A New Mixed-Valence Mn(II)Mn(III) Compound With Catalase and Superoxide Dismutase Activities

Abstract

The synthesis, X-ray molecular structure, physico-chemical characterization and dual antioxidant activity (catalase and superoxide dismutase) of a new polymeric mixed valence Mn(III)Mn(II) complex, containing the ligand H2BPClNOL (N-(2-hydroxybenzyl)-N-(2-pyridylmethyl)[(3-chloro)(2-hydroxy)] propylamine) is described. The monomeric unit is composed of a dinuclear Mn(II)Mn(III) moiety, [Mn(III)(μ-HBPClNOL)(μ-BPClNOL)Mn(II)(Cl)](ClO4)·2H2O, 1, in which the Mn ions are connected by two different bridging groups provided by two molecules of the ligand H2BPClNOL, a phenoxide and an alkoxide group. In the solid state, this mixed valence dinuclear unit is connected to its neighbors through chloro bridges. Magnetic measurements indicated the presence of ferromagnetic [J = +0.076(13) cm−1] and antiferromagnetic [J = −5.224(13) cm−1] interactions. The compound promotes O2•- dismutation in aqueous solution (IC50 = 0.370 μmol dm−3, kcat = 3.6x106 M−1 s−1). EPR studies revealed that a high-valent Mn(III)-O-Mn(IV) species is involved in the superoxide dismutation catalytic cycle. Complex 1 shows catalase activity only in the presence of a base, e.g., piperazine or triethylamine. Kinetic studies were carried out in the presence of piperazine and employing two different methods, resulting in kcat values of 0.58 ± 0.03 s−1 (detection of O2 production employing a Clark electrode) and 2.59 ± 0.12 s−1 (H2O2 consuption recorded via UV-Vis). EPR and ESI-(+)-MS studies indicate that piperazine induces the oxidation of 1, resulting in the formation of the catalytically active Mn(III)-O-Mn(IV) species.

Synthesis and Characterization of Manganese Dithiocarbamate Complexes: New Evidence of Dioxygen Activation

“…The electronic spectrum of 3 ( Supplementary Figure S10 ), collected in the region 800–200 nm in chloroform, is dominated by intense bands in the 235–300 nm UV range attributed to intra-ligand transitions and by a lower intensity band at 353 nm probably due to ligand–metal transitions. The Vis region shows a weak band at 464 nm, which could only be determined by analyzing a concentrated sample, attributed to d–d transitions compatible with a d 4 configuration of octahedral complexes of Mn(III) as reported in the literature [ 21 , 24 , 25 , 26 ]. The cyclic voltammetry carried out in a dichloromethane-based electrolyte, in agreement with AR Hendrickson et al [ 27 ], showed a quasi-reversible redox process (ratio of anodic and cathodic peak currents I p-ox /I p-red = 1.1; separation peaks ΔE = 140 mV) centered at +0.25 V vs. NHE relative to the pair Mn(III)/Mn(IV) ( Supplementary Figure S11 ).…”

“…The ES is dominated by an intense band in the 235–300 nm UV range attributed to intra-ligand transitions and a lower intensity band at 355 nm probably due to ligand–metal transitions. In the Vis region, we observed a weak band at 498 nm and a shoulder at 606 nm, which could only be analyzed with the concentrated sample, attributed to d–d transitions compatible with the assumed configuration of pseudo-octahedral complexes [ 21 , 22 ].…”

Quinol-containing ligands enable high superoxide dismutase activity by modulating coordination number, charge, oxidation states and stability of manganese complexes throughout redox cycling

“…In the case of a polymeric mixed valence Mn( iii )/Mn( ii ) complex, Mn( iii )–O–Mn( iv ) was identified as an intermediate in a reaction with superoxide by EPR spectroscopy. 105 Our explicit observation of Mn( iv )-oxo intermediates generated from mononuclear Mn( ii ) SOD mimetics by CSI-MS, UV/vis and EPR spectra is unique. Using the SOD activity of the Zn( ii ) analogue of 2 ( 4 ) as an estimate for the ligand-only contribution to the reactivity, we conclude that the metal-based inner-sphere SOD mechanism provides the more efficient pathway for superoxide degradation in Mn-quinol/phenol-based systems.…”

Insights into Second-Sphere Effects on Redox Potentials, Spectroscopic Properties, and Superoxide Dismutase Activity of Manganese Complexes with Schiff-Base Ligands

“…Besides, the SOD activities of these Mn-Schiff base complexes are in the same range as for Mn-amine/pyridine complexes. 15,48…”

“…At higher complex concentration, the formation of a Mn(III)Mn(II) mixed valence complex is also possible through the nucleophilic attack of the Mn(III)-peroxo to the Mn(II) center of 6 . 48,63 The disappearance of the broad signal in the EPR spectra taken at long times suggests that the mixed valence dimer finally yields a homovalent one, probably a Mn(III) dimer.…”