Impact of electrostatics in redox modulation of oxidative stress by Mn porphyrins:: Protection of SOD-deficient Escherichia coli via alternative mechanism where Mn porphyrin acts as a Mn carrier

Understanding the factors that determine the ability of Mn porphyrins to scavenge reactive species is essential for tuning their in vivo efficacy. We present herein the revised structure-activity relationships accounting for the critical importance of electrostatics in the Mn porphyrin-based redox modulation systems and show that the design of effective SOD mimics (per se) based on anionic porphyrins is greatly hindered by inappropriate electrostatics. A new strategy for the beta-octabromination of the prototypical anionic Mn porphyrins Mn(III) meso-tetrakis(p-carboxylatophenyl)porphyrin ([(MnTCPP)-T-III](3-) or MnTBAP(3-)) and Mn(III) meso-tetrakis(p-sulfonatophenyl)porphyrin ([(MnTSPP)-T-III](3-)), to yield the corresponding anionic analogues [(MnBr8TCPP)-Br-III](3-) and [(MnBr8TSPP)-Br-III](3-), respectively, is described along with characterization data, stability studies, and their ability to substitute for SOD in SOD-deficient Escherichia coli. Despite the Mn-III/Mn-II reduction potential of [(MnBr8TCPP)-Br-III](3-) and [(MnBr8TSPP)-Br-III](3-) being close to the SOD-enzyme optimum and nearly identical to that of the cationic Mn(III) meso-tetrakis(N-methylpyridinium-2-yl)porphyrin ((MnTM)-T-III-2-PyP5+), the SOD activity of both anionic brominated porphyrins ([(MnBr8TCPP)-Br-III](3-), E-1/2 = +213 mV vs NHE, log k(cat) = 5.07; [(MnBr8TCPP)-Br-III](3-), E-1/2 = +209 mV, log k(cat) = 5.56) is considerably lower than that of (MnTM)-T-III-2-PyP5+ (E-1/2 = +220 mV, log k(cat) = 7.79). This illustrates the impact of electrostatic guidance of O-2(-) toward the metal center of the mimic. With low k(cat), the [(MnTCPP)-T-III](3-), [(MnTSPP)-T-III](3-), and [(MnBr8TCPP)-Br-III](3-) did not rescue SOD-deficient E. coli. The striking ability of [(MnBr8TCPP)-Br-III](3-) to substitute for the SOD enzymes in the E coli model does not correlate with its log k(cat). In fact, the protectiveness of [(MnBr8TCPP)-Br-III](3-) is comparable to or better than that of the potent SOD mimic (MnTM)-T-III-2-PyP5+, even though the dismutation rate constant of the anionic complex is 170-fold smaller. Analyses of the medium and E. coli cell extract revealed that the major species in the [(MnBr8TCPP)-Br-III](3-) system is not the Mn complex, but the free-base porphyrin [H2Br8TSPP](4-) instead. Control experiments with extracellular MnCl2 showed the lack of E. coli protection, indicating that "free" Mn2+ cannot enter the cell to a significant extent. We proposed herein the alternative mechanism where a labile Mn porphyrin [(MnBr8TCPP)-Br-III](3-) is not an SOD mimic per se but carries Mn into the E coli cell. (C) 2008 Elsevier Inc. All rights reserved.