Pulse radiolysis, laser flash photolysis, and time-resolved resonance Raman spectroscopy have been used to study radicals derived by one-electron oxidation of 4-aminoresorcinol as nzodels for the active site free-radical intermediate in the catalytic cycle of copper amine oxidases containing the trihydroxyphenylalanine (TOPA) quinone cofactor. The 4-aminoresorcinol radical at neutral pH has an absorption maximum at 450 nm, which is similar to that of the enzyme radical. At pH 5 the resonance Raman spectrum of the radical from one-electron oxidation of 4-aminoresorcinol resembles that in the enzyme. The radical protonates at lower pH values with a pK(a) of 3.3 to give a species with a blue shifted absorption and different resonance Raman spectrum. Time-resolved resonance Raman spectroscopy shows that above pH 6.31 the radical from 4-aminoresorcinol deprotonates again to give a species that has a resonance Raman spectrum quite different from that of the enzyme radical. This second deprotonation is not immediately obvious from the transient absorption spectra obtained by pulse radiolysis. Interpretation of these spectra and comparison with related systems indicates that the enzyme intermediate is the singly deprotonated form of the radical,The results are discussed with respect to a recently proposed mechanism for the oxidative half reaction of copper amine oxidases (Su and Klinman, Biochemistry 1998, 37, 12513) in which the rate-limiting step is the oxidation by molecular oxygen of the fully reduced cofactor to the radical intermediate.