Direct Hydrogen-Atom Abstraction by Activated Bleomycin: An Experimental and Computational Study

Therapeutic Senolysis of Axitinib-Induced Senescent Human Lung Cancer Cells

BACKGROUND

Tyrosine kinase inhibitors (TKIs) inhibit receptor-mediated signals in cells. Axitinib is a TKI with high specificity for vascular endothelial growth factor receptors (VEGFRs).

AIM

We determined whether axitinib could induce senescence in human cancer cells and be lysed by the senolytic drug ABT-263.

METHODS

Human lung and breast adenocarcinoma cell lines were used. These cells were cultured with axitinib or a multi-target TKI lenvatinib. The expression of β-galactosidase, VEGFRs, Ki-67, reactive oxygen species (ROS) of cancer cells, and their BrdU uptake were evaluated by flow cytometry. The mRNA expression of p21 and IL-8 was examined by quantitative PCR. The effects of TKIs on phosphorylation of Akt and Erk1/2, as downstream molecules of VEGFR signaling, were examined by immunoblot. The in vivo anti-cancer effect was examined using a xenograft mice model.

RESULTS

Axitinib, but not lenvatinib, induced cellular senescence (increased cell size and enhanced expression of β-galactosidase) in all adenocarcinoma cell lines. Axitinib-induced senescence was unrelated to the expression of VEGFRs on cancer cells. ROS were involved in axitinib-induced senescence. Axitinib-induced senescent lung adenocarcinoma A549 cells were drastically lysed by ABT-263. In A549-xenografted mice, combination therapy with axitinib and ABT-263 significantly suppressed tumor growth with the induction of apoptotic cancer cells.

Mechanistic Insights into Amphoteric Reactivity of an Iron-Bispidine Complex

The reactivity of FeIII -alkylperoxido complexes has remained a riddle to inorganic chemists owing to their thermal instability and impotency towards organic substrates. These iron-oxygen adducts have been known as sluggish oxidants towards oxidative electrophilic and nucleophilic reactions. Herein, we report the synthesis and spectroscopic characterization of a relatively stable mononuclear high-spin FeIII -alkylperoxido complex supported by an engineered bispidine framework. Against the notion, this FeIII -alkylperoxido complex serves as a rare example of versatile reactivity in both electrophilic and nucleophilic reactions. Detailed mechanistic studies and computational calculations reveal a novel reaction mechanism, where a putative superoxido intermediate orchestrates the amphoteric property of the oxidant. The design of the backbone is pivotal to convey stability and reactivity to alkylperoxido and superoxido intermediates. Contrary to the well-known O-O bond cleavage that generates an FeIV -oxido species, the FeIII -alkylperoxido complex reported here undergoes O-C bond scission to generate a superoxido moiety that is responsible for the amphiphilic reactivity.

Systematic Electronic Tuning on the Property and Reactivity of Cobalt-(Hydro)peroxo Intermediates

A series of cobalt(III)-peroxo complexes, [Co^III(R₂-TBDAP)(O₂)]⁺ (1^R2; R₂ = Cl, H, and OMe), and cobalt(III)-hydroperoxo complexes, [Co^III(R₂-TBDAP)(O₂H)(CH₃CN)]²⁺ (2^R2), bearing electronically tuned tetraazamacrocyclic ligands (R₂-TBDAP = N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)-p-R₂-pyridinophane) were prepared from their cobalt(II) precursors and characterized by various physicochemical methods. The X-ray diffraction and spectroscopic analyses unambiguously showed that all 1^R2 compounds have similar octahedral geometry with a side-on peroxocobalt(III) moiety, but the O-O bond lengths of 1^Cl [1.398(3) Å] and 1^OMe [1.401(4) Å] were shorter than that of 1^H [1.456(3) Å] due to the different spin states. For 2^R2, the O-O bond vibration energies of 2^Cl and 2^OMe were identical at 853 cm^-1 (856 cm^-1 for 2^H), but their Co-O bond vibration frequencies were observed at 572 cm^-1 for 2^Cl and 550 cm^-1 for 2^OMe, respectively, by resonance Raman spectroscopy (560 cm^-1 for 2^H). Interestingly, the redox potentials (E_1/2) of 2^R2 increased in the order of 2^OMe (0.19 V) < 2^H (0.24 V) < 2^Cl (0.34 V) according to the electron richness of the R₂-TBDAP ligands, but the oxygen-atom-transfer reactivities of 2^R2 showed a reverse trend (k₂: 2^Cl < 2^H < 2^OMe) with a 13-fold rate enhancement at 2^OMe over 2^Cl in a sulfoxidation reaction with thioanisole. Although the reactivity trend contradicts the general consideration that electron-rich metal-oxygen species with low E_1/2 values have sluggish electrophilic reactivity, this could be explained by a weak Co-O bond vibration of 2^OMe in the unusual reaction pathway. These results provide considerable insight into the electronic nature-reactivity relationship of metal-oxygen species.

BesC Initiates C-C Cleavage through a Substrate-Triggered and Reactive Diferric-Peroxo Intermediate

BesC catalyzes the iron- and O₂-dependent cleavage of 4-chloro-l-lysine to form 4-chloro-l-allylglycine, formaldehyde, and ammonia. This process is a critical step for a biosynthetic pathway that generates a terminal alkyne amino acid which can be leveraged as a useful bio-orthogonal handle for protein labeling. As a member of an emerging family of diiron enzymes that are typified by their heme oxygenase-like fold and a very similar set of coordinating ligands, recently termed HDOs, BesC performs an unusual type of carbon-carbon cleavage reaction that is a significant departure from reactions catalyzed by canonical dinuclear-iron enzymes. Here, we show that BesC activates O₂ in a substrate-gated manner to generate a diferric-peroxo intermediate. Examination of the reactivity of the peroxo intermediate with a series of lysine derivatives demonstrates that BesC initiates this unique reaction trajectory via cleavage of the C4-H bond; this process represents the rate-limiting step in a single turnover reaction. The observed reactivity of BesC represents the first example of a dinuclear-iron enzyme that utilizes a diferric-peroxo intermediate to capably cleave a C-H bond as part of its native function, thus circumventing the formation of a high-valent intermediate more commonly associated with substrate monooxygenations.

DFT study of α-Keggin, lacunary Keggin, and ironII-VI substituted Keggin polyoxometalates: the effect of oxidation state and axial ligand on geometry, electronic structures and oxygen transfer

Herein, the geometry, electronic structure, Fe-ligand bonding nature and simulated IR spectrum of α-Keggin, lacunary Keggin, iron(ii/iii)-substituted and the important oxidized high-valent iron derivatives of Keggin type polyoxometalates have been studied using the density functional theory (DFT/OPTX-PBE) method and natural bond orbital (NBO) analysis. The effects of different Fe oxidation states (ii-vi) and H2O/OH-/O2- ligand interactions have been addressed concerning their geometry and electronic structures. It has been revealed that the d-atomic orbitals of Fe and 2p orbitals of polyoxometalate's oxygen-atoms contribute in ligand binding. Compared with other high valent species, the considered polyoxometalate system of [PW11O39(FeVO)]4-, possesses a high reactivity for oxygen transfer.

Kinetic analysis of amino acid radicals formed in H2O2-driven CuI LPMO reoxidation implicates dominant homolytic reactivity

Lytic polysaccharide monooxygenases (LPMOs) have been proposed to react with both [Formula: see text] and [Formula: see text] as cosubstrates. In this study, the [Formula: see text] reaction with reduced Hypocrea jecorina LPMO9A (Cu^I-HjLPMO9A) is demonstrated to be 1,000-fold faster than the [Formula: see text] reaction while producing the same oxidized oligosaccharide products. Analysis of the reactivity in the absence of polysaccharide substrate by stopped-flow absorption and rapid freeze-quench (RFQ) electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) yields two intermediates corresponding to neutral tyrosyl and tryptophanyl radicals that are formed along minor reaction pathways. The dominant reaction pathway is characterized by RFQ EPR and kinetic modeling to directly produce Cu^II-HjLPMO9A and indicates homolytic O-O cleavage. Both optical intermediates exhibit magnetic exchange coupling with the Cu^II sites reflecting facile electron transfer (ET) pathways, which may be protective against uncoupled turnover or provide an ET pathway to the active site with substrate bound. The reactivities of nonnative organic peroxide cosubstrates effectively exclude the possibility of a ping-pong mechanism.

Oxidative DNA Cleavage, Formation of μ-1,1-Hydroperoxo Species, and Cytotoxicity of Dicopper(II) Complex Supported by a p-Cresol-Derived Amide-Tether Ligand

Metal complexes to promote oxidative DNA cleavage by H₂O₂ are desirable as anticancer drugs. A dicopper(II) complex of known p-cresol-derived methylene-tether ligand Hbcc [Cu₂(bcc)]³⁺ did not promote DNA cleavage by H₂O₂. Here, we synthesized a new p-cresol-derived amide-tether one, 2,6-bis(1,4,7,10-tetrazacyclododecyl-1-carboxyamide)-p-cresol (Hbcamide). A dicopper(II) complex of the new ligand [Cu₂(μ-OH)(bcamide)]²⁺ was structurally characterized. This complex promoted the oxidative cleavage of supercoiled plasmid pUC19 DNA (Form I) with H₂O₂ at pH 6.0-8.2 to give Forms II and III. The reaction was largely accelerated in a high pH region. A μ-1,1-hydroperoxo species was formed as the active species and spectroscopically identified. The amide-tether complex is more effective in cytotoxicity against HeLa cells than the methylene-tether one.

Geometric and Electronic Structural Contributions to Fe/O2 Reactivity

While two classes of non-heme iron enzymes use ferric centers to activate singlet organic substrates for the spin forbidden reaction with ³O₂, most classes use high spin ferrous sites to activate dioxygen. These Fe^II active sites do not exhibit intense absorption bands and have an integer spin ground state thus are mostly EPR inactive. We have developed new spectroscopic methodologies that provide geometric and electronic structural insight into the ferrous centers and their interactions with cosubstrates for dioxygen activation and into the nature of the intermediates generated in these reactions. First, we present our variable-temperature variable-field magnetic circular dichroism (VTVH MCD) methodology to experimentally define the geometric and electronic structure of the high spin ferrous active site. Then, we focus on using Nuclear Resonance Vibrational Spectroscopy (NRVS, performed at SPring-8) to define geometric structure and VTVH MCD to define the electronic structure of the Fe^III-OOH and Fe^IV=O intermediates generated in O₂ activation and the spin state dependence of their frontier molecular orbitals (FMOs) in controlling reactivity. Experimentally validated reaction coordinates are derived for the anticancer drug bleomycin in its cleavage of DNA and for an alpha- ketoglutarate dependent dioxygenase in its selective halogenation over the thermodynamically favored hydroxylation of substrate.

H2O2 Oxidation by FeIII-OOH Intermediates and Its Effect on Catalytic Efficiency

The oxidation of the C–H and C=C bonds of hydrocarbons with H₂O₂ catalyzed by non-heme iron complexes with pentadentate ligands is widely accepted as involving a reactive Fe^IV=O species such as [(N4Py)Fe^IV=O]²⁺ formed by homolytic cleavage of the O–O bond of an Fe^III–OOH intermediate (where N4Py is 1,1-bis(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine). We show here that at low H₂O₂ concentrations the Fe^IV=O species formed is detectable in methanol. Furthermore, we show that the decomposition of H₂O₂ to water and O₂ is an important competing pathway that limits efficiency in the terminal oxidant and indeed dominates reactivity except where only sub-/near-stoichiometric amounts of H₂O₂ are present. Although independently prepared [(N4Py)Fe^IV=O]²⁺ oxidizes stoichiometric H₂O₂ rapidly, the rate of formation of Fe^IV=O from the Fe^III–OOH intermediate is too low to account for the rate of H₂O₂ decomposition observed under catalytic conditions. Indeed, with excess H₂O₂, disproportionation to O₂ and H₂O is due to reaction with the Fe^III–OOH intermediate and thereby prevents formation of the Fe^IV=O species. These data rationalize that the activity of these catalysts with respect to hydrocarbon/alkene oxidation is maximized by maintaining sub-/near-stoichiometric steady-state concentrations of H₂O₂, which ensure that the rate of the H₂O₂ oxidation by the Fe^III–OOH intermediate is less than the rate of the O–O bond homolysis and the subsequent reaction of the Fe^IV=O species with a substrate.

Nanosecond-Scale Isomerization of the 4'-Carboxonium Cation Oxidatively Produced in Pyrimidine Units of DNA

The long-standing puzzle of the chemistry producing the Stubbe-Kozarich abasic site, which is the minor product in the oxidation of 2'-deoxycytidine units of DNA by Fe(II)-bleomycin, has been computationally solved in this study. Scrupulous DFT-based calculations that included extensive screening of the potential energy surface of model-solvated nucleotides and the elucidation of the chemical structure of the located nucleotide cations via natural bond orbital analysis demonstrated that the 2'-deoxycytidine unit bearing the 2'-deoxyribose ring 2e-oxidized at the 4'-position undergoes carboxonium ion- iminium ion (C═O⁺-C → C═N⁺) isomerization. This 1,2-elimination of the carbonyl group 4'-C═O from the carboxonium cation fragment is associated with minimal spatial reorganization of the molecule and appears to be an ultrafast reaction. The calculated barrier Δ G₀^# of 2.7 kcal mol^-1 for this isomerization is lower than that reported for the addition of water to oxocarbenium ions. Thus, this unusual nucleotide transformation is the key chemical reaction that yields the Stubbe-Kozarich product. Such a product cannot be formed for purine nucleotide units in DNA. The isomerization of 4'-dehydro-2'-deoxyribose-4'-carboxonium cations formed in these DNA units is slower because it destroys the purine aromaticity, and the cations are intercepted by water molecules before they isomerize.

Acid-Triggered O-O Bond Heterolysis of a Nonheme FeIII (OOH) Species for the Stereospecific Hydroxylation of Strong C-H Bonds

A novel hydroperoxoiron(III) species [Fe^III (OOH)(MeCN)(PyNMe₃ )]²⁺ (3) has been generated by reaction of its ferrous precursor [Fe^II (CF₃ SO₃ )₂ (PyNMe₃ )] (1) with hydrogen peroxide at low temperatures. This species has been characterized by several spectroscopic techniques and cryospray mass spectrometry. Similar to most of the previously described low-spin hydroperoxoiron(III) compounds, 3 behaves as a sluggish oxidant and it is not kinetically competent for breaking weak C-H bonds. However, triflic acid addition to 3 causes its transformation into a much more reactive compound towards organic substrates that is capable of oxidizing unactivated C-H bonds with high stereospecificity. Stopped-flow kinetic analyses and theoretical studies provide a rationale for the observed chemistry, a triflic-acid-assisted heterolytic cleavage of the O-O bond to form a putative strongly oxidizing oxoiron(V) species. This mechanism is reminiscent to that observed in heme systems, where protonation of the hydroperoxo intermediate leads to the formation of the high-valent [(Porph^. )Fe^IV (O)] (Compound I).

Animal Models of Pulmonary Fibrosis

Pulmonary fibrosis is a debilitating disease and is often fatal. It may be the consequence of direct lung injury or the result of genetic defects and occupational, environmental, or drug-related exposures. In many cases the etiology is unknown. The pathogenesis of all forms of pulmonary fibrosis regardless of type of injury or etiology is incompletely understood. These disorders are characterized by the accumulation of extracellular matrix in the lung interstitium with a loss of lung compliance and impaired gas exchange that ultimately leads to respiratory failure. Animal models of pulmonary fibrosis have become indispensable in the improved understanding of these disorders. Multiple models have been developed each with advantages and disadvantages. In this chapter we discuss the application of two of the most commonly employed direct lung instillation models, namely, the induction of pulmonary fibrosis with bleomycin or fluorescein isothiocyanate (FITC). We provide details on design, materials, and methods and describe how these models can be best undertaken. We also discuss methods to induce fibrosis in aged mice using murine gamma-herpesvirus (γHV-68) and approaches to exacerbate bleomycin- or FITC-induced fibrosis using γHV-68.

Exchangeable proton ENDOR as a probe of the redox-active iron center in activated bleomycin and ferric bleomycin

Activated bleomycin (ABLM) is a drug-Fe(iii)-hydroperoxide complex kinetically competent in DNA attack (via H4' abstraction). This intermediate is relatively stable, but its spontaneous conversion to ferric bleomycin (Fe(iii)·BLM) is poorly characterized because no observable intermediate product accumulates. The Fe(iii)·BLM formed cryophotolytically from ABLM and kept at 77 K was remarkably similar by EPR and ENDOR criteria to Fe(iii)·BLM formed from Fe(iii) + BLM solution. The notable ENDOR criteria were the ENDOR frequencies and features of orientation-selected, strongly hyperfine-coupled, exchangeable protons associated with the environs of the iron within <3.5 Å of paramagnetic Fe(iii) in Fe(iii)·BLM and ABLM. Cryophotolytic conversion of activated bleomycin to its ferric bleomycin product in the frozen solid is a sign that the reaction requires only constrained local proton rearrangements. We have characterized the metal-proton distances and orientations of the protons in that rearrangement, especially noting that these protons are of mechanistic importance in the ambient temperature conversion of ABLM to Fe(iii)·BLM in concert with a directed radical-forming attack on DNA.

Modeling DNA damage-induced pneumopathy in mice: insight from danger signaling cascades

Radiation-induced pneumonitis and fibrosis represent severe and dose-limiting side effects in the radiotherapy of thorax-associated neoplasms leading to decreased quality of life or - as a consequence of treatment with suboptimal radiation doses - to fatal outcomes by local recurrence or metastatic disease. It is assumed that the initial radiation-induced damage to the resident cells triggers a multifaceted damage-signalling cascade in irradiated normal tissues including a multifactorial secretory program. The resulting pro-inflammatory and pro-angiogenic microenvironment triggers a cascade of events that can lead within weeks to a pronounced lung inflammation (pneumonitis) or after months to excessive deposition of extracellular matrix molecules and tissue scarring (pulmonary fibrosis).The use of preclinical in vivo models of DNA damage-induced pneumopathy in genetically modified mice has helped to substantially advance our understanding of molecular mechanisms and signalling molecules that participate in the pathogenesis of radiation-induced adverse late effects in the lung. Herein, murine models of whole thorax irradiation or hemithorax irradiation nicely reproduce the pathogenesis of the human disease with respect to the time course and the clinical symptoms. Alternatively, treatment with the radiomimetic DNA damaging chemotherapeutic drug Bleomycin (BLM) has frequently been used as a surrogate model of radiation-induced lung disease. The advantage of the BLM model is that the symptoms of pneumonitis and fibrosis develop within 1 month.Here we summarize and discuss published data about the role of danger signalling in the response of the lung tissue to DNA damage and its cross-talk with the innate and adaptive immune systems obtained in preclinical studies using immune-deficient inbred mouse strains and genetically modified mice. Interestingly we observed differences in the role of molecules involved in damage sensing (TOLL-like receptors), damage signalling (MyD88) and immune regulation (cytokines, CD73, lymphocytes) for the pathogenesis and progression of DNA damage-induced pneumopathy between the models of pneumopathy induced by whole thorax irradiation or treatment with the radiomimetic drug BLM. These findings underline the importance to pursue studies in the radiation model(s) if we are to unravel the mechanisms driving radiation-induced adverse late effects.A better understanding of the cross-talk of danger perception and signalling with immune activation and repair mechanisms may allow a modulation of these processes to prevent or treat radiation-induced adverse effects. Vice-versa an improved knowledge of the normal tissue response to injury is also particularly important in view of the increasing interest in combining radiotherapy with immune checkpoint blockade or immunotherapies to avoid exacerbation of radiation-induced normal tissue toxicity.

O2 Activation by Non-Heme Iron Enzymes

The non-heme Fe enzymes are ubiquitous in nature and perform a wide range of functions involving O₂ activation. These had been difficult to study relative to heme enzymes; however, spectroscopic methods that provide significant insight into the correlation of structure with function have now been developed. This Current Topics article summarizes both the molecular mechanism these enzymes use to control O₂ activation in the presence of cosubstrates and the oxygen intermediates these reactions generate. Three types of O₂ activation are observed. First, non-heme reactivity is shown to be different from heme chemistry where a low-spin Fe^III-OOH non-heme intermediate directly reacts with substrate. Also, two subclasses of non-heme Fe enzymes generate high-spin Fe^IV═O intermediates that provide both σ and π frontier molecular orbitals that can control selectivity. Finally, for several subclasses of non-heme Fe enzymes, binding of the substrate to the Fe^II site leads to the one-electron reductive activation of O₂ to an Fe^III-superoxide capable of H atom abstraction and electrophilic attack.

Differences and comparisons of the properties and reactivities of iron(III)-hydroperoxo complexes with saturated coordination sphere

Heme and nonheme monoxygenases and dioxygenases catalyze important oxygen atom transfer reactions to substrates in the body. It is now well established that the cytochrome P450 enzymes react through the formation of a high-valent iron(IV)-oxo heme cation radical. Its precursor in the catalytic cycle, the iron(III)-hydroperoxo complex, was tested for catalytic activity and found to be a sluggish oxidant of hydroxylation, epoxidation and sulfoxidation reactions. In a recent twist of events, evidence has emerged of several nonheme iron(III)-hydroperoxo complexes that appear to react with substrates via oxygen atom transfer processes. Although it was not clear from these studies whether the iron(III)-hydroperoxo reacted directly with substrates or that an initial O-O bond cleavage preceded the reaction. Clearly, the catalytic activity of heme and nonheme iron(III)-hydroperoxo complexes is substantially different, but the origins of this are still poorly understood and warrant a detailed analysis. In this work, an extensive computational analysis of aromatic hydroxylation by biomimetic nonheme and heme iron systems is presented, starting from an iron(III)-hydroperoxo complex with pentadentate ligand system (L5(2)). Direct C-O bond formation by an iron(III)-hydroperoxo complex is investigated, as well as the initial heterolytic and homolytic bond cleavage of the hydroperoxo group. The calculations show that [(L5(2))Fe(III)(OOH)](2+) should be able to initiate an aromatic hydroxylation process, although a low-energy homolytic cleavage pathway is only slightly higher in energy. A detailed valence bond and thermochemical analysis rationalizes the differences in chemical reactivity of heme and nonheme iron(III)-hydroperoxo and show that the main reason for this particular nonheme complex to be reactive comes from the fact that they homolytically split the O-O bond, whereas a heterolytic O-O bond breaking in heme iron(III)-hydroperoxo is found.

Electrochemical study of a nonheme Fe(ii) complex in the presence of dioxygen. Insights into the reductive activation of O2 at Fe(ii) centers

Recent efforts to model the reactivity of iron oxygenases have led to the generation of nonheme FeIII(OOH) and FeIV(O) intermediates from FeII complexes and O2 but using different cofactors. This diversity emphasizes the rich chemistry of nonheme Fe(ii) complexes with dioxygen. We report an original mechanistic study of the reaction of [(TPEN)FeII]2+ with O2 carried out by cyclic voltammetry. From this FeII precursor, reaction intermediates such as [(TPEN)FeIV(O)]2+, [(TPEN)FeIII(OOH)]2+ and [(TPEN)FeIII(OO)]+ have been chemically generated in high yield, and characterized electrochemically. These electrochemical data have been used to analyse and perform simulation of the cyclic voltammograms of [(TPEN)FeII]2+ in the presence of O2. Thus, several important mechanistic informations on this reaction have been obtained. An unfavourable chemical equilibrium between O2 and the FeII complex occurs that leads to the FeIII-peroxo complex upon reduction, similarly to heme enzymes such as P450. However, unlike in heme systems, further reduction of this latter intermediate does not result in O-O bond cleavage.

Synthesis, DNA binding profile and DNA cleavage pathway of divalent metal complexes

Divalent metal complexes of dipyridylamine ligand with an anthracene moiety induced considerable oxidative DNA cleavage in the presence hydrogen peroxide and dioxygen.

The pathogenesis of bleomycin-induced lung injury in animals and its applicability to human idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a devastating disease of unknown etiology, for which there is no curative pharmacological therapy. Bleomycin, an anti-neoplastic agent that causes lung fibrosis in human patients has been used extensively in rodent models to mimic IPF. In this review, we compare the pathogenesis and histological features of human IPF and bleomycin-induced pulmonary fibrosis (BPF) induced in rodents by intratracheal delivery. We discuss the current understanding of IPF and BPF disease development, from the contribution of alveolar epithelial cells and inflammation to the role of fibroblasts and cytokines, and draw conclusions about what we have learned from the intratracheal bleomycin model of lung fibrosis.

Hydrogen bonding tunes the early stage of hydrogen-atom abstracting reaction

The spontaneous and collision-assisted hydrogen-atom abstracting reaction (HA) dynamics of triplet benzil are investigated through the combination of transient absorption spectroscopy with TD-DFT calculations. HA dynamics exhibit a remarkable dependence on the hydrogen donor properties. The effects of the triplet-state hydrogen bonding on the reaction dynamics are illustrated. In particular, it is experimentally observed that strengthened triplet-state hydrogen bonding could accelerate the HA, whereas weakened triplet-state hydrogen bonding would postpone the HA. The triplet-state hydrogen bonding has great influences on the early stage of the HA reaction, while the bond dissociation energy of the hydrogen donors determines the subsequent reaction pathways. Protic solvents could sustain longer lifetimes of the excited-state intermediate formed after HA than non-protic solvents by 10 μs. This investigation provides insights into the HA dynamics and guidance to improve the product efficiency of photochemical reactions.

Efficient nuclear DNA cleavage in human cancer cells by synthetic bleomycin mimics

Iron complexes of N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)-methylamine (N4Py) have proven to be excellent synthetic mimics of the Bleomycins (BLMs), which are a family of natural antibiotics used clinically in the treatment of certain cancers. However, most investigations of DNA cleavage activity of these and related metal complexes were carried out in cell-free systems using plasmid DNA as substrate. The present study evaluated nuclear DNA cleavage activity and cell cytotoxicity of BLM and its synthetic mimics based on the ligand N4Py. The N4Py-based reagents induced nuclear DNA cleavage in living cells as efficiently as BLM and Fe(II)-BLM. Treatment of 2 cancer cell lines and 1 noncancerous cell line indicated improved cytotoxicity of N4Py when compared to BLM. Moreover, some level of selectivity was observed for N4Py on cancerous versus noncancerous cells. It was demonstrated that N4Py-based reagents and BLM induce cell death via different mechanistic pathways. BLM was shown to induce cell cycle arrest, ultimately resulting in mitotic catastrophe. In contrast, N4Py-based reagents were shown to induce apoptosis effectively. To the best of our knowledge, the present study is the first demonstration of efficient nuclear DNA cleavage activity of a synthetic BLM mimic within cells. The results presented here show that it is possible to design synthetic bioinorganic model complexes that are at least as active as the parent natural product and thereby are potentially interesting alternatives for BLM to induce antitumor activity.

Redox Properties of Iron Complexes with Pentadentate Bispidine Ligands

The solution coordination chemistry of iron complexes with the pentadentate bispidine ligands L1, L2, and L3 (dimethyl 9-oxo-2,4-di(pyridin-2-yl)-3,7-diazabicyclo[3.3.1]nonane-1,5-dicarboxylate derivatives) was examined. While in acetonitrile, (L1,2)FeII/III species have a preference for Cl– as co-ligand. The corresponding aqua and hydroxido complexes also prevail in the presence of Cl– in aqueous solution. The observed FeII/III potentials in water (cyclic voltammetry) and potentials of (L1–3)FeIV=O (buffered and unbuffered aqueous solutions) are strikingly similar, i.e. the latter are assigned to (L1–3)FeII/III potentials, and published potentials of FeIV=O complexes with other ligands with uncharged amine-pyridine donors, obtained by cyclic voltammetry, have to be considered with caution.

DNA cleavage by oxymyoglobin and cysteine-introduced metmyoglobin

DNA was cleaved oxidatively by oxygenated myoglobin, whereas Lys96Cys metmyoglobin functioned as an artificial nuclease under air by formation of an oxygenated species.

Geometric and electronic structure contributions to function in non-heme iron enzymes

Mononuclear non-heme Fe (NHFe) enzymes play key roles in DNA repair, the biosynthesis of antibiotics, the response to hypoxia, cancer therapy, and many other biological processes. These enzymes catalyze a diverse range of oxidation reactions, including hydroxylation, halogenation, ring closure, desaturation, and electrophilic aromatic substitution (EAS). Most of these enzymes use an Fe(II) site to activate dioxygen, but traditional spectroscopic methods have not allowed researchers to insightfully probe these ferrous active sites. We have developed a methodology that provides detailed geometric and electronic structure insights into these NHFe(II) active sites. Using these data, we have defined a general mechanistic strategy that many of these enzymes use: they control O2 activation (and limit autoxidation and self-hydroxylation) by allowing Fe(II) coordination unsaturation only in the presence of cosubstrates. Depending on the type of enzyme, O2 activation either involves a 2e(-) reduced Fe(III)-OOH intermediate or a 4e(-) reduced Fe(IV)═O intermediate. Nuclear resonance vibrational spectroscopy (NRVS) has provided the geometric structure of these intermediates, and magnetic circular dichroism (MCD) has defined the frontier molecular orbitals (FMOs), the electronic structure that controls reactivity. This Account emphasizes that experimental spectroscopy is critical in evaluating the results of electronic structure calculations. Therefore these data are a key mechanistic bridge between structure and reactivity. For the Fe(III)-OOH intermediates, the anticancer drug activated bleomycin (BLM) acts as the non-heme Fe analog of compound 0 in heme (e.g., P450) chemistry. However BLM shows different reactivity: the low-spin (LS) Fe(III)-OOH can directly abstract a H atom from DNA. The LS and high-spin (HS) Fe(III)-OOHs have fundamentally different transition states. The LS transition state goes through a hydroxyl radical, but the HS transition state is activated for EAS without O-O cleavage. This activation is important in one class of NHFe enzymes that utilizes a HS Fe(III)-OOH intermediate in dioxygenation. For Fe(IV)═O intermediates, the LS form has a π-type FMO activated for attack perpendicular to the Fe-O bond. However, the HS form (present in the NHFe enzymes) has a π FMO activated perpendicular to the Fe-O bond and a σ FMO positioned along the Fe-O bond. For the NHFe enzymes, the presence of π and σ FMOs enables enzymatic control in determining the type of reactivity: EAS or H-atom extraction for one substrate with different enzymes and halogenation or hydroxylation for one enzyme with different substrates.

Comparison of high-spin and low-spin nonheme Fe(III)-OOH complexes in O-O bond homolysis and H-atom abstraction reactivities

The geometric and electronic structures and reactivity of an S = 5/2 (HS) mononuclear nonheme (TMC)Fe(III)-OOH complex are studied by spectroscopies, calculations, and kinetics and compared with the results of previous studies of S = 1/2 (LS) Fe(III)-OOH complexes to understand parallels and differences in mechanisms of O-O bond homolysis and electrophilic H-atom abstraction reactions. The homolysis reaction of the HS [(TMC)Fe(III)-OOH](2+) complex is found to involve axial ligand coordination and a crossing to the LS surface for O-O bond homolysis. Both HS and LS Fe(III)-OOH complexes are found to perform direct H-atom abstraction reactions but with very different reaction coordinates. For the LS Fe(III)-OOH, the transition state is late in O-O and early in C-H coordinates. However, for the HS Fe(III)-OOH, the transition state is early in O-O and further along in the C-H coordinate. In addition, there is a significant amount of electron transfer from the substrate to the HS Fe(III)-OOH at transition state, but that does not occur in the LS transition state. Thus, in contrast to the behavior of LS Fe(III)-OOH, the H-atom abstraction reactivity of HS Fe(III)-OOH is found to be highly dependent on both the ionization potential and the C-H bond strength of the substrate. LS Fe(III)-OOH is found to be more effective in H-atom abstraction for strong C-H bonds, while the higher reduction potential of HS Fe(III)-OOH allows it to be active in electrophilic reactions without the requirement of O-O bond cleavage. This is relevant to the Rieske dioxygenases, which are proposed to use a HS Fe(III)-OOH to catalyze cis-dihydroxylation of a wide range of aromatic compounds.

Reactivity of compound II: electronic structure analysis of methane hydroxylation by oxoiron(IV) porphyrin complexes

The methane hydroxylation reaction by a Compound II (Cpd II) mimic PorFe(IV)=O and its hydrosulfide-ligated derivative [Por(SH)Fe(IV)=O](-) is investigated by density functional theory (DFT) calculations on the ground triplet and excited quintet spin-state surfaces. On each spin surface both the σ- and π-channels are explored. H-abstraction is invariably the rate-determining step. In the case of PorFe(IV)=O the H-abstraction reaction can proceed either through the classic π-channel or through the nonclassical σ-channel on the triplet surface, but only through the classic σ-mechanism on the quintet surface. The barrier on the quintet σ-pathway is much lower than on the triplet channels so the quintet surface cuts through the triplet surfaces and a two state reactivity (TSR) mechanism with crossover from the triplet to the quintet surface becomes a plausible scenario for C-H bond activation by PorFe(IV)=O. In the case of the hydrosulfide-ligated complex the H-abstraction follows a π-mechanism on the triplet surface: the σ* is too high in energy to make a σ-attack of the substrate favorable. The σ- and π-channels are both feasible on the quintet surface. As the quintet surface lies above the triplet surface in the entrance channel of the oxidative process and is highly destabilized on both the σ- and π-pathways, the reaction can only proceed on the triplet surface. Insights into the electron transfer process accompanying the H-abstraction reaction are achieved through a detailed electronic structure analysis of the transition state species and the reactant complexes en route to the transition state. It is found that the electron transfer from the substrate σ(CH) into the acceptor orbital of the catalyst, the Fe-O σ* or π*, occurs through a rather complex mechanism that is initiated by a two-orbital four-electron interaction between the σ(CH) and the low-lying, oxygen-rich Fe-O σ-bonding and/or Fe-O π-bonding orbitals of the catalyst.