20
1 Melanin Chemistry
Fig. 1.9 Formation of pheomelanin building monomers (1,4-benzothiazine, 1,3-benzothiazole, and
3-oxo-3,4-dihydro-1,4-benzothiazine)
distinguish the NaBH 4 reduction product of 1,4-benzothiazine-3-carboxylic acid
and DHBTCA, the use of NaBD 4 enabled selective detection of 1,4-benzothiazine3-carboxylic acid.
Pheomelanin shows an absorption spectrum with a peak around 300 nm that
attenuates toward longer wavelengths [98]. Although this peak position is slightly
different from the absorption maximum at 330 nm of benzothiazine, possible subsequent products, namely benzothiazole and 3-oxo-3,4-dihydro-1,4-benzothiazine (3oxo-3,4-dihydro-1,4-benzothiazine: ODHBT), have an absorption maximum near
300 nm. Therefore, pheomelanin was considered to also include benzothiazole and
ODHBT as building monomers as well as benzothiazine [98]. This was confirmed
by detailed analyses of tyrosinase-catalyzed oxidation of dopa in the presence of
cysteine [99]. This reaction resulted in accumulation of an intermediate DHBTCA
until the middle stage of the reaction, and then the further redox reactions gave
rise to benzothiazine-based pheomelanin, which showed gradual degradation into
benzothiazole-based pheomelanin. The formation of small amount of ODHBT was
also demonstrated by HPLC analyses of the reaction mixtures.
1 Melanin Chemistry
Fig. 1.9 Formation of pheomelanin building monomers (1,4-benzothiazine, 1,3-benzothiazole, and
3-oxo-3,4-dihydro-1,4-benzothiazine)
distinguish the NaBH 4 reduction product of 1,4-benzothiazine-3-carboxylic acid
and DHBTCA, the use of NaBD 4 enabled selective detection of 1,4-benzothiazine3-carboxylic acid.
Pheomelanin shows an absorption spectrum with a peak around 300 nm that
attenuates toward longer wavelengths [98]. Although this peak position is slightly
different from the absorption maximum at 330 nm of benzothiazine, possible subsequent products, namely benzothiazole and 3-oxo-3,4-dihydro-1,4-benzothiazine (3oxo-3,4-dihydro-1,4-benzothiazine: ODHBT), have an absorption maximum near
300 nm. Therefore, pheomelanin was considered to also include benzothiazole and
ODHBT as building monomers as well as benzothiazine [98]. This was confirmed
by detailed analyses of tyrosinase-catalyzed oxidation of dopa in the presence of
cysteine [99]. This reaction resulted in accumulation of an intermediate DHBTCA
until the middle stage of the reaction, and then the further redox reactions gave
rise to benzothiazine-based pheomelanin, which showed gradual degradation into
benzothiazole-based pheomelanin. The formation of small amount of ODHBT was
also demonstrated by HPLC analyses of the reaction mixtures.