High-valent nonheme FeIV–oxido species are key intermediates in biological oxidation, and their properties are proposed to be influenced by the unique microenvironments present in protein active ...sites. Microenvironments are regulated by noncovalent interactions, such as hydrogen bonds (H-bonds) and electrostatic interactions; however, there is little quantitative information about how these interactions affect crucial properties of high valent metal–oxido complexes. To address this knowledge gap, we introduced a series of FeIV–oxido complexes that have the same S = 2 spin ground state as those found in nature and then systematically probed the effects of noncovalent interactions on their electronic, structural, and vibrational properties. The key design feature that provides access to these complexes is the new tripodal ligand poat3–, which contains phosphinic amido groups. An important structural aspect of FeIVpoat(O)− is the inclusion of an auxiliary site capable of binding a Lewis acid (LAII); we used this unique feature to further modulate the electrostatic environment around the Fe–oxido unit. Experimentally, studies confirmed that H-bonds and LAII s can interact directly with the oxido ligand in FeIV–oxido complexes, which weakens the FeO bond and has an impact on the electronic structure. We found that relatively large vibrational changes in the Fe–oxido unit correlate with small structural changes that could be difficult to measure, especially within a protein active site. Our work demonstrates the important role of noncovalent interactions on the properties of metal complexes, and that these interactions need to be considered when developing effective oxidants.
Electron paramagnetic resonance (EPR) spectroscopy has long been a primary method for characterization of paramagnetic centers in materials and biological complexes. Transition metals in biological ...complexes have valence d-orbitals that largely define the chemistry of the metal centers. EPR spectra are distinctive for metal type, oxidation state, protein environment, substrates, and inhibitors. The study of many metal centers in proteins, enzymes, and biomimetic complexes has led to the development of a systematic methodology for quantitative interpretation of EPR spectra from a wide array of metal containing complexes. The methodology is now contained in the computer program SpinCount. SpinCount allows simulation of EPR spectra from any sample containing multiple species composed of one or two metals in any spin state. The simulations are quantitative, thus allowing determination of all species concentrations in a sample directly from spectra. This chapter will focus on applications to transition metals in biological systems using EPR spectra from multiple microwave frequencies and modes.
The structural and electronic properties of a series of manganese complexes with terminal oxido ligands are described. The complexes span three different oxidation states at the manganese center ...(III–V), have similar molecular structures, and contain intramolecular hydrogen-bonding networks surrounding the Mn–oxo unit. Structural studies using X-ray absorption methods indicated that each complex is mononuclear and that oxidation occurs at the manganese centers, which is also supported by electron paramagnetic resonance (EPR) studies. This gives a high-spin Mn ⱽ–oxo complex and not a Mn ᴵⱽ–oxy radical as the most oxidized species. In addition, the EPR findings demonstrated that the Fermi contact term could experimentally substantiate the oxidation states at the manganese centers and the covalency in the metal–ligand bonding. Oxygen-17–labeled samples were used to determine spin density within the Mn–oxo unit, with the greatest delocalization occurring within the Mn ⱽ–oxo species (0.45 spins on the oxido ligand). The experimental results coupled with density functional theory studies show a large amount of covalency within the Mn–oxo bonds. Finally, these results are examined within the context of possible mechanisms associated with photosynthetic water oxidation; specifically, the possible identity of the proposed high valent Mn–oxo species that is postulated to form during turnover is discussed.
Significance Metal complexes with terminal oxido ligands are important in a wide variety of transformations, including a high valent manganese-oxido unit that is involved in the O–O bond-forming step in photosynthetic water oxidation. Theoretical proposals suggest that a Mn ᴵⱽ–oxyl radical species is present, yet such species have not been observed experimentally. Using a combination of experimental measurements and theoretical calculations, we show here that the bonding within the Mn–oxido unit is best described as highly covalent, with 0.45 spins on the oxido ligand. These findings offer a counter explanation for the putative high valent manganese species in photosynthesis as an energetically accessible, high-spin Mn ⱽ–oxido unit instead of a Mn ᴵⱽ–oxyl radical species.
The magnetism of metal nanoclusters (NCs) with discrete electronic structures differs from that of metallic-state nanoparticles. Among the gold NCs, magnetic ones are rare. While 7e Au25(SR)180 (SR = ...thiolate) was an earlier reported one to be magnetic, its spin properties are yet to be understood. Here, we report the silver-doping effect on the magnetic properties of a series of Au25–x Ag x (SR)180 with x ranging from 1 to maximum 9 through analysis by electron paramagnetic resonance (EPR) spectroscopy. This series of M25 (M = Au/Ag) NCs reveals a linear decrease in axial splitting with increasing Ag doping owing to smaller spin–orbit coupling for Ag in comparison to Au, and X-ray crystallography analysis reveals the sites for Ag doping, which provides a basis for understanding the spin properties. The influence of aromatic vs nonaromatic ligands on the EPR signals has also been compared. The electronic structures of M25 NCs have been computed. Overall, this work demonstrates an effective strategy to manipulate the spin properties of NCs by atomically controlled Ag doping.
The unique active site of flavo-diiron proteins (FDPs) consists of a nonheme diiron-carboxylate site proximal to a flavin mononucleotide (FMN) cofactor. FDPs serve as the terminal components for ...reductive scavenging of dioxygen or nitric oxide to combat oxidative or nitrosative stress in bacteria, archaea, and some protozoan parasites. Nitric oxide is reduced to nitrous oxide by the four-electron reduced (FMNH2–FeIIFeII) active site. In order to clarify the nitric oxide reductase mechanism, we undertook a multispectroscopic presteady-state investigation, including the first Mössbauer spectroscopic characterization of diiron redox intermediates in FDPs. A new transient intermediate was detected and determined to be an antiferromagnetically coupled diferrous-dinitrosyl (S = 0, {FeNO}72) species. This species has an exchange energy, J ≥ 40 cm–1 (J S 1 ° S 2), which is consistent with a hydroxo or oxo bridge between the two irons. The results show that the nitric oxide reductase reaction proceeds through successive formation of diferrous-mononitrosyl (S = 1/2, FeII{FeNO}7) and the S = 0 diferrous-dinitrosyl species. In the rate-determining process, the diferrous-dinitrosyl converts to diferric (FeIIIFeIII) and by inference N2O. The proximal FMNH2 then rapidly rereduces the diferric site to diferrous (FeIIFeII), which can undergo a second 2NO → N2O turnover. This pathway is consistent with previous results on the same deflavinated and flavinated FDP, which detected N2O as a product ( Hayashi Biochemistry 2010, 49, 7040 ). Our results do not support other proposed mechanisms, which proceed either via “super-reduction” of {FeNO}72 by FMNH2 or through FeII{FeNO}7 directly to a diferric-hyponitrite intermediate. The results indicate that an S = 0 {FeNO}7}2 complex is a proximal precursor to N–N bond formation and N–O bond cleavage to give N2O and that this conversion can occur without redox participation of the FMN cofactor.
Thiol dioxygenases are non-heme mononuclear iron enzymes that catalyze the O2-dependent oxidation of free thiols (-SH) to produce the corresponding sulfinic acid (-SO2 –). Regardless of the ...phylogenic domain, the active site for this enzyme class is typically comprised of two major features: (1) a mononuclear ferrous iron coordinated by three protein-derived histidines and (2) a conserved sequence of outer Fe-coordination-sphere amino acids (Ser-His-Tyr) spatially adjacent to the iron site (∼3 Å). Here, we utilize a promiscuous 3-mercaptopropionic acid dioxygenase cloned from Azotobacter vinelandii (Av MDO) to explore the function of the conserved S-H-Y motif. This enzyme exhibits activity with 3-mercaptopropionic acid ( 3mpa ), l-cysteine ( cys ), as well as several other thiol-bearing substrates, thus making it an ideal system to study the influence of residues within the highly conserved S-H-Y motif (H157 and Y159) on substrate specificity and reactivity. The pK a values for these residues were determined by pH-dependent steady-state kinetics, and their assignments verified by comparison to H157N and Y159F variants. Complementary electron paramagnetic resonance and Mössbauer studies demonstrate a network of hydrogen bonds connecting H157–Y159 and Fe-bound ligands within the enzymatic Fe site. Crucially, these experiments suggest that the hydroxyl group of Y159 hydrogen bonds to Fe-bound NO and, by extension, Fe-bound oxygen during native catalysis. This interaction alters both the NO binding affinity and rhombicity of the 3mpa -bound iron–nitrosyl site. In addition, Fe coordination of cys is switched from thiolate only to bidentate (thiolate/amine) for the Y159F variant, indicating that perturbations within the S-H-Y proton relay network also influence cys Fe binding denticity.
An Fe(V)(O) complex has been synthesized from equimolar solutions of (Et4N)2Fe(III)(Cl)(biuret-amide) and mCPBA in CH3CN at room temperature. The Fe(V)(O) complex has been characterized by UV-vis, ...EPR, Mössbauer, and HRMS and shown to be capable of oxidizing a series of alkanes having C-H bond dissociation energies ranging from 99.3 kcal mol(-1) (cyclohexane) to 84.5 kcal mol(-1) (cumene). Linearity in the Bell-Evans-Polayni graph and the finding of a large kinetic isotope effect suggest that hydrogen abstraction is engaged the rate-determining step.
The unique properties of entirely aliphatic TAML activator FeIII{(Me2CNCOCMe2 NCO)2CMe2}OH2− (3), namely the increased steric bulk of the ligand and the unmatched resistance to the acid-induced ...demetalation, enables the generation of high-valent iron derivatives in pure water at any pH. An iron(V)oxo species is readily produced with NaClO at pH values from 2 to 10.6 without any observable intermediate. This is the first reported example of iron(V)oxo formed in pure water. At pH 13, iron(V)oxo is not formed and NaClO oxidizes 3 to an iron(IV)oxo derivative.
Fe-TAML/peroxide catalysis provides simple, powerful, ultradilute approaches for removing micropollutants from water. The typically rate-determining interactions of H2O2 with Fe-TAMLs (rate constant ...k I) are sharply pH-sensitive with rate maxima in the pH 9–10 window. Fe-TAML design or process design that shifts the maximum rates to the pH 6–8 window of most wastewaters would make micropollutant eliminations even more powerful. Here, we show how the different pH dependencies of the interactions of Fe-TAMLs with peroxide or hypochlorite to form active Fe-TAMLs (k I step) illuminate why moving from H2O2 (pK a, ca. 11.6) to hypochlorite (pK a, 7.5) shifts the pH of the fastest catalysis to as low as 8.2. At pH 7, hypochlorite catalysis is 100–1000 times faster than H2O2 catalysis. The pH of maximum catalytic activity is also moderated by the pK a’s of the Fe-TAML axial water ligands, 8.8, 9.3, and 10.3, respectively, for Fe{4-NO2C6H3-1,2-(NCOCMe2NSO2)2CHMe}(H2O) n − (2) n = 1–2, Fe{4-NO2C6H3-1,2-(NCOCMe2NCO)2CF2}(H2O) n − (1b), and Fe{C6H4-1,2-(NCOCMe2NCO)2CMe2}(H2O) n − (1a). The new bis(sulfonamido)-bis(carbonamido)-ligated 2 exhibits the lowest pK a and delivers the largest hypochlorite over peroxide catalytic rate advantage. The fast Fe-TAML/hypochlorite catalysis is accompanied by slow noncatalytic oxidations of Orange II.
The first step of the kynurenine pathway for l-tryptophan (l-Trp) degradation is catalyzed by heme-dependent dioxygenases, tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase. In this ...work, we employed stopped-flow optical absorption spectroscopy to study the kinetic behavior of the Michaelis complex of Cupriavidus metallidurans TDO (cmTDO) to improve our understanding of oxygen activation and initial oxidation of l-Trp. On the basis of the stopped-flow results, rapid freeze-quench (RFQ) experiments were performed to capture and characterize this intermediate by Mössbauer spectroscopy. By incorporating the chlorite dismutase–chlorite system to produce high concentrations of solubilized O2, we were able to capture the Michaelis complex of cmTDO in a nearly quantitative yield. The RFQ–Mössbauer results confirmed the identity of the Michaelis complex as an O2-bound ferrous species. They revealed remarkable similarities between the electronic properties of the Michaelis complex and those of the O2 adduct of myoglobin. We also found that the decay of this reactive intermediate is the rate-limiting step of the catalytic reaction. An inverse α-secondary substrate kinetic isotope effect was observed with a k H/k D of 0.87 ± 0.03 when (indole-d 5)-l-Trp was employed as the substrate. This work provides an important piece of spectroscopic evidence of the chemical identity of the Michaelis complex of bacterial TDO.
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