Photolysis of Pheomelanin Precursors: An Esr‐spin Trapping Study

The photolysis of 5‐S‐cysteinyldopa, 2,5‐S,S‐dicysteinyldopa, 4‐hydroxybenzothiazole and cysteinyldopa melanins has been studied by ESR‐spin trapping methods using 5.5‐dimethyl‐1‐pyrroline‐1‐oxide and 2‐methyl‐2‐nitrosopropane as spin traps for hydrated electrons, hydrogen atoms, and carbon‐centered alanyl radicals. The photochemistry of these materials is shown to resemble dopa in that both photoionization and photohomolysis occur. However, unlike dopa, the cysteinyldopa also forms the carbon‐centered alanyl radicals via photodecomposition of the cysteinyl group. Action spectra and quantum yields are reported. Mechanism(s) of radical formation are discussed.