THE SUPEROXIDE RADICAL REACTS WITH TYROSINE-DERIVED PHENOXYL RADICALS BY ADDITION RATHER THAN BY ELECTRON-TRANSFER

Radiolytically generated azide radicals have been used for the formation of tyrosyl radical, TyrO., from tyrosine. The TyrO. radicals combine (2k = 4.5 x 10(8) dm3 mol-1 s-1, determined by pulse radiolysis) yielding bityrosine in a > 90% yield. Bityrosine formation is not suppressed in the presence of oxygen [k(TyrO. + O2) < 1 x 10(3) dm3 mol-1 s-1]. When TyrO. and O2.- radicals are generated side by side in a 1:1.2 ratio, bityrosine formation is strongly suppressed and (2S,3aR,7aS)- and (2S,3aS,7aR)-3a-hydroxy-6-oxo-2,3,3a,6,7,7a-hexahydro-1H-indole-2-carboxylic acids 10 become the major final products. Their hydroperoxidic precursor is only short-lived (t1/2 = 4.2 h at room temperature and pH 8). Upon its decay H2O2 is released. Product 10 is believed to be formed by the addition of O2.- to the ortho- and paraposition of the phenoxyl radical, followed by protonation, ring closure and hydrolysis. Based on material balance considerations an electron transfer from O2.- to TyrO., although thermodynamically feasible, must play a minor role (less-than-or-equal-to 10%). The rate constant k(O2.- + TyrO.) has been determined by pulse radiolysis to be 1.5 x 10(9) dm3 mol-1 s-1.