PHOTOCHEMISTRY OF MELANIN PRECURSORS: DOPA, 5‐S‐CYSTEINYLDOPA AND 2,5‐S,S'‐DICYSTEINYLDOPA

The photochemistries of the melanin precursors dopa, 5‐S‐cysteinyldopa (5‐SCD) and 2.5‐S,S'‐dicysteinyldopa (2.5‐SCD) were investigated by 265‐nm laser flash photolysis. The quantum yield of hydrated electron following flash photolysis of dopa (9.1%) was half the yield of dopasemiquinone (19.6%), implying that dopasemiquinone is formed via two primary photochemical mechanisms: photionisation (giving e) or photohomolysis (giving H˙). Dopasemiquinone rapidly disproportionates to form dopaquinone and re‐form dopa. Dopaquinone in turn decays via a base‐catalysed unimolecular cyclisation eventually to form dopachrome. Assignment of the transient species was confirmed by previous pulse radiolysis studies of the one‐electron oxidation of dopa. In contrast, flash photolysis of the cysteinyldopas, 5‐SCD and 2,5‐SCD results in lower photoionisation quantum yields and the production of initial transient species whose absorption spectra were markedly different from their semiquinone absorption spectra previously determined pulse radiolytically. These observations indicate that the primary cysteinyldopa photochemical species is not such a semiquinone, but rather results from S‐CH2 bond photohomolysis. Absorption spectra and rate constants for the formation and decay of various transient species are reported.

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