Chlorine dioxide oxidations of tyrosine, N-acetyltyrosine, and dopa.

The reactions of aqueous ClO2 with tyrosine, N-acetyltyrosine, and dopa (3,4-dihydroxyphenylalanine) are investigated from pH 4 to 7. The reaction rates increase greatly with pH to give a series of oxidized products. Tyrosine and N-acetyltyrosine have similar reactivities with second-order rate constants (25.0 °C) for their phenoxide forms equal to 1.8 × 108 and 7.6 × 107 M-1 s-1, respectively. Both species generate phenoxyl radicals that react rapidly with a second ClO2 at the 3 position to give observable but short-lived adducts with proposed C(H)OClO bonding. The decay of these phenoxyl−ClO2 adducts also is rapid and is base-assisted to form dopaquinone (from tyrosine) and N-acetyldopaquinone (from N-acetyltyrosine) as initial products. The consumption of two ClO2 molecules corresponds to a four-electron oxidation that gives ClO2- in the first step and HOCl in the second step. The reaction between ClO2 and the deprotoned-catechol form of dopa is extremely fast (2.8 × 109 M-1 s-1). Dopa consumes two ClO...