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 β-octabromination of the prototypical anionic Mn porphyrins Mn(III) meso-tetrakis(p-carboxylatophenyl)porphyrin ([MnIIITCPP]3- or MnTBAP3-) and Mn(III) meso-tetrakis(p-sulfonatophenyl)porphyrin ([MnIIITSPP]3-), to yield the corresponding anionic analogues [MnIIIBr8TCPP]3- and [MnIIIBr8TSPP]3-, respectively, is described along with characterization data, stability studies, and their ability to substitute for SOD in SOD-deficient Escherichia coli. Despite the MnIII/MnII reduction potential of [MnIIIBr8TCPP]3- and [MnIIIBr8TSPP]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 (MnIIITM-2-PyP5+), the SOD activity of both anionic brominated porphyrins ([MnIIIBr8TCPP]3-, E1/2 = + 213 mV vs NHE, log kcat = 5.07; [MnIIIBr8TSPP]3-, E1/2 = + 209 mV, log kcat = 5.56) is considerably lower than that of MnIIITM-2-PyP5+ (E1/2 = + 220 mV, log kcat = 7.79). This illustrates the impact of electrostatic guidance of O2– toward the metal center of the mimic. With low kcat, the [MnIIITCPP]3-, [MnIIITSPP]3-, and [MnIIIBr8TCPP]3- did not rescue SOD-deficient E. coli. The striking ability of [MnIIIBr8TSPP]3- to substitute for the SOD enzymes in the E. coli model does not correlate with its log kcat. In fact, the protectiveness of [MnIIIBr8TSPP]3- is comparable to or better than that of the potent SOD mimic MnIIITM-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 [MnIIIBr8TSPP]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 [MnIIIBr8TSPP]3- is not an SOD mimic per se but carries Mn into the E. coli cell.