Electron Paramagnetic Resonance Spectroscopic Studies Research Articles

Structural and Spectroscopic Investigations of pH-Dependent Mo(V) Species in a Bacterial Sulfite-Oxidizing Enzyme.

Mono-pyranopterin-containing sulfite-oxidizing enzymes (SOEs), including eukaryotic sulfite oxidases and homologous prokaryotic sulfite dehydrogenases (SDHs), are molybdenum enzymes that exist in almost all forms of life, where they catalyze the direct oxidation of sulfite into sulfate, playing a key role in protecting cells and organisms against sulfite-induced damage. To decipher their catalytic mechanism, we have previously provided structural and spectroscopic evidence for direct coordination of HPO42- to the Mo atom at the active site of the SDH from the hyperthermophilic bacterium Thermus thermophilus (TtSDH), mimicking the proposed sulfate-bound intermediate proposed to be formed during catalysis. In this work, by solving the X-ray crystallographic structure of the unbound enzyme, we resolve the changes in the hydrogen bonding network in the molybdenum environment that enable the stabilization of the previously characterized phosphate adduct. In addition, electron paramagnetic resonance spectroscopic study of the enzyme over a wide pH range reveals the formation of pH-dependent Mo(V) species, a characteristic feature of eukaryotic SOEs. The combined use of HYSCORE, H2O/D2O exchange, and density functional theory calculations allows the detailed characterization of a typical low pH Mo(V) species previously unreported in bacterial SOEs, underlining the conservation of the active site properties of SOEs irrespective of their source organism.

Synthesis of copper complexes for potential use in the diagnosis of Alzheimer's disease

Background: Since there is currently no cure for Alzheimer's disease (AD), it is important to develop methods that could contribute to its diagnosis and propose new therapeutic approaches for its treatment. Positron emission tomography (PET) is a medical imaging technique that can be used to diagnose Alzheimer's disease, as some radiotracers have been developed to detect amyloid plaques, one of the hallmarks of the disease. However, the radiotracers already used for the diagnosis of AD have a low half-life, which limits their use over the time and requires the presence of a cyclotron close to the examination site. The use of copper complexes could be a good alternative for the development of new radiotracers, since they can be used in PET imaging and have a longer half-life than the other radiotracers already used for AD diagnosis. Aim of the study: Design and synthesize copper complexes that could be used as PET radiotracers able to cross the blood brain barrier and detecting amyloid plaques.Material and methods: The first objective was to design a series of new copper complexes with the capacity to target amyloid plaques inside the brain and contribute to the rational synthesis of such complexes. To this end, theoretical models were used to predict the ability of the complex to cross the blood-brain barrier (BBB) which separates brain from de blood and is main gatekeeper for drugs entering the brain. The models employed consider the physicochemical properties of the molecules. Based on these models, one of the molecules was synthesized in ten steps. The key step in the synthetic strategy was the coupling of a monopicolinate-N-alkylated cyclam-based ligand with a moiety capable of recognizing Aβ plaques via a Buchwald-Hartwig coupling reaction. Potentiometric, spectrophotometric and electron paramagnetic resonance spectroscopic studies were performed to determine the structure and thermodynamic stability of the complex. In addition, the ability to target amyloid plaques was evaluated using brain sections from Alzheimer's disease patients and the cytotoxicity was evaluated using human neuronal cells.Results: A first complex has been designed and synthesized. The physicochemical studies performed indicate that this complex seem to be thermodynamically stable. In addition, cytotoxicity tests on human neuronal cells indicate low toxicity and labeling tests on brain sections from Alzheimer's patients suggest that the complex is capable of recognizing amyloid plaques. Conclusion: We have developed a novel copper complex that is able to detect recognize amyloid plaques. In the future this molecule could be used in PET and contribute to the diagnosis of Alzheimer's disease. From these results, we also propose to develop a new copper complex for tau PET imaging.

Abstract Tu123: Redox Biology of the Mitochondrial Protein mitoNEET and Ascorbate

Background: It has been proposed that mitoNEET is an Fe-S transfer/repair enzyme, playing a role under conditions of oxidative stress. mitoNEET protects mitochondria against oxidative stress in cardiomyocytes. Loss of mitoNEET is linked with age-related heart failure. Oxidized mitoNEET, [2Fe-2S] 2+ , exports Fe-S cluster from mitochondria to cytosol. Redox control of mitochondrion is essential for the proper functioning of the organelle/mitochondria. However, the molecular mechanisms involved in the redox biology of mitoNEET is not fully studied. Hypothesis: Ascorbate regulates redox reactions and Fe-S cluster transport function of mitoNEET in the mitochondria. Aims: The objective of this work is to evaluate the redox reactions between mitoNEET and ascorbate. Methods: Room temperature and low temperature (77 K) electron paramagnetic resonance (EPR) spectroscopy techniques were used to measure the oxidation of ascorbate and reduced mitoNEET, respectively. Results: EPR spectroscopic study shows that mitoNEET oxidizes ascorbate to ascorbate radical. The formation of ascorbate radical increases with increasing concentration of ascorbate. The formation of ascorbate radical is decreased in the presence of menadione. Low temperature EPR study shows that one electron reduction of mitoNEET by ascorbate. Conclusion: This study suggest that ascorbate is a substrate/cofactor for mitoNEET. Ascorbate regulates the Fe-S cluster transport function of mitoNEET. Ascorbate plays an important role in the redox biology of mitoNEET under physiological and pathophysiological conditions.

The Role of Ovalbumin in Manganese Homeostasis during Chick Embryogenesis: An EPR Spectroscopic Study.

Ovalbumin (OVA), a protein vital for chick embryo nutrition, hydration, and antimicrobial protection, together with other egg-white proteins, migrates to the amniotic fluid and is orally absorbed by the embryo during embryogenesis. Recently, it has been shown that for optimal eggshell quality, the hen diet can be supplemented with manganese. Although essential for embryonic development, manganese in excess causes neurotoxicity. This study investigates whether OVA may be involved in the regulation of manganese levels. The binding of Mn(II) to OVA was investigated using electron paramagnetic resonance (EPR) spectroscopy. The results show that OVA binds a maximum of two Mn(II) ions, one with slightly weaker affinity, even in a 10-fold excess, suggesting it may have a protective role from Mn(II) overload. It seems that the binding of Mn(II), or the presence of excess Mn(II), does not affect OVA's tertiary structure, as evidenced from fluorescence and UV/vis measurements. Comparative analysis with bovine and human serum albumins revealed that they exhibit higher affinities for Mn(II) than OVA, most likely due to their essentially different physiological roles. These findings suggest that OVA does not play a role in the transport and storage of manganese; however, it may be involved in embryo protection from manganese-induced toxicity.

Uranium mineralization in the Thrace Basin, NW Türkiye: Evidence from radiation-induced defects in detrital quartz and synchrotron XRF/XANES analysis

The Paleogene-Neogene Thrace Basin in northwestern Türkiye has long been known to host economic gas and oil resources and has recently been reported to potentially host sandstone-type uranium deposits in the Oligocene Süloğlu Formation. The latter discovery raises questions about the source and deposition mechanism of uranium mineralization in the basin. This contribution reports on the results of a detailed electron paramagnetic resonance (EPR) spectroscopic study of detrital quartz from four sandstone and one mudstone samples in the Süloğlu Formation and documents the distribution and speciation of uranium using combined microbeam synchrotron X-ray fluorescence maps (μsXRF) and microbeam X-ray near edge structure spectroscopy (μsXANES). The EPR spectra of quartz separates are characterized by the presence of diagnostic radiation-induced defects (i.e., silicon-vacancy hole centers H′3, H′4, and H′7 with gmax = 2.049, 2.034, and 2.018, respectively, and the oxygen-vacancy electron center E′1), formed by the bombardment of alpha particles emitted from uranium, thorium, and their unstable progenies. Moreover, notable decreases in the intensity of silicon-vacancy hole centers in the EPR spectra of quartz separates after partial dissolution with hydrofluoric acid, provide compelling evidence for the circulation of uranium-bearing fluids in the Thrace Basin. The μsXRF and μsXANES data reveal the occurrences of mixed U6+ and U4+ species in hematite partially replacing pyrite aggregates but dominantly U4+ in disseminated pyrite and illite in sandstones of the Süloğlu Formation. These results provide new insights into uranium transport, reduction, and deposition mechanisms, with important implications for better understanding sandstone-type uranium deposits in general and further exploration in the Thrace Basin.

An EPR study of the marbles from quarries of the Denizli region (Turkey): A contribution to the provenance assessment of materials with close relationships

The analytical and spectroscopic discrimination of marbles coming from quarries used in historical times is a task object of a wide interest in archaeometric investigations. This task is even more difficult, when the goal of the provenance assessment is focused on marbles coming from historical quarries located in a close geographic area. In this paper, we present the results of a systematic Electron Paramagnetic Resonance spectroscopy study aimed at assessing the discrimination criteria among 5 quarries located in the Denizli region (Anatolia, Turkey) that were benefited in Hellenistic and Roman period to provide materials for the buildings in the nearby city of Hierapolis of Phrygia (Turkey). The resulting EPR characterisation is used, in combination with the results of isotope geochemistry and petrological observation, to define criteria able to discriminate the provenance of marble samples from the considered quarries. The criteria arose from the analysis operated through robust compositional statistical techniques over the results of the experimental investigation. In this approach, the internal structure of the multimethodic dataset was unravelled. The results here presented provide evidence of a good discriminating ability of the proposed approach.

Streptomycin generates oxidative stress in melanin-producing cells: In vitro study with EPR spectroscopy evidence

Streptomycin (STR) is an aminoglycoside antibiotic with a broad-spectrum of activity and ototoxic potential. The mechanism of STR-induced inner ear damage has not been fully elucidated. It was previously found that STR binds to melanin, which may result in the accumulation of the drug in melanin-containing tissues. Melanin pigment is present in various parts of the inner ear, including the cochlea and vestibular organ.The present study aimed to assess if streptomycin generates oxidative stress and affects melanogenesis in normal human melanocytes. Moreover the variation of free radical concentration in STR-treated melanocytes was examined by electron paramagnetic resonance spectroscopy (EPR). We found that STR decreases cell metabolic activity and reduces melanin content. The observed changes in the activity of antioxidant enzymes activity in HEMn-DPs treated with streptomycin may suggest that the drug affects redox homeostasis in melanocytes. In this work EPR study expanded knowledge about free radicals in interactions of STR and melanin in melanocytes. The results may help elucidate the mechanisms of STR toxicity on pigment cells, including melanin-producing cells in the inner ear. This is important because understanding the mechanism of STR-induced ototoxicity would be helpful in developing new therapeutic strategies to protect patients' hearing.

Evaluation of PEGylation efficacy of curcumin-loaded nanoemulsions using complementary methods to assess protein interactions and physicochemical properties

Nanoemulsions (NEs) are frequently used in the food, cosmetics and pharmaceutical industries to deliver nutraceuticals, pharmaceutical or cosmetic active ingredients. When administering NEs parenterally, various stabilisers are added to prevent rapid plasma clearance and to successfully deliver the active ingredient to the target site. For this purpose, PEGylation is often used to prolong the circulation time of the droplets. However, the problem is to determine the optimal concentration of the PEGylating agent – the PEGylation efficacy – that ensures adequate surface protection. This is a particular challenge when the active ingredient is incorporated into the stabilising layer, where any changes could disrupt the stability of the droplet. For this reason, we aimed to determine the optimal concentration of PEG2000-DSPE for surface protection of curcumin-loaded NEs for parenteral administration using electron paramagnetic resonance (EPR) spectroscopy. NEs were prepared using the high pressure homogenisation technique with 0.1 %, 0.3 % or 0.6 % of the PEGylated phospholipid. A droplet size of approximately 100 nm and polydispersity index below 0.25 indicated suitability for parenteral application. EPR analysis showed that PEG2000-DSPE had a stabilising effect on selected NEs, which was most pronounced in the part of the stabilising layer closest to the aqueous phase. To confirm these results, protein interaction studies were carried out using dynamic light scattering, UV–Vis spectroscopy, atomic force microscopy and release studies from protein-enriched media – bovine serum albumin (BSA) or foetal bovine serum (FBS) in phosphate-buffered saline. These analyses confirmed that the addition of PEG2000-DSPE reduced protein binding to the droplets as a function of concentration, with 0.3 % providing the best protection for the droplets. Our conclusions from the EPR spectroscopy study demonstrate the usefulness of EPR in determining the optimal concentrations of PEGylating agents for surface coverage and its usefulness in the formulation development phase.

Disentangling the Unusual Magnetic Anisotropy of the Near‐Room‐Temperature Ferromagnet Fe4GeTe2

AbstractIn the quest for 2D conducting materials with high ferromagnetic ordering temperature the new family of the layered FenGeTe2 compounds, especially the near‐room‐temperature ferromagnet Fe4GeTe2, receives a significant attention. Fe4GeTe2 features a peculiar spin reorientation transition at TSR ≈ 110 K suggesting a non‐trivial temperature evolution of the magnetic anisotropy (MA)—one of the main contributors to the stabilization of the magnetic order in the low‐dimensional systems. An electron spin resonance (ESR) spectroscopic study reported here provides quantitative insights into the unusual magnetic anisotropy of Fe4GeTe2. At high temperatures the total MA is mostly given by the demagnetization effect with a small contribution of the counteracting intrinsic magnetic anisotropy of an easy‐axis type, whose growth below a characteristic temperature Tshape ≈ 150 K renders the sample seemingly isotropic at TSR. Below one further temperature Td ≈ 50 K the intrinsic MA becomes even more complex. Importantly, all the characteristic temperatures found in the ESR experiment match those observed in transport measurements, suggesting an inherent coupling between magnetic and electronic degrees of freedom in Fe4GeTe2. This finding together with the observed signatures of the intrinsic two‐dimensionality should facilitate optimization routes for the use of Fe4GeTe2 in the magneto‐electronic devices, potentially even in the monolayer limit.

ZnAl2O4:Er3+ Upconversion Nanophosphor for SPECT Imaging and Luminescence Modulation via Defect Engineering.

This work reports an "all-in-one" theranostic upconversion luminescence (UCL) system having potential for both diagnostic and therapeutic applications. Despite considerable efforts in designing upconversion nanoparticles (UCNPs) for multimodal imaging and tumor therapy, there are few reports investigating dual modality SPECT/optical imaging for theranostics. Especially, research focusing on in vivo biodistribution studies of intrinsically radiolabeled UCNPs after intravenous injection is of utmost importance for the potential clinical translation of such formulations. Here, we utilized the gamma emission from 169Er and 171Er radionuclides for the demonstration of radiolabeled ZnAl2O4:171/169Er3+ as a potent agent for dual-modality SPECT/optical imaging. No uptake of radio nanoformulation was detected in the skeleton after 4 h of administration, which evidenced the robust integrity of ZnAl2O4:169/171Er3+. Combining the therapeutics using the emission of β- particulates from 169Er and 171Er will be promising for the radio-theranostic application of the synthesized ZnAl2O4:169/171Er3+ nanoformulation. Cell toxicity studies of ZnAl2O4:1%Er3+ nanoparticles were examined by an MTT assay in B16F10 mouse melanoma cell lines, which demonstrated good biocompatibility. In addition, we explored the mechanism of UCL modulation via defect engineering by Bi3+ codoping in the ZnAl2O4:Er3+ upconversion nanophosphor. The UCL color tuning was successfully achieved from the red to the green region as a function of Bi3+ codoping concentrations. Further, we tried to establish a correlation of UCL tuning with the intrinsic oxygen and cation vacancy defects as a function of Bi3+ codoping concentrations with the help of electron paramagnetic resonance (EPR) and positron annihilation lifetime spectroscopy (PALS) studies. This study contributes to building a bridge between nature of defects and UC luminescence that is crucial for the design of advanced UCNPs for theranostics.

Adsorption and degradation of tetracycline hydrochloride in aqueous solutions using activated carbon with Fe/Ag nanoparticles

In this study, a new adsorbent denoted as nAg/nZVI/AC was developed using activated carbon (AC) loaded with nanosized zero-valent iron (nZVI) and nanosilver (nAg). Then, the adsorption and degradation of tetracycline hydrochloride (TCH) in aqueous solutions using nAg/nZVI/AC was examined. At pH 3.8, the adsorbed amount of TCH on nAg/nZVI/AC was 191.18 mg/g, which is 78.95 mg/g higher than that adsorbed on the pristine AC support. The adsorption process could be explained adequately using the Freundlich (R2 = 0.9641) and pseudo-second-order kinetic models (R2 = 0.9967). Thermodynamic analyses revealed that TCH removal is a spontaneous chemisorption-dominated process. Free-radical-quenching experiments and electron paramagnetic resonance spectroscopic studies revealed that hydroxyl (·OH) and superoxide (·O2−) radicals are the main species involved in the degradation of TCH. Finally, the intermediates generated during the degradation were examined using liquid chromatography-mass spectrometry, and possible deterioration pathways are proposed.

Characterization of ferredoxins involved in electron transfer pathways for nitrogen fixation implicates differences in electronic structure in tuning 2[4Fe[sbnd]4S] Fd activity

Ferredoxins (Fds) are small proteins which shuttle electrons to pathways like biological nitrogen fixation. Physical properties tune the reactivity of Fds with different pathways, but knowledge on how these properties can be manipulated to engineer new electron transfer pathways is lacking. Recently, we showed that an evolved strain of Rhodopseudomonas palustris uses a new electron transfer pathway for nitrogen fixation. This pathway involves a variant of the primary Fd of nitrogen fixation in R. palustris, Fer1, in which threonine at position 11 is substituted for isoleucine (Fer1T11I). To understand why this substitution in Fer1 enables more efficient electron transfer, we used in vivo and in vitro methods to characterize Fer1 and Fer1T11I. Electrochemical characterization revealed both Fer1 and Fer1T11I have similar redox transitions (−480 mV and − 550 mV), indicating the reduction potential was unaffected despite the proximity of T11 to an iron‑sulfur (FeS) cluster of Fer1. Additionally, disruption of hydrogen bonding around an FeS cluster in Fer1 by substituting threonine with alanine (T11A) or valine (T11V) did not increase nitrogenase activity, indicating that disruption of hydrogen bonding does not explain the difference in activity observed for Fer1T11I. Electron paramagnetic resonance spectroscopy studies revealed key differences in the electronic structure of Fer1 and Fer1T11I, which indicate changes to the high spin states and/or spin-spin coupling between the FeS clusters of Fer1. Our data implicates these electronic structure differences in facilitating electron flow and sets a foundation for further investigations to understand the connection between these properties and intermolecular electron transfer.

Designing surface exposed sites on Bacillus subtilis lipase A for spin-labeling and hydration studies

Spin-labeling with electron paramagnetic resonance spectroscopy (EPR) is a facile method for interrogating macromolecular flexibility, conformational changes, accessibility, and hydration. Within we present a computationally based approach for the rational selection of reporter sites in Bacillus subtilis lipase A (BSLA) for substitution to cysteine residues with subsequent modification with a spin-label that are expected to not significantly perturb the wild-type structure, dynamics, or enzymatic function. Experimental circular dichroism spectroscopy, Michaelis-Menten kinetic parameters and EPR spectroscopy data validate the success of this approach to computationally select reporter sites for future magnetic resonance investigations of hydration and hydration changes induced by polymer conjugation, tethering, immobilization, or amino acid substitution in BSLA. Analysis of molecular dynamic simulations of the impact of substitutions on the secondary structure agree well with experimental findings. We propose that this computationally guided approach for choosing spin-labeled EPR reporter sites, which evaluates relative surface accessibility coupled with hydrogen bonding occupancy of amino acids to the catalytic pocket via atomistic simulations, should be readily transferable to other macromolecular systems of interest including selecting sites for paramagnetic relaxation enhancement NMR studies, other spin-labeling EPR studies or any method requiring a tagging method where it is desirable to not alter enzyme stability or activity.

Visible light mediated photocatalysis by lanthanide metal-organic frameworks: enhanced specificity and mechanistic insights.

The utilization of earth-abundant photosensitizers with visible light absorption to enable sustainable photocatalysis is a long-standing challenge. Overcoming such a challenge, in this work, two lanthanide (Ln3+ = Tb, Eu) based metal-organic frameworks (Ln-MOFs) have been synthesized utilizing a Co3+-based metalloligand. Both Ln-MOFs function as remarkable photocatalysts for the selective oxidation of assorted alcohols and sulfides to their corresponding aldehydes/ketones and sulfoxides using visible light. The photophysical behavior of both Ln-MOFs and mechanism of photocatalysis is comprehensively investigated using time-resolved transient absorption spectroscopy, electrochemical impedance spectroscopy, electron paramagnetic resonance spectroscopy, photoluminescence and phosphorescence studies. In both Ln-MOFs, a metalloligand acts as a light-harvester, being excited by visible light, while Ln3+ ions endow the resulting MOFs with long-lived triplet excited states. Ultrafast transient absorption spectroscopy, further supported by electron paramagnetic resonance spectra, revealed excited-state electron transfer from metalloligands to the Ln3+ ions and transient generation of Ln2+ sites alongside the facilitation of intersystem crossing. The excited Ln2+ ions transfer energy to the ground-state triplet oxygen (3O2) to generate singlet oxygen (1O2). The HOMO-LUMO positions of both Ln-MOFs support the generation of ˙O2 - and 1O2 but inhibit strongly-oxidizing yet non-selective ˙OH radicals. Scavenger experiments, 1O2 traps and electron paramagnetic resonance spectra confirmed the generation of singlet oxygen. The heavy-metal effect of a lanthanide ion in Ln-MOFs for the generation of triplet excitons is confirmed by the synthesis of a non-heavy-metal analogue involving a zinc ion via a single-crystal-to-single-crystal transformation strategy. The present results are noteworthy and may aid in the development of other earth-abundant metalloligand-based photocatalysts for challenging yet sustainable catalysis.

Heterobimetallic 3d-4f complexes supported by a Schiff-base tripodal ligand.

Complexes featuring multiple metal centres are of growing interest regarding metal-metal cooperation and its tuneability. Here the synthesis and characterisation of heterobimetallic complexes of a 3d metal (4: Mn, 5: Co) and lanthanum supported by a (1,1,1-tris[(3-methoxysalicylideneamino)methyl]ethane) ligand is reported, as well as discussion of their electronic structure via electron paramagnetic resonance (EPR) spectroscopy, electrochemical experiments and computational studies. Competitive binding experiments of the ligand and various metal salts unequivocally demonstrate that in these heterobimetallic complexes the 3d metal (Mn, Co) selectively occupies the κ6-N3O3 binding site of the ligand, whilst La occupies the κ6-O6 metal binding site in line with their relative oxophilicities. EPR spectroscopy supported by density functional theory analysis indicates that the 3d metal is high spin in both cases (S = 5/2 (Mn), 3/2 (Co)). Cyclic voltammetry studies on the Mn/La and Co/La bimetallic complexes revealed a quasi-reversible Mn2+/3+ redox process and poorly-defined irreversible oxidation events respectively.

An Isolable Phosphinogermylyne as a Synthon of One‐Coordinate GeI Radical

Comprehensive SummaryReduction of chlorogermylene MsFluindtBu‐GeCl 1 with potassium graphite (KC8) afforded putative germylyne radical MsFluindtBu‐Ge 2 as confirmed by electron paramagnetic resonance (EPR) spectroscopy. However, it slowly decayed via C—H bond activation at the fluorenyl moiety to yield a bis(germylene) 3 at room temperature. By using a Lewis base to stabilize the unoccupied p orbital at the GeI radical center, acyclic two‐coordinate GeI radicals MsFluindtBu‐Ge(IMe4) 4 (IMe4 = 1,3,4,5‐tetramethyl‐imidazolin‐2‐ylidene), MsFluindtBu‐Ge(IiPr) 5 (IiPr = 1,3‐diisopropyl‐4,5‐dimethyl‐imidazolin‐2‐ylidene), MsFluindtBu‐Ge(PMe3) 6 were isolated in crystalline forms. The unpaired electron in 4—6 mainly resides at the Ge 4p orbital as revealed by EPR spectroscopic studies and theoretical calculations. Interestingly, facile ligand exchange of PMe3 in 6 with IMe4 and IiPr was observed to afford 4 and 5, respectively. Moreover, phosphinogermylyne 6 reacted with PhEEPh (E = S, Se), 4‐tetrabutylphenylacetylene (Ar'CCH), [CpMo(CO)3]2 and nBu3SnH to furnish germylenes MsFluindtBu‐GeEPh (E = S 7, Se 8), MsFluindtBu‐GeCH=CHAr’ 9, a germylyne complex MsFluindtBu‐Ge≡Mo(CO)2Cp 10 and a Ge(IV) compound MsFluindtBu‐GeH2SnnBu3 11, respectively. The reactivity studies demonstrate that 6 can act as a synthon of one‐coordinate germylyne radical attributing to labile coordination of trimethylphosphine.

Steering Single-Electron Metal-Metal Bonds and Hyperfine Coupling between a Transition Metal-Lanthanide Heteronuclear Bimetal Confined in Carbon Cages.

Metal complexes bearing single-electron metal-metal bonds (SEMBs) exhibit unusual electronic structures evoking strong magnetic coupling, and such bonds can be stabilized in the form of dimetallofullerenes (di-EMFs) in which two metals are confined in a carbon cage. Up to now, only a few di-EMFs containing SEMBs are reported, which are all based on a high-symmetry icosahedral (Ih) C80 cage embedding homonuclear rare-earth bimetals, and a chemical modification of the Ih-C80 cage is required to stabilize the SEMB. Herein, by introducing 3d-block transition metal titanium (Ti) along with 4f-block lanthanum (La) into the carbon cage, we synthesized the first crystallographically characterized SEMB-containing 3d-4f heteronuclear di-EMFs based on pristine fullerene cages. Four novel La-Ti heteronuclear di-EMFs were isolated, namely, LaTi@D3h(5)-C78, LaTi@Ih(7)-C80, LaTi@D5h(6)-C80, and LaTi@C2v(9)-C82, and their molecular structures were unambiguously determined by single-crystal X-ray diffraction. Upon increasing the cage size from C78 to C82, the La-Ti distance decreases from 4.31 to 3.97 Å, affording fine-tuning of the metal-metal bonding and hyperfine coupling, as evidenced by an electron spin resonance (ESR) spectroscopic study. Density functional theory (DFT) calculations confirm the existence of SEMB in all four LaTi@C2n di-EMFs, and the accumulation of electron density between La and Ti atoms shifts gradually from the proximity of the Ti atom inside C78 to the center of the LaTi bimetal inside C82 due to the decrease of the La-Ti distance. The electronic properties of LaTi@C2n heteronuclear dimetallofullerenes differ apparently from their homonuclear La2@C2n counterparts, revealing the peculiarity of heteronuclear dimetallofullerenes with the involvement of 3d-block transition metal Ti.

N-Sulfenylation of β-Lactams: Radical Reaction of N-Bromo-azetidinones by TEMPO Catalysis.

Azetidinones with a sulfenyl group on the β-lactam nitrogen atom show interesting biological activities as antimicrobial agents and enzyme inhibitors. We report in the present study a versatile synthesis of N-sulfenylated azetidinones starting from the corresponding N-bromo derivatives by means of the (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) radical as the catalyst and disulfides. Preparation of N-halo-azetidinones was studied and optimized. The reactivity of N-bromo-azetidinone 2a as a model compound in the presence of TEMPO radical was investigated by NMR and electron paramagnetic resonance (EPR) spectroscopy studies. Optimization of the reaction conditions allowed the access of N-alkylthio- or N-arylthio-azetidinones from 55 to 92% yields, and the method exhibited a good substrate scope.

Realism-based assessment of the efficacy of potassium peroxymonosulphate on Stenotrophomonas maltophilia biofilm control

The potential of pentapotassium bis(peroxymonosulphate) bis(sulphate) (OXONE) to control biofilms in drinking water distribution systems (DWDS) was evaluated and compared to chlorine disinfection. Mature biofilms of drinking water (DW)-isolated Stenotrophomonas maltophilia were formed using a simulated DWDS with a rotating cylinder reactor (RCR). After 30 min of exposure, OXONE at 10 × minimum bactericidal concentration (MBC) caused a significant 4 log reduction of biofilm culturability in comparison to the unexposed biofilms and a decrease in the number of non-damaged cells below the detection limit (4.8 log cells/cm2). The effects of free chlorine were restricted to approximately 1 log reduction in both biofilm culturability and non-damaged cells. OXONE in synthetic tap water (STW) at 25 ºC was more stable over 40 days than free chlorine in the same conditions. OXONE solution exhibited a disinfectant decrease of about 10% of the initial concentration during the first 9 days, and after this time the values remained stable. Whereas possible reaction of chlorine with inorganic and organic substances in STW contributed to free chlorine depletion of approximately 48% of the initial concentration. Electron paramagnetic resonance (EPR) spectroscopy studies confirmed the presence of singlet oxygen and other free radicals during S. maltophilia disinfection with OXONE. Overall, OXONE constitutes a relevant alternative to conventional DW disinfection for effective biofilm control in DWDS.

A Conserved Second Sphere Residue Tunes Copper Site Reactivity in Lytic Polysaccharide Monooxygenases.

Lytic polysaccharide monooxygenases (LPMOs) are powerful monocopper enzymes that can activate strong C-H bonds through a mechanism that remains largely unknown. Herein, we investigated the role of a conserved glutamine/glutamate in the second coordination sphere. Mutation of the Gln in NcAA9C to Glu, Asp, or Asn showed that the nature and distance of the headgroup to the copper fine-tune LPMO functionality and copper reactivity. The presence of Glu or Asp close to the copper lowered the reduction potential and decreased the ratio between the reduction and reoxidation rates by up to 500-fold. All mutants showed increased enzyme inactivation, likely due to changes in the confinement of radical intermediates, and displayed changes in a protective hole-hopping pathway. Electron paramagnetic resonance (EPR) and X-ray absorption spectroscopic (XAS) studies gave virtually identical results for all NcAA9C variants, showing that the mutations do not directly perturb the Cu(II) ligand field. DFT calculations indicated that the higher experimental reoxidation rate observed for the Glu mutant could be reconciled if this residue is protonated. Further, for the glutamic acid form, we identified a Cu(III)-hydroxide species formed in a single step on the H2O2 splitting path. This is in contrast to the Cu(II)-hydroxide and hydroxyl intermediates, which are predicted for the WT and the unprotonated glutamate variant. These results show that this second sphere residue is a crucial determinant of the catalytic functioning of the copper-binding histidine brace and provide insights that may help in understanding LPMOs and LPMO-inspired synthetic catalysts.