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.

Self-decomposition of the nickel(III) doubly deprotonated peptide complex of Gly2HisGly occurs by base-assisted oxidation of the peptide. At < or =p[H+] 7.0, the major pathway is a four-electron oxidation (via 4 Ni(III) complexes) at the alpha carbon of the N-terminal glycyl residue. The product of this oxidation is oxamylglycylhistidylglycine, which hydrolyzes to yield ammonia and oxalylglycylhistidylglycine. Both of these peptide products decompose to give isocyanatoacetylhistidylglycine. A small amount (2%) of oxidative decarboxylation also is observed. In another major pathway above p[H+] 7.0, two Ni(III)-peptide complexes coordinate via an oxo bridge in the axial positions to form a reactive dimer species. This dimer generates two Ni(II)-peptide radical intermediates that cross-link at the alpha carbons of the N-terminal glycyl residues. In 0.13 mM Ni(III)-peptide at p[H+] 10.3, this pathway accounts for 60% of the reaction. The cross-linked peptide is subject to oxidation via atmospheric O2, where the 2,3-diaminobutanedioic acid is converted to a 2,3-diaminobutenedioic acid. The products observed at

The decomposition kinetics of the Ni(III) complexes of Gly(2)HisGly and Gly(2)Ha are studied from p[H(+)] 3.5 to 10, where His is l-histidine and Ha is histamine. In these redox reactions, at least two Ni(III) complexes are reduced to Ni(II) while oxidizing a single peptide ligand. The rate of Ni(III) loss is first order at low pH, mixed order from pH 7.0 to 8.5, and second order at higher pH. The transition from first- to second-order kinetics is attributed to the formation of an oxo-bridged Ni(III)-peptide dimer. The rates of decay of the Ni(III) complexes are general-base assisted with Brønsted beta values of 0.62 and 0.59 for Ni(III)Gly(2)HisGly and Ni(III)Gly(2)Ha, respectively. The coordination of Gly(2)HisGly and Gly(2)Ha to Ni(II) are examined by UV-vis and CD spectroscopy. The square planar Ni(II)(H(-2)Gly(2)HisGly)(-) and Ni(II)(H(-2)Gly(2)Ha) complexes lose an additional proton from an imidazole nitrogen at high pH with pK(a) values of 11.74 and 11.54, respectively. The corresponding Ni(III) complexes have axially coordinated water molecules with pK(a) values of 9.37 and 9.44. At higher pH an additional proton is lost from the imidazole nitrogen with a pK(a) value of 10.50 to give Ni(III)(H(-3)Gly(2)Ha)(H(2)O)(OH)(2-).

The doubly-deprotonated Ni(III) complex of Gly(2)Ha (where Ha is histamine) undergoes base-assisted oxidative self-decomposition of the peptide. At