Photooxidation and Decarboxylation of Tyrosine Studied by E.P.R. and C.I.D.N.P. Techniques

The photooxidation of deoxygenated solutions of tyrosine in water at pH 8.5-12.6 has been studied at room temperature using e.p.r. spectroscopy. By means of e.p.r., four radicals were detected during irradiation with the full spectrum of a Hg-Xe arc lamp. These were identified as the tyrosyl radical, and two radicals derived from dopa. A tentative mechanism suggested for formation involves the reaction of tyrosine with the hydrated electron, which has been ejected from another tyrosine molecule in one of its excited electronic states. A mechanism similar to the one proposed for the photooxidation of phenol has been suggested for the formation of the radicals derived from dopa. Support for this mechanism is provided by the observation of proton nuclear spin polarization during irradiation.