Electronic Structure of Activated Bleomycin: Oxygen Intermediates in Heme versus Non-Heme Iron

In Vitro and In Vivo Drug Release from a Nano-Hydroxyapatite Reinforced Resorbable Nanofibrous Scaffold for Treating Female Pelvic Organ Prolapse

Pelvic prolapse stands as a substantial medical concern, notably impacting a significant segment of the population, predominantly women. This condition, characterized by the descent of pelvic organs, such as the uterus, bladder, or rectum, from their normal positions, can lead to a range of distressing symptoms, including pelvic pressure, urinary incontinence, and discomfort during intercourse. Clinical challenges abound in the treatment landscape of pelvic prolapse, stemming from its multifactorial etiology and the diverse array of symptoms experienced by affected individuals. Current treatment options, while offering relief to some extent, often fall short in addressing the full spectrum of symptoms and may pose risks of complications or recurrence. Consequently, there exists a palpable need for innovative solutions that can provide more effective, durable, and patient-tailored interventions for pelvic prolapse. We manufactured an integrated polycaprolactone (PCL) mesh, reinforced with nano-hydroxyapatite (nHA), along with drug-eluting poly(lactic-co-glycolic acid) (PLGA) nanofibers for a prolapse scaffold. This aims to offer a promising avenue for enhanced treatment outcomes and improved quality of life for individuals grappling with pelvic prolapse. Solution extrusion additive manufacturing and electrospinning methods were utilized to prepare the nHA filled PCL mesh and drug-incorporated PLGA nanofibers, respectively. The pharmaceuticals employed included metronidazole, ketorolac, bleomycin, and estrone. Properties of fabricated resorbable scaffolds were assessed. The in vitro release characteristics of various pharmaceuticals from the meshes/nanofibers were evaluated. Furthermore, the in vivo drug elution pattern was also estimated on a rat model. The empirical data show that nHA reinforced PCL mesh exhibited superior mechanical strength to virgin PCL mesh. Electrospun resorbable nanofibers possessed diameters ranging from 85 to 540 nm, and released effective metronidazole, ketorolac, bleomycin, and estradiol, respectively, for 9, 30, 3, and over 30 days in vitro. Further, the mesh/nanofiber scaffolds also liberated high drug levels at the target site for more than 28 days in vivo, while the drug concentrations in blood remained low. This discovery suggests that resorbable scaffold can serve as a viable option for treating female pelvic organ prolapse.

Efficient and durable MoFeNi hydroxide anode: Room temperature recrystallization regulated morphology-, valence- and crystallinity-dependent water oxidation performance

The formation of crystal-amorphous (c-a) interfaces by modulating the crystallinity of the material is a promising strategy for the oxygen evolution reaction (OER). Herein, a recrystallization growth at room temperature to regulate the crystallinity of catalysts is reported. The MoFeNi hydroxide precursor was synthesized by the solvothermal method, and then the crystallinity of the material was controlled by adjusting the concentration of Na2S in the immersion solution. These c-a heterogeneous interfaces significantly improved the OER activity of the catalysts while ensuring structural stability. The best catalyst exhibited a low overpotential of 195 mV to reach 10 mA cm-2 in 1 M KOH. It also showed good stability, operating stably at high current densities for 96 h without significant degradation. In addition, the anode of the two-electrode water splitting electrolyzer required only 1.46 V to reach 10 mA cm-2 and operated for a long time without significant degradation. This method will provide new insights and perspectives for developing efficient and stable OER catalysts.

Second-sphere tuning of analogues for the ferric-hydroperoxoheme form of Mycobacterium tuberculosis MhuD

Mycobacterium tuberculosis MhuD catalyzes the oxygenation of heme to mycobilin; experimental data presented here elucidates the novel hydroxylation reaction catalyzed by this enzyme. Analogues for the critical ferric-hydroperoxoheme (MhuD-heme-OOH) intermediate of this enzyme were characterized using UV/Vis absorption (Abs), circular dichroism (CD), and magnetic CD (MCD) spectroscopies. In order to extract electronic transition energies from these spectroscopic data, a novel global fitting model was developed for analysis of UV/Vis Abs, CD, and MCD data. A variant of MhuD was prepared, N7S, which weakens the affinity of heme-bound enzyme for a hydroperoxo analogue, azide, without significantly altering the protein secondary structure. Global fitting of spectroscopic data acquired in this study revealed that the second-sphere N7S substitution perturbs the electronic structure of two analogues for MhuD-heme-OOH: azide-inhibited MhuD (MhuD-heme-N₃) and cyanide-inhibited MhuD (MhuD-heme-CN). The ground state electronic structures of MhuD-heme-N₃ and MhuD-heme-CN were assessed using variable-temperature, variable-field MCD. Altogether, these data strongly suggest that there is a hydrogen bond between the Asn7 side-chain and the terminal oxygen of the hydroperoxo ligand in MhuD-heme-OOH. As discussed herein, this finding supports a novel hydroxylation reaction mechanism where the Asn7 side-chain guides a transient hydroxyl radical derived from homolysis of the OO bond in MhuD-heme-OOH to the β- or δ-meso carbon of the porphyrin ligand yielding β- or δ-meso-hydroxyheme, respectively.

Copper ion and G-quadruplex-mediated fluorescent sensor for highly selective detection of bleomycin in actual samples

Improper dosage of Bleomycin (BLM) can easily lead to a series of side effects such as pulmonary fibrosis and pulmonary toxicity. Therefore, detecting the content of BLM in actual sample is very helpful to make full use of its therapeutic efficacy and reduce its toxicity. Herein, we constructed a copper ion and G-quadruplex mediated label-free sensor to detect BLM. The strategy mainly relies on the chelation of BLM to copper ions, which makes the copper ions lose the quenching ability to the fluorescent dye N-methylmesoporphyrin (NMM) after chelation. With the assistance of the G-quadruplex, the BLM content in the sample can be detected by observing the change in fluorescence. A good linear relationship can be clearly observed within the BLM concentration range of 0.1 nM-75 nM, and the limit of detection was derived as 0.1 nM. This sensor did not involve any labeling or addition of Fe²⁺, even in the presence of 10 different antibiotics and 11 different metal ions, it still has a good monitoring effect, and can be successfully applied to the detection of BLM in serum and wastewater. Thus, we believe that this work hold great potential in antibiotic monitoring and environmental protection.

Detection of bleomycin and its hydrolase by the cationic surfactant-doped liquid crystal-based sensing platform

Bleomycin (BLM) is a broadly used antibiotic to treat different types of cancer. It can be hydrolyzed by bleomycin hydrolase (BLMH), which eventually influences the anti-tumor efficacy of BLM. Therefore, it is particularly important to detect BLM and BLMH. Herein, we demonstrated highly sensitive detection of BLM and BLMH by a simple and convenient liquid crystal (LC)-based sensing platform for the first time. 5CB (a nematic LC) doped with the cationic surfactant OTAB was working as the sensing platform. When the OTAB-laden 5CB interface was in contact with an aqueous solution of ssDNA, LCs displayed a bright image due to disruption of the arrangement of OTAB monolayers by ssDNA, indicating the planar orientation of LCs at the aqueous/LC interface. When BLM·Fe(II) and ssDNA were both present in the aqueous solution, ssDNA underwent irreversible cleavage, which prevented disruption of the arrangement of OTAB monolayers. Accordingly, LCs showed a dark image, suggesting the homeotropic orientation of LCs at the aqueous/LC interface. However, when BLM·Fe(II) was enzymatically hydrolyzed by BLMH, LCs remained the bright image. This approach showed high sensitivity for the detection of BLM and BLMH with the limits of detection of 0.2 nM and 0.3 ng/mL, respectively. Besides, the detection of BLM and BLMH was successfully achieved in human serum. This method has the advantages of high sensitivity, robust stability, simple operation, low cost, and easy detection through naked eyes, which makes it a potential candidate for applications in clinical analysis.

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.

Characterization of the Earliest Intermediate of Fe-N2 Protonation: CW and Pulse EPR Detection of an Fe-NNH Species and Its Evolution to Fe-NNH2

Iron diazenido species (Fe(NNH)) have been proposed as the earliest intermediates of catalytic N₂-to-NH₃ conversion (N₂RR) mediated by synthetic iron complexes and relatedly as intermediates of N₂RR by nitrogenase enzymes. However, direct identification of such iron species, either during or independent of catalysis, has proven challenging owing to their high degree of instability. The isolation of more stable silylated diazenido analogues, Fe(NNSiR₃), and also of further downstream intermediates (e.g., Fe(NNH₂)), nonetheless points to Fe(NNH) as the key first intermediate of protonation in synthetic systems. Herein we show that low-temperature protonation of a terminally bound Fe-N₂^- species, supported by a bulky trisphosphinoborane ligand (^ArP₃^B), generates an S = 1/2 terminal Fe(NNH) species that can be detected and characterized by continuous-wave (CW) and pulse EPR techniques. The ¹H-hyperfine for ^ArP₃^BFe(NNH) derived from the presented ENDOR studies is diagnostic for the distally bound H atom ( a_iso = 16.5 MHz). The Fe(NNH) species evolves further to cationic [Fe(NNH₂)]⁺ in the presence of additional acid, the latter being related to a previously characterized [Fe(NNH₂)]⁺ intermediate of N₂RR mediated by a far less encumbered iron tris(phosphine)borane catalyst. While catalysis is suppressed in the present sterically very crowded system, N₂-to-NH₃ conversion can nevertheless be demonstrated. These observations in sum add support to the idea that Fe(NNH) plays a central role as the earliest intermediate of Fe-mediated N₂RR in a synthetic system.

Mapping chromosomal instability induced by small-molecular therapeutics in a yeast model

The yeast Saccharomyces cerevisiae has been widely used as a model system for studying the physiological and pharmacological action of small-molecular drugs. Here, a heterozygous diploid S. cerevisiae strain QSS4 was generated to determine whether drugs could induce chromosomal instability by determining the frequency of mitotic recombination. Using the combination of a custom SNP microarray and yeast screening system, the patterns of chromosomal instability induced by drugs were explored at the whole genome level in QSS4. We found that Zeocin (a member of the bleomycin family) treatment increased the rate of genomic alterations, including aneuploidy, loss of heterozygosity (LOH), and chromosomal rearrangement over a hundred-fold. Most recombination events are likely to be initiated by DNA double-stand breaks directly generated by Zeocin. Another remarkable finding is that G4-motifs and low GC regions were significantly underrepresented within the gene conversion tracts of Zeocin-induced LOH events, indicating that certain DNA regions are less preferred Zeocin-binding sites in vivo. This study provides a novel paradigm for evaluating genetic toxicity of small-molecular drugs using yeast models.

What factors tune the chemical equilibrium between metal-iodosylarene oxidants and high-valent metal-oxo ones?

Metal-iodosylarene complexes (1) and high-valent metal-oxo complexes (2) are two key reactive intermediates in oxygenation reactions.

The genome-wide sequence specificity of DNA cleavage by bleomycin analogues in human cells

Bleomycin (BLM) is a cancer chemotherapeutic agent that cleaves cellular DNA at specific sequences. Using next-generation Illumina sequencing, the genome-wide sequence specificity of DNA cleavage by two BLM analogues, 6'-deoxy-BLM Z and zorbamycin (ZBM), was determined in human HeLa cells and compared with BLM. Over 200 million double-strand breaks were examined for each sample, and the 50,000 highest intensity cleavage sites were analysed. It was found that the DNA sequence specificity of the BLM analogues in human cells was different to BLM, especially at the cleavage site (position "0") and the "+1" position. In human cells, the 6'-deoxy-BLM Z had a preference for 5'-GTGY*MC (where * is the cleavage site, Y is C or T, M is A or C); it was 5'-GTGY*MCA for ZBM; and 5'-GTGT*AC for BLM. With cellular DNA, the highest ranked tetranucleotides were 5'-TGC*C and 5'-TGT*A for 6'-deoxy-BLM Z; 5'-TGC*C, 5'-TGT*A and 5'-TGC*A for ZBM; and 5'-TGT*A for BLM. In purified human genomic DNA, the DNA sequence preference was 5'-TGT*A for 6'-deoxy-BLM, 5'-RTGY*AYR (where R is G or A) for ZBM, and 5'-TGT*A for BLM. Thus, the sequence specificity of the BLM analogue, 6'-deoxy-BLM Z, was similar to BLM in purified human DNA, while ZBM was different.

The Interaction of the Metallo-Glycopeptide Anti-Tumour Drug Bleomycin with DNA

The cancer chemotherapeutic drug, bleomycin, is clinically used to treat several neoplasms including testicular and ovarian cancers. Bleomycin is a metallo-glycopeptide antibiotic that requires a transition metal ion, usually Fe(II), for activity. In this review, the properties of bleomycin are examined, especially the interaction of bleomycin with DNA. A Fe(II)-bleomycin complex is capable of DNA cleavage and this process is thought to be the major determinant for the cytotoxicity of bleomycin. The DNA sequence specificity of bleomycin cleavage is found to at 5′-GT* and 5′-GC* dinucleotides (where * indicates the cleaved nucleotide). Using next-generation DNA sequencing, over 200 million double-strand breaks were analysed, and an expanded bleomycin sequence specificity was found to be 5′-RTGT*AY (where R is G or A and Y is T or C) in cellular DNA and 5′-TGT*AT in purified DNA. The different environment of cellular DNA compared to purified DNA was proposed to be responsible for the difference. A number of bleomycin analogues have been examined and their interaction with DNA is also discussed. In particular, the production of bleomycin analogues via genetic manipulation of the modular non-ribosomal peptide synthetases and polyketide synthases in the bleomycin gene cluster is reviewed. The prospects for the synthesis of bleomycin analogues with increased effectiveness as cancer chemotherapeutic agents is also explored.

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).

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.

The DNA sequence specificity of bleomycin cleavage in a systematically altered DNA sequence

Bleomycin is an anti-tumour agent that is clinically used to treat several types of cancers. Bleomycin cleaves DNA at specific DNA sequences and recent genome-wide DNA sequencing specificity data indicated that the sequence 5'-RTGT*AY (where T* is the site of bleomycin cleavage, R is G/A and Y is T/C) is preferentially cleaved by bleomycin in human cells. Based on this DNA sequence, we constructed a plasmid clone to explore this bleomycin cleavage preference. By systematic variation of single nucleotides in the 5'-RTGT*AY sequence, we were able to investigate the effect of nucleotide changes on bleomycin cleavage efficiency. We observed that the preferred consensus DNA sequence for bleomycin cleavage in the plasmid clone was 5'-YYGT*AW (where W is A/T). The most highly cleaved sequence was 5'-TCGT*AT and, in fact, the seven most highly cleaved sequences conformed to the consensus sequence 5'-YYGT*AW. A comparison with genome-wide results was also performed and while the core sequence was similar in both environments, the surrounding nucleotides were different.

Bleomycin analogues preferentially cleave at the transcription start sites of actively transcribed genes in human cells

Bleomycin (BLM) is a cancer chemotherapeutic agent that is used in the treatment of several types of tumours. The cytotoxicity of three BLM analogues, BLM Z, 6'-deoxy-BLM Z and zorbamycin (ZBM), was determined in human HeLa cells in comparison with BLM. It was found that the IC₅₀ values were 2.9μM for 6'-deoxy-BLM Z, 3.2μM for BLM Z, 4.4μM for BLM and 7.9μM for ZBM in HeLa cells. Using next-generation Illumina DNA sequencing techniques, the genome-wide cleavage of DNA by the BLM analogues was determined in human HeLa cells and compared with BLM. It was ascertained that BLM, 6'-deoxy-BLM Z and ZBM preferentially cleaved at the transcription start sites of actively transcribed genes in human cells. The degree of preferential cleavage at the transcription start sites was quantified and an inverse correlation with the IC₅₀ values was observed. This indicated that the degree of preferential cleavage at transcription start sites is an important component in determining the cytotoxicity of BLM analogues.

Reactivity of a Cobalt(III)-Hydroperoxo Complex in Electrophilic Reactions

The reactivity of mononuclear metal-hydroperoxo adducts has fascinated researchers in many areas due to their diverse biological and catalytic processes. In this study, a mononuclear cobalt(III)-peroxo complex bearing a tetradentate macrocyclic ligand, [Co^III(Me₃-TPADP)(O₂)]⁺ (Me₃-TPADP = 3,6,9-trimethyl-3,6,9-triaza-1(2,6)-pyridinacyclodecaphane), was prepared by reacting [Co^II(Me₃-TPADP)(CH₃CN)₂]²⁺ with H₂O₂ in the presence of triethylamine. Upon protonation, the cobalt(III)-peroxo intermediate was converted into a cobalt(III)-hydroperoxo complex, [Co^III(Me₃-TPADP)(O₂H)(CH₃CN)]²⁺. The mononuclear cobalt(III)-peroxo and -hydroperoxo intermediates were characterized by a variety of physicochemical methods. Results of electrospray ionization mass spectrometry clearly show the transformation of the intermediates: the peak at m/z 339.2 assignable to the cobalt(III)-peroxo species disappears with concomitant growth of the peak at m/z 190.7 corresponding to the cobalt(III)-hydroperoxo complex (with bound CH₃CN). Isotope labeling experiments further support the existence of the cobalt(III)-peroxo and -hydroperoxo complexes. In particular, the O-O bond stretching frequency of the cobalt(III)-hydroperoxo complex was determined to be 851 cm^-1 for ¹⁶O₂H samples (803 cm^-1 for ¹⁸O₂H samples), and its Co-O vibrational energy was observed at 571 cm^-1 for ¹⁶O₂H samples (551 cm^-1 for ¹⁸O₂H samples; 568 cm^-1 for ¹⁶O₂²H samples) by resonance Raman spectroscopy. Reactivity studies performed with the cobalt(III)-peroxo and -hydroperoxo complexes in organic functionalizations reveal that the latter is capable of conducting oxygen atom transfer with an electrophilic character, whereas the former exhibits no oxygen atom transfer reactivity under the same reaction conditions. Alternatively, the cobalt(III)-hydroperoxo complex does not perform hydrogen atom transfer reactions, while analogous low-spin Fe(III)-hydroperoxo complexes are capable of this reactivity. Density functional theory calculations indicate that this lack of reactivity is due to the high free energy cost of O-O bond homolysis that would be required to produce the hypothetical Co(IV)-oxo product.

Magnetic Circular Dichroism Evidence for an Unusual Electronic Structure of a Tetracarbene-Oxoiron(IV) Complex

In biology, high valent oxo-iron(IV) species have been shown to be pivotal intermediates for functionalization of C-H bonds in the catalytic cycles of a range of O₂-activating iron enzymes. This work details an electronic-structure investigation of [Fe^IV(O)(L^NHC)(NCMe)]²⁺ (L^NHC = 3,9,14,20-tetraaza-1,6,12,17-tetraazoniapenta-cyclohexacosane-1(23),4,6(26),10,12(25),15,17(24),21-octaene, complex 1) using helium tagging infrared photodissociation (IRPD), absorption, and magnetic circular dichroism (MCD) spectroscopy, coupled with DFT and highly correlated wave function based multireference calculations. The IRPD spectrum of complex 1 reveals the Fe-O stretching vibration at 832 ± 3 cm^-1. By analyzing the Franck-Condon progression, we can determine the same vibration occurring at 616 ± 10 cm^-1 in the E(d_xy → d_xz,yz) excited state. Both values are similar to those measured for [Fe^IV(O)(TMC)(NCMe)]²⁺ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). The low-temperature MCD spectra of complex 1 exhibit three pseudo A-term signals around 12 500, 17 000, and 24 300 cm^-1. We can unequivocally assign them to the ligand field transitions of d_xy → d_xz,yz, d_xz,yz → d_z2, and d_xz,yz → d_x2-y2, respectively, through direct calculations of MCD spectra and independent determination of the MCD C-term signs from the corresponding electron donating and accepting orbitals. In comparison with the corresponding transitions observed for [Fe^IV(O) (SR-TPA)(NCMe)]²⁺ (SR-TPA = tris(3,5-dimethyl-4-methoxypyridyl-2-methy)amine), the excitations within the (FeO)²⁺ core of complex 1 have similar transition energies, whereas the excitation energy for d_xz,yz → d_x2-y2 is significantly higher (∼12 000 cm^-1 for [Fe^IV(O)(SR-TPA)(NCMe)]²⁺). Our results thus substantiate that the tetracarbene ligand (L^NHC) of complex 1 does not significantly affect the bonding in the (FeO)²⁺ unit but strongly destabilizes the d_x2-y2 orbital to eventually lift it above d_z2. As a consequence, this unusual electron configuration leads to an unprecedentedly larger quintet-triplet energy separation for complex 1, which largely rules out the possibility that the H atom transfer reaction may take place on the quintet surface and hence quenches two-state reactivity. The resulting mechanistic implications are discussed.

Spectroscopic and Computational Studies of Nitrile Hydratase: Insights into Geometric and Electronic Structure and the Mechanism of Amide Synthesis

Nitrile hydratases (NHases) are mononuclear nonheme enzymes that catalyze the hydration of nitriles to amides. NHase is unusual in that it utilizes a low-spin (LS) Fe^III center and a unique ligand set comprised of two deprotonated backbone amides, cysteine-based sulfenic acid (RSO(H)) and sulfinic acid (RSO₂^-), and an unmodified cysteine trans to an exogenous ligand site. Electron paramagnetic resonance (EPR), magnetic circular dichroism (MCD) and low-temperature absorption (LT-Abs) spectroscopies are used to determine the geometric and electronic structures of butyrate-bound (NHaseBA) and active (NHaseAq) NHase. These data calibrate DFT models, which are then extended to explore the mechanism of nitrile hydration by NHase. In particular, the nitrile is activated by coordination to the LS Fe^III and the sulfenate group is found to be deprotonated and a significantly better nucleophile than water that can attack the coordinated nitrile to form a cyclic species. Attack at the sulfenate S atom of the cyclic species is favorable and leads to a lower kinetic barrier than attack by water on coordinated, uncyclized nitrile, while attack at the C of the cyclic species is unfavorable. The roles of the unique ligand set and low-spin nature of the NHase active site in function are also explored. It is found that the oxidized thiolate ligands are crucial to maintaining the LS state, which is important in the binding and activation of nitrile susbtrates. The dominant role of the backbone amidate ligands appears to be as a chelate in keeping the sulfenate properly oriented for nucleophilic attack on the coordinated substrate.

Magnetic circular dichroism and computational study of mononuclear and dinuclear iron(IV) complexes

High-valent iron(IV)-oxo species are key intermediates in the catalytic cycles of a range of O2-activating iron enzymes. This work presents a detailed study of the electronic structures of mononuclear ([FeIV(O)(L)(NCMe)]2+, 1, L = tris(3,5-dimethyl-4-methoxylpyridyl-2-methyl)amine) and dinuclear ([(L)FeIV(O)(μ-O)FeIV(OH)(L)]3+, 2) iron(IV) complexes using absorption (ABS), magnetic circular dichroism (MCD) spectroscopy and wave-function-based quantum chemical calculations. For complex 1, the experimental MCD spectra at 2-10 K are dominated by a broad positive C-term band between 12000 and 18000 cm-1. As the temperature increases up to ~20 K, this feature is gradually replaced by a derivative-shaped signal. The computed MCD spectra are in excellent agreement with experiment, which reproduce not only the excitation energies and the MCD signs of key transitions but also their temperature-dependent intensity variations. To further corroborate the assignments suggested by the calculations, the individual MCD sign for each transition is independently determined from the corresponding electron donating and accepting orbitals. Thus, unambiguous assignments can be made for the observed transitions in 1. The ABS/MCD data of complex 2 exhibit ten features that are assigned as ligand-field transitions or oxo- or hydroxo-to-metal charge transfer bands, based on MCD/ABS intensity ratios, calculated excitation energies, polarizations, and MCD signs. In comparison with complex 1, the electronic structure of the FeIV=O site is not significantly perturbed by the binding to another iron(IV) center. This may explain the experimental finding that complexes 1 and 2 have similar reactivities toward C-H bond activation and O-atom transfer.

The peroxidase activity of bleomycin-Fe3+ is associated with damage to biological components

In this study, bleomycin-Fe(3+) steadily oxidized tetramethylbenzidine (TMB) in the presence of peroxides. However, the ability of bleomycin-Fe(3+) to function as a peroxidase was extremely low compared with that of other peroxidases. A characteristic property of bleomycin-Fe(3+) different from that observed for other peroxidases is its ability to oxidize TMB at the similar rate at both a pH 5 and 8 in the presence of lipid hydroperoxide (LOOH). In the present experiments, hydroxyl radicals (HO•) were generated only when bleomycin-Fe(3+) was incubated with H2O2 at a pH of 5. No generation of HO• was observed during the incubation of bleomycin-Fe(3+) with LOOH. Meanwhile, bleomycin-Fe(3+) induced the formation of LOOH from linoleic acid and alcohol dehydrogenase was inactivated by bleomycin-Fe(3+) with peroxides. Thiobarbituric acid reactive substances were formed from DNA by bleomycin-Fe(3+) with H2O2, and strand breaks were caused by bleomycin-Fe(3+) with LOOH. The oxidative substrates for bleomycin-Fe(3+) blocked the damage to biological components induced by bleomycin-Fe(3+). These results suggest that compound I-like species contribute to the process of damage to biological components induced by bleomycin-Fe(3+).

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.

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.

Understanding the oxidative relationships of the metal oxo, hydroxo, and hydroperoxide intermediates with manganese(IV) complexes having bridged cyclams: correlation of the physicochemical properties with reactivity

Multiple transition metal functional groups including metaloxo, hydroxo, and hydroperoxide groups play significant roles in various biological and chemical oxidations such as electron transfer, oxygen transfer, and hydrogen abstraction. Further studies that clarify their oxidative relationships and the relationship between their reactivity and their physicochemical properties will expand our ability to predict the reactivity of the intermediate in different oxidative events. As a result researchers will be able to provide rational explanations of poorly understood oxidative phenomena and design selective oxidation catalysts. This Account summarizes results from recent studies of oxidative relationships among manganese(IV) molecules that include pairs of hydroxo/oxo ligands. Changes in the protonation state may simultaneously affect the net charge, the redox potential, the metal-oxygen bond order (M-O vs M═O), and the reactivity of the metal ion. In the manganese(IV) model system, [Mn(IV)(Me(2)EBC)(OH)(2)](PF(6))(2), the Mn(IV)-OH and Mn(IV)═O moieties have similar hydrogen abstraction capabilities, but Mn(IV)═O abstracts hydrogen at a more than 40-fold faster rate than the corresponding Mn(IV)-OH. However, after the first hydrogen abstraction, the reduction product, Mn(III)-OH(2) from the Mn(IV)-OH moiety, cannot transfer a subsequent OH group to the substrate radical. Instead the Mn(III)-OH from the Mn(IV)═O moiety reforms the OH group, generating the hydroxylated product. In the oxygenation of substrates such as triarylphosphines, the reaction with the Mn(IV)═O moiety proceeds by concerted oxygen atom transfer, but the reaction with the Mn(IV)-OH functional group proceeds by electron transfer. In addition, the manganese(IV) species with a Mn(IV)-OH group has a higher redox potential and demonstrates much more facile electron transfer than the one that has the Mn(IV)═O group. Furthermore, an increase in the net charge of the Mn(IV)-OH further accelerates its electron transfer rate. But its influence on hydrogen abstraction is minor because charge-promoted electron transfer does not enhance hydrogen abstraction remarkably. The Mn(IV)-OOH moiety with an identical coordination environment is a more powerful oxidant than the corresponding Mn(IV)-OH and Mn(IV)═O moieties in both hydrogen abstraction and oxygen atom transfer. With this full understanding of the oxidative reactivity of the Mn(IV)-OH and Mn(IV)═O moieties, we have clarified the correlation between the physicochemical properties of these active intermediates, including net charge, redox potential, and metal-oxygen bond order, and their reactivities. The reactivity differences between the metal oxo and hydroxo moieties on these manganese(IV) functional groups after the first hydrogen abstraction have provided clues for understanding their occurrence and functions in metalloenzymes. The P450 enzymes require an iron(IV) oxo form rather than an iron(IV) hydroxo form to perform substrate hydroxylation. However, the lipoxygenases use an iron(III) hydroxo group to dioxygenate unsaturated fatty acids rather than an iron(III) oxo species, a moiety that could facilitate hydroxylation reactions. These distinctly different physicochemical properties and reactivities of the metal oxo and hydroxo moieties could provide clues to understand these elusive oxidation phenomena and provide the foundation for the rational design of novel oxidation catalysts.

Definition of the intermediates and mechanism of the anticancer drug bleomycin using nuclear resonance vibrational spectroscopy and related methods

Bleomycin (BLM) is a glycopeptide anticancer drug capable of effecting single- and double-strand DNA cleavage. The last detectable intermediate prior to DNA cleavage is a low spin Fe(III) peroxy level species, termed activated bleomycin (ABLM). DNA strand scission is initiated through the abstraction of the C-4' hydrogen atom of the deoxyribose sugar unit. Nuclear resonance vibrational spectroscopy (NRVS) aided by extended X-ray absorption fine structure spectroscopy and density functional theory (DFT) calculations are applied to define the natures of Fe(III)BLM and ABLM as (BLM)Fe(III)─OH and (BLM)Fe(III)(η(1)─OOH) species, respectively. The NRVS spectra of Fe(III)BLM and ABLM are strikingly different because in ABLM the δFe─O─O bending mode mixes with, and energetically splits, the doubly degenerate, intense O─Fe─N(ax) transaxial bends. DFT calculations of the reaction of ABLM with DNA, based on the species defined by the NRVS data, show that the direct H-atom abstraction by ABLM is thermodynamically favored over other proposed reaction pathways.

Reaction coordinate of isopenicillin N synthase: oxidase versus oxygenase activity

Isopenicillin N synthase (IPNS) can have both oxidase and oxygenase activity depending on the substrate. For the native substrate, ACV, oxidase activity exists; however, for the substrate analogue ACOV, which lacks an amide nitrogen, IPNS exhibits oxygenase activity. The potential energy surfaces for the O-O bond elongation and cleavage were calculated for three different reactions: homolytic cleavage via traditional Fenton chemistry, heterolytic cleavage, and nucleophilic attack. These surfaces show that the hydroperoxide-ferrous intermediate, formed by O(2)-activated H atom abstraction from the substrate, can exploit different reaction pathways and that interactions with the substrate govern the pathway. The hydrogen bonds from hydroperoxide to the amide nitrogen of ACV polarize the sigma* orbital of the peroxide toward the proximal oxygen, facilitating heterolytic cleavage. For the substrate analogue ACOV, this hydrogen bond is no longer present, leading to nucleophilic attack on the substrate intermediate C-S bond. After cleavage of the hydroperoxide, the two reaction pathways proceed with minimal barriers, resulting in the closure of the beta-lactam ring for the oxidase activity (ACV) or formation of the thiocarboxylate for oxygenase activity (ACOV).