Acid–base and electrochemical properties of manganese meso(ortho- and meta-N-ethylpyridyl)porphyrins: potentiometric, spectrophotometric and spectroelectrochemical study of protolytic and redox equilibria

Abstract

The difference in electrostatics and reduction potentials between manganese ortho-tetrakis(N-ethylpyridinium-2-yl)porphyrin (MnTE-2-PyP) and manganese meta-tetrakis(N-ethylpyridinium-2-yl)porphyrin (MnTE-3-PyP) is a challenging topic, particularly because of the high likelihood for their clinical development. Hence, a detailed study of the protolytic and electrochemical speciation of MnII–IVTE-2-PyP and MnII-IVTE-3-PyP in a broad pH range has been performed using the combined spectrophotometric and potentiometric methods. The results reveal that in aqueous solutions within the pH range ~2–13 the following species exist: (H2O)MnIITE-m-PyP4+, (HO)MnIITE-m-PyP3+, (H2O)2MnIIITE-m-PyP5+, (H2O)(HO)MnIIITE-m-PyP4+, (H2O)(O=)MnIIITE-m-PyP3+, (H2O)(O=)MnIVTE-m-PyP4+ and (HO)(O=)MnIVTE-m-PyP3+ (m = 2, 3). All the protolytic equilibrium constants that include the accessible species as well as the thermodynamic parameters for each particular protolytic equilibrium have been determined. The corresponding formal reduction potentials related to the reduction of the above species and the thermodynamic parameters describing the accessible reduction couples were calculated as well.

Electrochemistry of redox-active Mn porphyrin-based SOD mimic MnTnBuOE-2-PyP5+ - Study of Redox Species Involved in ROS/RNS Scavenging

“…[15,26,29]. These results are in agreement with the previously established scheme of behavior of MnPs in aqueous solutions [26,30]. At pH = 7.8 only a single-electron reduction of Mn III P can be observed, shown by eq.…”

Differential Coordination Demands in Fe versus Mn Water-Soluble Cationic Metalloporphyrins Translate into Remarkably Different Aqueous Redox Chemistry and Biology

“…8, 24, 27, 29-31 These pK a sets indicate that all FePs have lost a proton from the coordinated axial water to yield the hydroxo-aqua complex as the major species at pH 7.8. Conversely, the acid-base equilibrium of the Mn analogues described by the same equations [2] and [3], with pK a1 and pK a2 values for the (H 2 O) 2 MnTM-2(or 3, or 4)-PyP 5+ 8, 25 and (H 2 O) 2 MnTE-2(or 3)-Pyp 5+ 26 in the region of ~10.5-11.4 ( ortho isomers) and ~11.5-13.5 ( meta isomers), indicate that the Mn porphyrins exist as the diaqua species at pH 7.8. …”

“…K a1 >> K a2 and K a1 >> K’ a1 ), already demonstrated for the corresponding MnP isomers. 25, 26 Specifically, the lengthening of the alkyl chain does not affect axial protonation equilibria: the pK a1 and pK a2 for Mn(III) methyl to n-octyl ortho analogs differ insignificantly, by only 0.5 and 0.3 units, respectively 25 . Therefore, a simple expression given in equation [1] could be applied to calculate the pK a1 values (Table 4) for other Fe III P analogues described in this work: $${\mathrm{pK}}_{\mathrm{normala}}=7.8-\Delta E∕0.059,$$ where Δ E = E 1/2 (pH=2) – E 1/2 (pH=7.8).…”

“…25, 26 The existence of diaqua (i.e. fully protonated) species of either ortho , meta or para N -methylpyridyl Fe(III) porphyrins at pH=2 has been documented in the literature, 8, 24, 25, 27-31 as determined by spectroscopic and electrochemical methods.…”

Diverse functions of cationic Mn(III) N-substituted pyridylporphyrins, recognized as SOD mimics

“…The electron density of the metal site may be best described in terms of metal-centered reduction potential, E 1/2 for Mn III P/Mn II P redox couple, which parallels the proton dissociation constants, p K a of the axially bound waters. The detailed spectral and electrochemical studies of ortho and meta MnTE-2(or 3)-PyP 5+ with Mn in +2, +3, and +4 oxidation states, which are relevant for the in vivo redox reactions of MnPs, have recently been reported [91,92]. …”