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

Mn porphyrins are among the most efficient SOD mimics with potency approaching that of SOD enzymes. The most potent ones, Mn(III) N  -alkylpyridylporphyrins bear positive charges in a close proximity to the metal site, affording thermodynamic and kinetic facilitation for the reaction with negatively charged superoxide. The addition of electron-withdrawing bromines onto β-pyrrolic positions dramatically improves thermodynamic facilitation for the O2
• - dismutation. We have previously characterized the para isomer, MnIIBr8TM-4-PyP4+ [Mn(II) β-octabromo-meso-tetrakis(N-methylpyridinium-4-yl)porphyrin]. Herein we fully characterized its meta analogue, MnIIBr8TM-3-PyP4+ with respect to UV/vis spectroscopy, electron spray mass spectrometry, electrochemistry, O2
• - dismutation, metal–ligand stability, and the ability to protect SOD-deficient Escherichia coli in comparison with its para analogue. The increased electron-deficiency of the metal center stabilizes Mn in its +2 oxidation state. The metal-centered MnIII/MnII reduction potential, E½ = +468 mV vs NHE, is increased by 416 mV with respect to non-brominated analogue, MnIIITM-3-PyP5+ and is only 12 mV less positive than for para isomer. Yet, the complex is significantly more stable towards the loss of metal than its para analogue. As expected, based on the structure–activity relationships, an increase in E½ results in a higher catalytic rate constant for the O2
• - dismutation, log kcat ⩾ 8.85; 1.5-fold increase with respect to the para isomer. The IC50 was calculated to be ⩽3.7 nM. Manipulation of the electron-deficiency of a cationic porphyrin resulted, therefore, in the highest kcat ever reported for a metalloporphyrin, being essentially identical to the kcat of superoxide dismutases (log kcat = 8.84–9.30). The positive kinetic salt effect points to the unexpected, unique and first time recorded behavior of Mn β-octabrominated porphyrins when compared to other Mn porphyrins studied thus far. When species of opposing charges react, the increase in ionic strength invariably results in the decreased rate constant; with brominated porphyrins the opposite was found to be true. The effect is 3.5-fold greater with meta than with para isomer, which is discussed with respect to the closer proximity of the quaternary nitrogens of the meta isomer to the metal center than that of the para isomer. The potency of MnIIBr8TM-3-PyP4+ was corroborated by in vivo studies, where 500 nM allows SOD-deficient E. coli to grow >60% of the growth of wild type; at concentrations ⩾5 μM it exhibits toxicity. Our work shows that exceptionally high kcat for the O2
• - disproportionation can be achieved not only with an N5-type coordination motif, as rationalized previously for aza crown ether (cyclic polyamines) complexes, but also with a N4-type motif as in the Mn porphyrin case; both motifs sharing “up-down-up-down” steric arrangement.