PHOTOOXIDATION OF 3, 4‐DIHYDROXYPHENYLALANINE BY HEMATOPORPHYRIN IN AQUEOUS SOLUTIONS: AN ELECTRON SPIN RESONANCE STUDY USING 2, 2, 6, 6‐TETRAMETHYL‐4‐PIPERIDONE‐1‐OXYL (TEMPONE)

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Abstract
Abstract— The oxidation of 3, 4‐dihydroxyphenylalanine (dopa) photosensitized by hematoporphyrin has been studied by ESR spectroscopy. The nitroxide free radical 2, 2, 6, 6‐tetramethyl‐4‐piperidone‐I‐oxyl (TEMPONE) is shown to be a convenient probe for monitoring oxygen consumption during the reaction. Isotope and azide effects on oxygen consumption suggest that photooxidation in D2O proceeds mainly by a singler oxygen mechanism, whereas in H2O both singlet oxygen and free radical mechanisms are important. The relative importance of these mechanisms can change as oxygen is consumed during the photoreaction. TEMPONE also can be used as an indicator of the presence of reducing radicals (in this case hematoporphyrin radical anions) in the system, but only at low oxygen levels: in aerated solutions efficient reaction of these radical ions with oxygen is proposed.