The superoxide radical reacts with tyrosine-derived phenoxyl radicals by addition rather than by electron transfer

Abstract

Radiolytically generated azide radicals have been used for the formation of tyrosyl radical, TyrO˙ from tyrosine. The TyrO˙ radicals combine (2k= 4.5 × 10⁸ 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 × 10³ 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 (t_½= 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 para-position 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 (⩽10%). The rate constant k(O₂˙^–+ TyrO˙) has been determined by pulse radiolysis to be 1.5 × 10⁹ dm³ mol^–1 S^–1.