Relationship among Redox Potentials, Proton Dissociation Constants of Pyrrolic Nitrogens, and in Vivo and in Vitro Superoxide Dismutating Activities of Manganese(III) and Iron(III) Water-Soluble Porphyrins

Abstract

The log k cat values for the dismutation of O2 •- by a series of monohydroxoiron(III) and aquamanganese(III) porphyrins, including ortho, meta, and para isomers of 5,10,15,20-tetrakis(N-alkylpyridiniumyl)porphyrins, were found to vary linearly with the metal-centered redox potentials (E 1/2) for the M(III)/M(II) couple. Each 120 mV increase in E 1/2 imparted a 10-fold increase in k cat. The observed behavior is in accord with the Marcus equation for outer-sphere electron-transfer reactions, suggesting that the same mechanism is operative for iron and manganese porphyrins. The Marcus plot enabled us to estimate the self-exchange rate constants of monohydroxoiron porphyrins to be ∼1 order of magnitude higher than those of aquamanganese porphyrins. Furthermore, E 1/2 values for all of the metalloporhyrins investigated were linearly related to the acid dissociation constants (pK a3) of the pyrrolic nitrogen of the metal-free porphyrins, indicating that either E 1/2, or the more readily measured pK a3, may be useful in predicting SOD activity in vitro. The most potent compounds investigated, with respect to SOD activity, are those of the ortho N-alkylpyridiniumyl series. Ortho N-alkylpyridiniumyl groups are more electron withdrawing than are the meta or para groups, thus imparting a more positive redox potential and a correspondingly higher SOD activity. Sufficiently positive potentials, or sufficiently low pK a3 values, are necessary for useful SOD activity, but so is the absence of toxicity. Despite their favorable redox potentials and SOD activities, all Fe(III) porphyrins investigated were toxic to Escherichia coli under both aerobic and anaerobic conditions and to both SOD-deficient and SOD-proficient strains. Only the ortho and meta manganese isomers of the N-alkylpyridiniumyl series (MnIIITE-2-PyP5+, MnIIITM-2-PyP5+ and MnIIITM-3-PyP5+) significantly protected SOD-deficient E. coli and allowed growth in an aerobic minimal medium. In previous work, we established that the lower toxicity of these compounds is due to diminished ability to bind to nucleic acids. The Mn(III) complexes are preferable to the Fe(III) complexes for SOD mimics possibly due to a lower tendency for axial ligation. We propose E 1/2 ≥ +0.05 V vs NHE and/or pK a3 ≤ 2.0 as necessary requirements for Mn porphyrins to be considered useful SOD mimics.

SOD-like activity of Mn(II) β-octabromo-meso-tetrakis(N-methylpyridinium-3-yl)porphyrin equals that of the enzyme itself

“…Our first report on a potent ortho isomer, Mn III TM-2-PyP 5+ , in protecting SOD-deficient E. coli indicated that the meta isomer, Mn III TM-3-PyP 5+ , although ~10-fold inferior with respect to in vitro antioxidant potency [1] may still be effective in vivo [26]. Indeed, meta isomer was comparable to the ortho analogue in providing protection to SOD-deficient E. coli to grow aerobically in restricted five amino acid-medium while para isomer, MnTM-4-PyP 5+ , was toxic [26].…”

“…The cyclic voltammetry was carried out on a CH Instruments Model 600 Voltammetry Analyzer [1]. A three-electrode system consisted of a 3 mm-diameter glassy carbon button working electrode (Bioanalytical Systems), the Ag/AgCl reference electrode, and a Pt auxiliary electrode was used in a small volume cell (3.0 mL).…”

“…The potentials were standardized against Mn III TE-2-PyP 5+ (chloride salt) (E ½ = +228 mV vs NHE [1]). Also another experiment was done where the voltammogram was measured first in 0.09 M phosphate buffer only, then solid NaCl was added to yield a 0.1 M Cl − concentration and the voltammogram was recorded again.…”

“…We have been developing Mn porphyrin-based SOD mimics for over a decade. We have established structure activity relationship (SAR) for both Mn and Fe porphyrins [1] between the metalcentered redox potential and the rate constant for O 2 •− dismutation and refined it recently to account for the differently charged/sized Mn porphyrins [2][3][4][5]. The introduction of both electron-withdrawing groups and positive charges onto the porphyrin core affords thermodynamic and kinetic/electrostatic facilitation for the approach of superoxide to the redox-cycling metal site [1][2][3][4].…”

“…Para isomer was toxic due to its interactions with nucleic acids [26]. Guided by these collective in vitro, in vivo, and SAR results [1][2][3][4][5]12,[24][25][26], we resumed herein our studies on the meta Mn(III) Nalkylpyridylporphyrins by preparing a brominated derivative of Mn III TM-3-PyP 5+ . This is the first report on the synthesis and characterization of Mn II Br 8 TM-3-PyP 4+ (Fig.…”