New heme-dioxygen and carbon monoxide adducts using pyridyl or imidazolyl tailed porphyrins

The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.

Ferric Heme Superoxide Reductive Transformations to Ferric Heme (Hydro)Peroxide Species: Spectroscopic Characterization and Thermodynamic Implications for H-Atom Transfer (HAT)

A new end-on low-spin ferric heme peroxide, [(PIm )FeIII -(O22- )]- (PIm -P), and subsequently formed hydroperoxide species, [(PIm )FeIII -(OOH)] (PIm -HP) are generated utilizing the iron-porphyrinate PIm with its tethered axial base imidazolyl group. Measured thermodynamic parameters, the ferric heme superoxide [(PIm )FeIII -(O2⋅- )] (PIm -S) reduction potential (E°') and the PIm -HP pKa value, lead to the finding of the OO-H bond-dissociation free energy (BDFE) of PIm -HP as 69.5 kcal mol-1 using a thermodynamic square scheme and Bordwell relationship. The results are validated by the observed oxidizing ability of PIm -S via hydrogen-atom transfer (HAT) compared to that of the F8 superoxide complex, [(F8 )FeIII -(O2.- )] (S) (F8 =tetrakis(2,6-difluorophenyl)porphyrinate, without an internally appended axial base imidazolyl), as determined from reactivity comparison of superoxide complexes PIm -S and S with the hydroxylamine (O-H) substrates TEMPO-H and ABNO-H.

Heme-FeIII Superoxide, Peroxide and Hydroperoxide Thermodynamic Relationships: FeIII-O2•- Complex H-Atom Abstraction Reactivity

Establishing redox and thermodynamic relationships between metal-ion-bound O2 and its reduced (and protonated) derivatives is critically important for a full understanding of (bio)chemical processes involving dioxygen processing. Here, a ferric heme peroxide complex, [(F8)FeIII-(O22-)]- (P) (F8 = tetrakis(2,6-difluorophenyl)porphyrinate), and a superoxide complex, [(F8)FeIII-(O2•-)] (S), are shown to be redox interconvertible. Using Cr(η-C6H6)2, an equilibrium state where S and P are present is established in tetrahydrofuran (THF) at -80 °C, allowing determination of the reduction potential of S as -1.17 V vs Fc+/0. P could be protonated with 2,6-lutidinium triflate, yielding the low-spin ferric hydroperoxide species, [(F8)FeIII-(OOH)] (HP). Partial conversion of HP back to P using a derivatized phosphazene base gave a P/HP equilibrium mixture, leading to the determination of pKa = 28.8 for HP (THF, -80 °C). With the measured reduction potential and pKa, the O-H bond dissociation free energy (BDFE) of hydroperoxide species HP was calculated to be 73.5 kcal/mol, employing the thermodynamic square scheme and Bordwell relationship. This calculated O-H BDFE of HP, in fact, lines up with an experimental demonstration of the oxidizing ability of S via hydrogen atom transfer (HAT) from TEMPO-H (2,2,6,6-tetramethylpiperdine-N-hydroxide, BDFE = 66.5 kcal/mol in THF), forming the hydroperoxide species HP and TEMPO radical. Kinetic studies carried out with TEMPO-H(D) reveal second-order behavior, kH = 0.5, kD = 0.08 M-1 s-1 (THF, -80 °C); thus, the hydrogen/deuterium kinetic isotope effect (KIE) = 6, consistent with H-atom abstraction by S being the rate-determining step. This appears to be the first case where experimentally derived thermodynamics lead to a ferric heme hydroperoxide OO-H BDFE determination, that FeIII-OOH species being formed via HAT reactivity of the partner ferric heme superoxide complex.

Tuning the Geometric and Electronic Structure of Synthetic High-Valent Heme Iron(IV)-Oxo Models in the Presence of a Lewis Acid and Various Axial Ligands

High-valent ferryl species (e.g., (Por)FeIV═O, Cmpd-II) are observed or proposed key oxidizing intermediates in the catalytic cycles of heme-containing enzymes (P-450s, peroxidases, catalases, and cytochrome c oxidase) involved in biological respiration and oxidative metabolism. Herein, various axially ligated iron(IV)-oxo complexes were prepared to examine the influence of the identity of the base. These were generated by addition of various axial ligands (1,5-dicyclohexylimidazole (DCHIm), a tethered-imidazole system, and sodium derivatives of 3,5-dimethoxyphenolate and imidazolate). Characterization was carried out via UV-vis, electron paramagnetic resonance (EPR), 57Fe Mössbauer, Fe X-ray absorption (XAS), and 54/57Fe resonance Raman (rR) spectroscopies to confirm their formation and compare the axial ligand perturbation on the electronic and geometric structures of these heme iron(IV)-oxo species. Mössbauer studies confirmed that the axially ligated derivatives were iron(IV) and six-coordinate complexes. XAS and 54/57Fe rR data correlated with slight elongation of the iron-oxo bond with increasing donation from the axial ligands. The first reported synthetic H-bonded iron(IV)-oxo heme systems were made in the presence of the protic Lewis acid, 2,6-lutidinium triflate (LutH+), with (or without) DCHIm. Mössbauer, rR, and XAS spectroscopic data indicated the formation of molecular Lewis acid ferryl adducts (rather than full protonation). The reduction potentials of these novel Lewis acid adducts were bracketed through addition of outer-sphere reductants. The oxidizing capabilities of the ferryl species with or without Lewis acid vary drastically; addition of LutH+ to F8Cmpd-II (F8 = tetrakis(2,6-difluorophenyl)porphyrinate) increased its reduction potential by more than 890 mV, experimentally confirming that H-bonding interactions can increase the reactivity of ferryl species.

Synthetic Fe/Cu Complexes: Toward Understanding Heme-Copper Oxidase Structure and Function

Heme-copper oxidases (HCOs) are terminal enzymes on the mitochondrial or bacterial respiratory electron transport chain, which utilize a unique heterobinuclear active site to catalyze the 4H+/4e- reduction of dioxygen to water. This process involves a proton-coupled electron transfer (PCET) from a tyrosine (phenolic) residue and additional redox events coupled to transmembrane proton pumping and ATP synthesis. Given that HCOs are large, complex, membrane-bound enzymes, bioinspired synthetic model chemistry is a promising approach to better understand heme-Cu-mediated dioxygen reduction, including the details of proton and electron movements. This review encompasses important aspects of heme-O2 and copper-O2 (bio)chemistries as they relate to the design and interpretation of small molecule model systems and provides perspectives from fundamental coordination chemistry, which can be applied to the understanding of HCO activity. We focus on recent advancements from studies of heme-Cu models, evaluating experimental and computational results, which highlight important fundamental structure-function relationships. Finally, we provide an outlook for future potential contributions from synthetic inorganic chemistry and discuss their implications with relevance to biological O2-reduction.

A Six-Coordinate Peroxynitrite Low-Spin Iron(III) Porphyrinate Complex-The Product of the Reaction of Nitrogen Monoxide (·NO(g)) with a Ferric-Superoxide Species

Peroxynitrite (-OON═O, PN) is a reactive nitrogen species (RNS) which can effect deleterious nitrative or oxidative (bio)chemistry. It may derive from reaction of superoxide anion (O2•-) with nitric oxide (·NO) and has been suggested to form an as-yet unobserved bound heme-iron-PN intermediate in the catalytic cycle of nitric oxide dioxygenase (NOD) enzymes, which facilitate a ·NO homeostatic process, i.e., its oxidation to the nitrate anion. Here, a discrete six-coordinate low-spin porphyrinate-FeIII complex [(PIm)FeIII(-OON═O)] (3) (PIm; a porphyrin moiety with a covalently tethered imidazole axial "base" donor ligand) has been identified and characterized by various spectroscopies (UV-vis, NMR, EPR, XAS, resonance Raman) and DFT calculations, following its formation at -80 °C by addition of ·NO(g) to the heme-superoxo species, [(PIm)FeIII(O2•-)] (2). DFT calculations confirm that 3 is a six-coordinate low-spin species with the PN ligand coordinated to iron via its terminal peroxidic anionic O atom with the overall geometry being in a cis-configuration. Complex 3 thermally transforms to its isomeric low-spin nitrato form [(PIm)FeIII(NO3-)] (4a). While previous (bio)chemical studies show that phenolic substrates undergo nitration in the presence of PN or PN-metal complexes, in the present system, addition of 2,4-di-tert-butylphenol (2,4DTBP) to complex 3 does not lead to nitrated phenol; the nitrate complex 4a still forms. DFT calculations reveal that the phenolic H atom approaches the terminal PN O atom (farthest from the metal center and ring core), effecting O-O cleavage, giving nitrogen dioxide (·NO2) plus a ferryl compound [(PIm)FeIV═O] (7); this rebounds to give [(PIm)FeIII(NO3-)] (4a).The generation and characterization of the long sought after ferriheme peroxynitrite complex has been accomplished.

Isocyanide or nitrosyl complexation to hemes with varying tethered axial base ligand donors: synthesis and characterization

A series of ferrous-heme 2,6-dimethylphenyl isocyanide (DIMPI) and ferrous-heme mononitrosyl complexes have been synthesized and characterized. The heme portion of the complexes studied is varied with respect to the nature of the axial ligand, including complexes, where it is covalently tethered to the porphyrinate periphery. Reduced heme complexes, [(F8)Fe(II)], [(P(Py))Fe(II)], [(P(Im))Fe(II)], and [(P(ImH))Fe(II)], where F8 = tetrakis(2,6-difluorophenyl)-porphyrinate and P(Py), P(Im), and P(ImH) are partially fluorinated tetraaryl porphyrinates with covalently appended axial base pyridyl/imidazolyl or histamine moieties, were employed; P(ImH) is a new construct. Room temperature addition of DIMPI to these iron(II) complexes affords the bis-isocyanide species [(F8)Fe(II)-(DIMPI)2] in the case of [(F8)Fe(II)], while for the other hemes, mono-DIMPI compounds are obtained, [(P(Py))Fe(II)-(DIMPI)] [(2)-DIMPI], [(P(Im))Fe(II)-(DIMPI)] [(3)-DIMPI], and [(P(ImH))Fe(II)-(DIMPI)] [(4)-DIMPI]. The structures of complexes (3)-DIMPI and (4)-DIMPI have been determined by single crystal X-ray crystallography, where interesting H…F(porphryinate aryl group) interactions are observed. (19)F-NMR spectra determined for these complexes suggest that H…F(porphyrinate aryl groups) attractions also occur in solution, the H atom coming either from the DIMPI methyl groups or from a porphyinate axial base imidazole or porphyrinate pyrrole. Similarly, we have used nitrogen monoxide to generate ferrous-nitrosyl complexes, a five-coordinate species for F8, [(F8)Fe(II)-(NO)], or low-spin six-coordinate compounds [(P(Py))Fe(II)-(NO)], [(P(Im))Fe(II)-(NO)], and [(P(ImH))Fe(II)-(NO)]. The DIMPI and mononitrosyl complexes have also been characterized using UV-Vis, IR, (1)H-NMR, and EPR spectroscopies.

Reactions of a heme-superoxo complex toward a cuprous chelate and •NO(g): CcO and NOD chemistry

Following up on the characterization of a new (heme)FeIII-superoxide species formed from the cryogenic oxygenation of a ferrous-heme (PPy)FeII (1) (PPy = a tetraarylporphyrinate with a covalently tethered pyridine group as a potential axial base), giving (PPy)FeIII-O2•- (2) (Li Y et al., Polyhedron 2013; 58: 60-64), we report here on (i) its use in forming a cytochrome c oxidase (CcO) model compound, or (ii) in a reaction with nitrogen monoxide (•NO; nitric oxide) to mimic nitric oxide dioxygenase (NOD) chemistry. Reaction of (2) with the cuprous chelate [CuI(AN)][B(C6F5)4] (AN = bis[3-(dimethylamino) propyl]amine) gives a meta-stable product [(PPy)FeIII-([Formula: see text])-CuII(AN)][B(C6F5)4] (3a), possessing a high-spin iron(III) and Cu(II) side-on bridged peroxo moiety with a μ-η2:η2-binding motif. This complex thermally decays to a corresponding μ-oxo complex [(PPy)FeIII-(O2-)-CuII(AN)][B(C6F5)4] (3). Both (3) and (3a) have been characterized by UV-vis, 2H NMR and EPR spectroscopies. When (2) is exposed to •NO(g), a ferric heme nitrato compound forms; if 2,4-di-tert-butylphenol is added prior to •NO(g) exposure, phenol ortho-nitration occurs with the iron product being the ferric hydroxide complex (PPy) FeIII(OH) (5). The latter reactions mimic the action of NOD's.

Transition metal complexes and the activation of dioxygen

In order to address how diverse metalloprotein active sites, in particular those containing iron and copper, guide O₂binding and activation processes to perform diverse functions, studies of synthetic models of the active sites have been performed. These studies have led to deep, fundamental chemical insights into how O₂coordinates to mono- and multinuclear Fe and Cu centers and is reduced to superoxo, peroxo, hydroperoxo, and, after O-O bond scission, oxo species relevant to proposed intermediates in catalysis. Recent advances in understanding the various factors that influence the course of O₂activation by Fe and Cu complexes are surveyed, with an emphasis on evaluating the structure, bonding, and reactivity of intermediates involved. The discussion is guided by an overarching mechanistic paradigm, with differences in detail due to the involvement of disparate metal ions, nuclearities, geometries, and supporting ligands providing a rich tapestry of reaction pathways by which O₂is activated at Fe and Cu sites.