of pheomelanins will be discussed