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 degrees C) for their phenoxide forms equal to 1.8x10(8) and 7.6x10(7) 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 (2.8x10(9) M-1 s-1). Dopa consumes two ClO2 to give dopaquinone and 2ClO2- as products. Above pH 4, dopaquinone cyclizes to give cyclodopa, which in turn is rapidly oxidized to dopachrome. A resolved first-order rate constant of 249 s-1 is evaluated for the cyclization of the basic form of dopaquinone that leads to dopachrome as a product with strong absorption bands at 305 and 485 nm.
Napolitano
Michael J
MJ
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-2084, USA.
Green
Brandon J
BJ
Nicoson
Jeffrey S
JS
Margerum
Dale W
DW
D017606:Chlorine Compounds / Q000737:chemistry*; D002851:Chromatography, High Pressure Liquid; D004295:Dihydroxyphenylalanine / Q000737:chemistry*; D010084:Oxidation-Reduction; D010087:Oxides / Q000737:chemistry*; D013055:Spectrophotometry, Infrared; D013056:Spectrophotometry, Ultraviolet; D014443:Tyrosine / Q000031:analogs & derivatives* / Q000737:chemistry*
D017606:Chlorine Compounds; D010087:Oxides; D014443:Tyrosine; D004295:Dihydroxyphenylalanine; C025109:chlorine dioxide; C025787:N-acetyltyrosine