Electron spin resonance-spin stabilization of semiquinones produced during oxidation of epinephrine and its analogues

The ESR-spin stabilization approach has been employed to detect and characterize o-semiquinone radicals from the oxidation of epinephrine and related materials (norepinephrine, 3,4-dihydroxynorephedrine, isoproterenol, and adrenalone) in aqueous solutions. Semiquinones were generated by various oxidative procedures--enzymatic oxidation (with horseradish peroxidase/H2O2), chemical oxidation (with Ag2O) and photooxidation--and subsequently kinetically stabilized through complexation with Zn2+ ions. This "spin stabilization" affords high radical concentrations, which has allowed unambiguous identification of the radical intermediates. Where appropriate, spectral assignments have been supported by deuterium substitution experiments and computer simulations of spectra. Two types of free radical have been identified: primary "open chain" semiquinones, formed by one-electron oxidation of the parent catecholamines, and secondary semiquinones, formed subsequent to cyclization reactions. The mechanism of formation of the secondary radicals is discussed, and it is concluded that they are derived from product aminochromes. Thus, oxidation of adrenochrome gives a radical identical to the secondary species observed from oxidation of epinephrine.