Get access

The “Benzothiazine” Chromophore of Pheomelanins: A Reassessment

Authors

  • Alessandra Napolitano,

    Corresponding author
    1. Department of Organic Chemistry and Biochemistry, University of Naples “Federico II” Complesso Universitario Monte S. Angelo, Naples, Italy
      *email: alesnapo@unina.it (Alessandra Napolitano)
    Search for more papers by this author
  • Maria De Lucia,

    1. Department of Organic Chemistry and Biochemistry, University of Naples “Federico II” Complesso Universitario Monte S. Angelo, Naples, Italy
    Search for more papers by this author
  • Lucia Panzella,

    1. Department of Organic Chemistry and Biochemistry, University of Naples “Federico II” Complesso Universitario Monte S. Angelo, Naples, Italy
    2. Dermatology Unit, Department of Systematic Pathology, University of Naples “Federico II”, Naples, Italy
    Search for more papers by this author
  • Marco D’Ischia

    1. Department of Organic Chemistry and Biochemistry, University of Naples “Federico II” Complesso Universitario Monte S. Angelo, Naples, Italy
    Search for more papers by this author

  • This invited paper is part of the Symposium-in-Print: Melanins.

*email: alesnapo@unina.it (Alessandra Napolitano)

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 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 (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.

Get access to the full text of this article

Ancillary