Photoionization of Melanin Precursors: An Electron Spin Resonance Investigation Using the Spin Trap 5,5‐dimethyl‐1‐pyrroline‐1‐oxide (Dmpo)

The photoionization of 3,4‐dihydroxyphenylalanine (dopa) and catechol has been studied by electron spin resonance spectroscopy using the free radical scavenger 5,5‐dimethyI‐1‐pyrroline‐1 ‐oxide as a spin trap for hydrated electrons and hydrogen atoms. The photochemistry of these materials is shown to resemble tyrosine in that both photoionization and photohomolysis (to give H) occur, with photoionization predominating (by a factor of 2.6 for dopa). Ionization of one of the phenolic hydroxyl groups increases the yield of radicals by a factor of 2. Action spectra and quantum yields for radical production are reported.