Iron(III) Complexes as Superoxide Dismutase Mimics: Synthesis, Characterization, Crystal Structure, and Superoxide Dismutase (SOD) Activity of Iron(III) Complexes Containing Pentaaza Macrocyclic Ligands

Computational Analysis of the Superoxide Dismutase Mimicry Exhibited by a Zinc(II) Complex with a Redox-Active Organic Ligand

Previously, we found that a Zn(II) complex with the redox-active ligand N-(2,5-dihydroxybenzyl)-N,N',N'-tris(2-pyridinylmethyl)-1,2-ethanediamine (H2qp1) was able to act as a functional mimic of superoxide dismutase, despite its lack of a redox-active transition metal. As the complex catalyzes the dismutation of superoxide to form O2 and H2O2, the quinol in the ligand is believed to cycle between three oxidation states: quinol, quinoxyl radical, and para-quinone. Although the metal is not the redox partner, it nonetheless is essential to the reactivity since the free ligand by itself is inactive as a catalyst. In the present work, we primarily use calculations to probe the mechanism. The calculations support the inner-sphere decomposition of superoxide, suggest that the quinol/quinoxyl radical couple accounts for most of the catalysis, and elucidate the many roles that proton transfer between the zinc complexes and buffer has in the reactivity. Acid/base reactions involving the nonmetal-coordinating hydroxyl group on the quinol are predicted to be key to lowering the energy of the intermediates. We prepared a Zn(II) complex with N-(2-hydroxybenzyl)-N,N',N'-tris(2-pyridinylmethyl)-1,2-ethanediamine (Hpp1) that lacks this functional group and found that it could not catalyze the dismutation of superoxide; this confirms the importance of the second, distal hydroxyl group of the quinol.

Superoxide dismutase activity enabled by a redox-active ligand rather than metal

Reactive oxygen species are integral to many physiological processes. Although their roles are still being elucidated, they seem to be linked to a variety of disorders and may represent promising drug targets. Mimics of superoxide dismutases, which catalyse the decomposition of O₂^•- to H₂O₂ and O₂, have traditionally used redox-active metals, which are toxic outside of a tightly coordinating ligand. Purely organic antioxidants have also been investigated but generally require stoichiometric, rather than catalytic, doses. Here, we show that a complex of the redox-inactive metal zinc(II) with a hexadentate ligand containing a redox-active quinol can catalytically degrade superoxide, as demonstrated by both reactivity assays and stopped-flow kinetics studies of direct reactions with O₂^•- and the zinc(II) complex. The observed superoxide dismutase catalysis has an important advantage over previously reported work in that it is hastened, rather than impeded, by the presence of phosphate, the concentration of which is high under physiological conditions.

Enhancement of SOD activity in boehmite supported nanoreceptors

Binuclear Cu²⁺ complexes of a pyridinophane polyamine ligand grafted to boehmite nanoparticles display a remarkable SOD activity enhancement.

Six- and seven-coordinate Fe(ii) and Zn(ii) compounds ligated by unsymmetric xanthene-based ligands: characterization and magnetic properties

We present new ligand platforms for pentagonal bipyramidal metal complexes, and the characterization of the corresponding Zn(ii)- and Fe(ii)-containing compounds.

Mononuclear nonheme iron(iii) complexes that show superoxide dismutase-like activity and antioxidant effects against menadione-mediated oxidative stress

Iron(iii) complexes with pentadentate monocarboxylamido ligands display excellent SOD-like antioxidant activity to reduce superoxide radicals in cultured cells.

Role of Fe(IV)-oxo intermediates in stoichiometric and catalytic oxidations mediated by iron pyridine-azamacrocycles

An iron(II) complex with a pyridine-containing 14-membered macrocyclic (PyMAC) ligand L1 (L1 = 2,7,12-trimethyl-3,7,11,17-tetra-azabicyclo[11.3.1]heptadeca-1(17),13,15-triene), 1, was prepared and characterized. Complex 1 contains low-spin iron(II) in a pseudo-octahedral geometry as determined by X-ray crystallography. Magnetic susceptibility measurements (298 K, Evans method) and Mössbauer spectroscopy (90 K, δ = 0.50(2) mm/s, ΔE(Q) = 0.78(2) mm/s) confirmed that the low-spin configuration of Fe(II) is retained in liquid and frozen acetonitrile solutions. Cyclic voltammetry revealed a reversible one-electron oxidation/reduction of the iron center in 1, with E(1/2)(Fe(III)/Fe(II)) = 0.49 V vs Fc(+)/Fc, a value very similar to the half-wave potentials of related macrocyclic complexes. Complex 1 catalyzed the epoxidation of cyclooctene and other olefins with H(2)O(2). Low-temperature stopped-flow kinetic studies demonstrated the formation of an iron(IV)-oxo intermediate in the reaction of 1 with H(2)O(2) and concomitant partial ligand oxidation. A soluble iodine(V) oxidant, isopropyl 2-iodoxybenzoate, was found to be an excellent oxygen atom donor for generating Fe(IV)-oxo intermediates for additional spectroscopic (UV-vis in CH(3)CN: λ(max) = 705 nm, ε ≈ 240 M(-1) cm(-1); Mössbauer: δ = 0.03(2) mm/s, ΔE(Q) = 2.00(2) mm/s) and kinetic studies. The electrophilic character of the (L1)Fe(IV)═O intermediate was established in rapid (k(2) = 26.5 M(-1) s(-1) for oxidation of PPh(3) at 0 °C), associative (ΔH(‡) = 53 kJ/mol, ΔS(‡) = -25 J/K mol) oxidation of substituted triarylphosphines (electron-donating substituents increased the reaction rate, with a negative value of Hammet's parameter ρ = -1.05). Similar double-mixing kinetic experiments demonstrated somewhat slower (k(2) = 0.17 M(-1) s(-1) at 0 °C), clean, second-order oxidation of cyclooctene into epoxide with preformed (L1)Fe(IV)═O that could be generated from (L1)Fe(II) and H(2)O(2) or isopropyl 2-iodoxybenzoate. Independently determined rates of ferryl(IV) formation and its subsequent reaction with cyclooctene confirmed that the Fe(IV)-oxo species, (L1)Fe(IV)═O, is a kinetically competent intermediate for cyclooctene epoxidation with H(2)O(2) at room temperature. Partial ligand oxidation of (L1)Fe(IV)═O occurs over time in oxidative media, reducing the oxidizing ability of the ferryl species; the macrocyclic nature of the ligand is retained, resulting in ferryl(IV) complexes with Schiff base PyMACs. NH-groups of the PyMAC ligand assist the oxygen atom transfer from ferryl(IV) intermediates to olefin substrates.

Manganese(II) complexes of scorpiand-like azamacrocycles as MnSOD mimics

Mn(II) complexes of scorpiand-type azamacrocycles constituted by a tretrazapyridinophane core appended with an ethylamino tail including 2- or 4-quinoline functionalities show very appealing in vitro SOD activity. The observed behaviour is related to structural and electrochemical parameters.

Understanding the influence of the protein environment on the Mn(II) centers in Superoxide Dismutases using High-Field Electron Paramagnetic Resonance

One of the most puzzling questions of manganese and iron superoxide dismutases (SODs) is what is the basis for their metal-specificity. This review summarizes our findings on the Mn(II) electronic structure of SODs and related synthetic models using high-field high-frequency electron paramagnetic resonance (HFEPR), a technique that is able to achieve a very detailed and quantitative information about the electronic structure of the Mn(II) ions. We have used HFEPR to compare eight different SODs, including iron, manganese and cambialistic proteins. This comparative approach has shown that in spite of their high structural homology each of these groups have specific spectroscopic and biochemical characteristics. This has allowed us to develop a model about how protein and metal interactions influence protein pK, inhibitor binding and the electronic structure of the manganese center. To better appreciate the thermodynamic prerequisites required for metal discriminatory SOD activity and their relationship to HFEPR spectroscopy, we review the work on synthetic model systems that functionally mimic Mn-and FeSOD. Using a single ligand framework, it was possible to obtain metal-discriminatory "activity" as well as variations in the HFEPR spectra that parallel those found in the proteins. Our results give new insights into protein-metal interactions from the perspective of the Mn(II) and new steps towards solving the puzzle of metal-specificity in SODs.

Cyano-Bridged 2D CuII−CrIII Coordination Polymers: Structural Evidence for Formation of a Polymeric Macrocyclic Metallic Compound

Two unique cyano-bridged 2D coordination polymers have been synthesized and characterized structurally and magnetically. The complexes contain two polyaza Cu(II) units and one novel macromolecular Cu(II) moiety, which have been synthesized via one-pot metal template condensation reactions involving ethylenediamine (en) and formaldehyde. Self-assembly of the polyaza Cu(II) mixture with [Cr(CN)(6)](3)(-) gave rise to two layered complexes. One complex contains unprecedented covalently linked polymeric Cu(II) chains and cyano-bridged Cu(II)(-)Cr(III) coordination chains, which are interwoven to form a novel layer. The other complex shows intriguing encapsulation of [Cr(CN)(6)](3)(-) anions. Intermetallic ferromagnetic coupling is operative within the bridged 2D layer. The magnetic susceptibilities of both complexes were simulated using approximate models.

Manganese-based complexes of radical scavengers as neuroprotective agents

Manganese was incorporated in the structure of the selected antioxidants to mimic the superoxide dismutase (SOD) and to increase radical scavenging ability. Five manganese complexes (1-5) showed potent SOD activity in vitro with IC(50) of 1.18-1.84 microM and action against lipid peroxidation in vitro with IC(50) of 1.97-8.00 microM greater than their ligands and trolox. The manganese complexes were initially tested in vivo at 50 mg/kg for antagonistic activity on methamphetamine (MAP)-induced hypermotility resulting from dopamine release in the mice brain. Only manganese complexes of kojic acid (1) and 7-hydroxyflavone (3) exhibited the significant suppressions on MAP-induced hypermotility and did not significantly decrease the locomotor activity in normal condition. Manganese complex 3 also showed protective effects against learning and memory impairment in transient cerebral ischemic mice. These results supported the brain delivery and the role of manganese in SOD activity as well as in the modulation of brain neurotransmitters in the aberrant condition. Manganese complex 3 from 7-hydroxyflavone was the promising candidate for radical implicated neurodegenerative diseases.

A detailed mechanistic study of the substitution behavior of an unusual seven-coordinate iron(III) complex in aqueous solution

A detailed mechanistic study of the substitution behavior of a 3d metal heptacoordinate complex, with a rare pentagonal-bipyramidal structure, was undertaken to resolve the solution chemistry of this system. The kinetics of the complex-formation reaction of [Fe(dapsox)(H(2)O)(2)]ClO(4) (H(2)dapsox = 2,6-diacetylpyridine-bis(semioxamazide)) with thiocyanate was studied as a function of thiocyanate concentration, pH, temperature, and pressure. The reaction proceeds in two steps, which are both base-catalyzed due to the formation of an aqua-hydroxo complex (pK(a1) = 5.78 +/- 0.04 and pK(a2) = 9.45 +/- 0.06 at 25 degrees C). Thiocyanate ions displace the first coordinated water molecule in a fast step, followed by a slower reaction in which the second thiocyanate ion coordinates trans to the N-bonded thiocyanate. At 25 degrees C and pH <4.5, only the first reaction step can be observed, and the kinetic parameters (pH 2.5: k(f(I)) = 2.6 +/- 0.1 M(-1) s(-1), DeltaH(#)(f(I)) = 62 +/- 3 kJ mol(-1), DeltaS(#)(f(I)) = -30 +/- 10 J K(-1) mol(-1), and DeltaV(#)(f(I)) = -2.5 +/- 0.2 cm(3) mol(-1)) suggest the operation of an I(a) mechanism. In the pH range 2.5 to 5.2 this reaction step involves the participation of both the diaqua and aqua-hydroxo complexes, for which the complex-formation rate constants were found to be 2.19 +/- 0.06 and 1172 +/- 22 M(-1) s(-1) at 25 degrees C, respectively. The more labile aqua-hydroxo complex is suggested to follow an I(d) or D substitution mechanism on the basis of the reported kinetic data. At pH > or =4.5, the second substitution step also can be monitored (pH 5.5 and 25 degrees C: k(f(II)) = 21.1 +/- 0.5 M(-1) s(-1), DeltaH(#)(f(II)) = 60 +/- 2 kJ mol(-1), DeltaS(#)(f(II)) = -19 +/- 6 J K(-1) mol(-1), and DeltaV(#)(f(II)) = +8.8 +/- 0.3 cm(3) mol(-1)), for which an I(d) or D mechanism is suggested. The results are discussed in terms of known structural parameters and in comparison to relevant structural and kinetic data from the literature.

Peroxynitrite: reactive, invasive and enigmatic

The reactive oxygen and nitrogen species superoxide ion, nitric oxide, nitrogen dioxide and peroxynitrite ion all react with biological target molecules. Some of these interactions are carefully orchestrated segments of signal transduction cascades or part of the armamentarium of the immune system, others are pathological events and may lie at the root of many diseases. As a result of these small reactive molecules, proteins, particularly metalloproteins, can be altered with loss of function, DNA can be cleaved and lipid components can be oxidized to disrupt membranes. The interactions of these species with each other and their aftermath can be sensed by the cell, resulting in a variety of responses including gene regulation and transcription. Indeed, there is recent, tantalizing evidence that the currency of reactive oxygen and nitrogen species is central to the life and death cellular decisions in homeostasis or the initiation of apoptosis. New families of metallopharmaceuticals may serve both to probe the nature and mechanisms of these events and to effect the outcome.