Reactions of Manganese Porphyrins with Peroxynitrite and Carbonate Radical Anion

Abstract

We have studied the reaction kinetics of ten manganese porphyrins, differing in their meso substituents, with peroxynitrite (ONOO – ) and carbonate radical anion ( CO 3 · ¯ ) using stopped-flow and pulse radiolysis, respectively. Rate constants for the reactions of Mn(III) porphyrins with ONOO – ranged from 1 × 10 5 to 3.4 × 10 7 m –1 s – 1 and correlated well with previously reported kinetic and thermodynamic data that reflect the resonance and inductive effects of the substituents on the porphyrin ring. Rate constants for the reactions of Mn(III) porphyrins with CO 3 · ¯ ranged from 2 × 10 8 to 1.2 × 10 9 m –1 s – 1 at pH ≤ 8.5 and increased with pH as a consequence of the ionization of the complexes. Mn(II) porphyrins reacted with CO 3 · ¯ with rate constants ranging from 1 × 10 9 to 5 × 10 9 m –1 s – 1 at pH 10.4. Hence, fast scavenging of ONOO – and CO 3 · ¯ by manganese porphyrins could occur in vivo because of the catalytic reduction at the expense of a number of cellular reductants. Additionally, we determined the p K a of the axial water molecules of the Mn(III) complexes at pH 7.5–13.2 by spectrophotometric titration. Results were consistent with two acid-base equilibria for most of the complexes studied. The p K a values also correlated with the resonance and inductive effects of the substituents. The correlations of E ½ with the rate constants with ONOO – and with the p K a values display a deviation from linearity when N -alkylpyridinium substituents included N -alkyl moieties longer than ethyl, which is interpreted in terms of a decrease in the local dielectric constant.