Low-temperature Electron Paramagnetic Resonance Research Articles

4f/5d Hybridization Induced Single-Electron Delocalization in an Azide-Bridged Dicerium Complex.

Dilanthanide complexes with one-electron delocalization are important targets for understanding the specific 4f/5d-bonding feature in lanthanide chemistry. Here, we report an isolable azide-bridged dicerium complex 3 [{(TrapenTMS)Ce}2(μ-N3)]• [Trapen = tris (2-aminobenzyl)amine; TMS = SiMe3], which is synthesized by the reaction of tripodal ligand-supported (TrapenTMS)CeIVCl complex 2 with NaN3. The structure and bonding nature of 3 are fully characterized by X-ray crystal diffraction analysis, electron paramagnetic resonance (EPR), magnetic measurement, cyclic voltammetry, X-ray absorption spectroscopy, and quantum-theoretical studies. Complex 3 presents a trans-bent central Ce-N3-Ce unit with a single electron of two mixed-valent Ce atoms. The unique low-temperature (2 K) anisotropic EPR signals [g = 1.135, 2.003, and 3.034] of 3 indicate that its spin density is distributed on the central Ce-N3-Ce unit with marked electron delocalization. Quantum chemical analyses show strong 4f/5d orbital mixing in the singly occupied molecular orbital of 3, which allows for the unpaired electron to extend throughout the cerium-azide-cerium unit via a multicentered one-electron (Ce-N3-Ce) interaction. This work extends the family of mixed-valent dilanthanide complexes and provides a paradigm for understanding the bonding motif of ligand-bridged dilanthanide complexes.

Hollow AMn2O4-δ (A = Co, Zn, Ni) nanotube for direct photo-oxidation of methane to C1 and C2 alcohols at atmospheric pressure and room temperature

Methane (CH4) conversion is brought into research hotspot in the field of environment, and direct photo-oxidation of methane into C1 and C2 alcohols under moderate conditions remains a huge challenge. Herein, we report a series of hollow AMn2O4-δ (A = Co, Zn, Ni) spinel nanotubes (HNTs) for the direct photo-oxidation of methane (DPOM) reaction. The results of catalytic performance demonstrated that the CH4 conversion and the yields of high value-added liquid products arrange in order of ZnMn2O4-δ HNTs > CoMn2O4-δ HNTs > NiMn2O4-δ HNTs. The ZnMn2O4-δ HNTs exhibited a superior CH4 conversion of 53.9 %, a liquid product yield of 241.7 μmol g−1h−1 (CH3OH 40.1 μmol g−1h−1, CH3CH2OH 200.9 μmol g−1h−1) and a selectivity of 100 % under light irradiation at atmospheric pressure and room temperature conditions. Xray photoelectron spectroscopy (XPS) and low-temperature electron paramagnetic resonance (EPR) analysis showed that the substitution of Zn ion at the A-site of manganate spinel significantly improved surface chemisorbed oxygen (Oα) and oxygen vacancy activity, which were beneficial to the formation of •CH3. Multi-technology photocatalytic activity characterizations results showed that ZnMn2O4-δ HNTs possessed a narrower band gap and greatly favored the separation of photogenerated carriers. Moreover, the DPOM reaction mechanism involving the formation and dehydrogenation of alkyl and alkoxy intermediates was proposed over AMn2O4-δ (A = Zn, Co, Ni) HNTs, which was investigated through temperature-programmed desorption of CH4 (CH4-TPD), in situ EPR, in situ Fourier transform infrared spectroscopy (FT-IR) and density functional theoretical (DFT) calculations. It emphasized that CH3OH was formed via the combination of •CH3 and •OH. And it was more inclined to generate CH3CH2OH through the intermediates *CH2CH3/*CH3O. This study could broaden the avenue toward the application of manganese-based spinel in the direct photo-oxidation of methane into alcohols and offer a disparate perspective on the role of the substitution of metal ion at the A-site in enhancing the photocatalytic performance.

Fabrication of OVs enriched BiOCl microflowers doped with Fe3+ for effective destruction of two typical contaminants

ABSTRACT In this study, a one-step solvothermal method was used to fabricate Fe3+ doped BiOCl microflowers with abundant oxygen vacancies (OVs) in the presence of glacial acetic acid. Various analytical techniques were employed to characterize the structural, morphological, and optical properties of the prepared samples. The presence of OVs was confirmed by low temperature electron paramagnetic resonance (EPR) analysis. The photocatalytic results show that Fe3+ doped BiOCl photocatalysts have higher activity than the bare BiOCl, and 10% Fe3+/BiOCl exhibits the highest photocatalytic performance, the photocatalytic efficiency of this sample is 2.3 and 1.1 times higher than that of the blank BiOCl toward photocatalytic degradation of perfluorooctanoic acid (PFOA) and rhodamine B (RhB), respectively. Furthermore, Fe3+ doped BiOCl demonstrates excellent reusability. Based on the experimental observations, an enhancement mechanism for the photocatalytic activity of Fe3+ doped BiOCl was reasonably elucidated.

Unsaturated Penta-Coordinated Mo5c5+ Sites Enabled Low-Temperature Oxidation of C-H Bonds in Ethers.

Selective oxidation of C-H bonds under mild conditions is one of the most important and challenging issues in utilization of energy-related molecules. Molybdenum oxide nanostructures containing Mo5+ species are effective for these reactions, but the accurate identification of the structure of active Mo5+ species and the catalytic mechanism remain unclear. Herein, unsaturated penta-coordinated Mo5c5+ with a high fraction in MoOx fabricated by the hydrothermal method were identified as the active sites for low-temperature oxidation of dimethyl ether (DME) by the deep correlation of characterizations, density functional theory calculations, and activity results, giving a methyl formate selectivity of 96.3% and DME conversion of 12.5% at unreported 110 °C. Low-temperature electron spin resonance (ESR) and quasi in situ X-ray photoelectron spectra (XPS) with the designed experiments confirm that the Mo5c5+ species can be formed in situ. Molybdenum located at the pentachronic site is preferable to significantly promote the oxidation of the C-H bond in CH3O* at lower temperatures.

Elucidating diatomic catalysis for contaminants degradation: Role of multi-functional Fe/Co sites in peroxydisulfate activation

The application of catalysts with highly available active sites, such as single-atom catalysts (SACs), is a growing trend in peroxysulfate activation for organic contaminant degradation. However, the single functional active site of SACs limits its performance in such reactions involving multiple steps or reactants. For this issue, a Fe-Co diatomic catalyst (FeCo-N-C) with multi-functional active sites is synthesized to abate bisphenol A (BPA) using peroxydisulfate (PDS) as an oxidant. X-ray Absorption Fine Structure (XAFS) measurement confirms the successful construction of diatomic Fe-N4/Co-N4 sites. The FeCo-N-C exhibits a higher normalized reaction rate (2.07 × 105 min−1 mol−1) in BPA removal than previously reported SACs. Low-temperature electron paramagnetic resonance analysis reveals that Fe-N4 and Co-N4 process different intrinsic properties (spin states), which may lead to their diverse functions in the PDS activation process. The Co sites can significantly accelerate electron transfer and enhance the PDS adsorption abilities. As a complement, Fe sites perform better in the electrostatic interaction and destabilization of PDS. This study not only explains the synergy of multi-functional diatomic sites from the whole PDS activation process but provides a reliable reference for the elimination of emerging organic contaminants.

Field-induced Co(II) single-ion magnet in octahedral [S6] coordination environment

The soft donor ligands are of great importance to the pursuit of 3d transition-metal complexes with large magnetic anisotropy. Herein, a soft tripodal ligand, hydrotris(mercaptobenzothiazolyl)borate (Tbz) was used to construct a high-spin mononuclear complex [CoII(Tbz)2] (1), having an octahedral geometry in an S6 donor set. The combination of direct-current (dc) magnetic susceptibility and Low-temperature high-field electron paramagnetic resonance (HFEPR) reveals the easy-axis magnetic anisotropy of 1. Complex 1 shows field-induced slow relaxation of magnetization at an applied magnetic field of 1000 Oe, indicating the behavior of single-molecule magnet. The foregoing result constitutes the first instance of Co(II)-based single-ion magnet containing a CoS6 unit.

Selective C-H Bond Cleavage with a High-Spin FeIV-Oxido Complex.

Non-heme Fe monooxygenases activate C-H bonds using intermediates with high-spin FeIV-oxido centers. To mimic these sites, a new tripodal ligand [pop]3- was prepared that contains three phosphoryl amido groups that are capable of stabilizing metal centers in high oxidation states. The ligand was used to generate [FeIVpop(O)]-, a new FeIV-oxido complex with an S = 2 spin ground state. Spectroscopic measurements, which included low-temperature absorption and electron paramagnetic resonance spectroscopy, supported the assignment of a high-spin FeIV center. The complex showed reactivity with benzyl alcohol as the external substrate but not with related compounds (e.g., ethyl benzene and benzyl methyl ether), suggesting the possibility that hydrogen bonding interaction(s) between the substrate and [FeIVpop(O)]- was necessary for reactivity. These results exemplify the potential role of the secondary coordination sphere in metal-mediated processes.

Trans Ligand Determines the Stability of Paramagnetic Manganese(II) Hydrides of the Type trans-[MnH(L)(dmpe)2]+ Where L is PMe3, C2H4, or CO.

Paramagnetic metal hydride (PMH) complexes play important roles in catalytic applications and bioinorganic chemistry. 3d PMH chemistry has largely focused on Ti, Mn, Fe, and Co. Various MnII PMHs have been proposed as intermediates in catalysis, but isolated MnII PMHs are limited to dimeric high-spin MnII structures with bridging hydrides. In this paper, a series of the first low-spin monomeric MnII PMH complexes are generated by chemical oxidation of their MnI analogues. This series is of the type trans-[MnH(L)(dmpe)2]+/0 where the trans ligand L is PMe3, C2H4, or CO [dmpe is 1,2-bis(dimethylphosphino)ethane], and the thermal stability of the MnII hydride complexes was found to be strongly dependent on the identity of the trans ligand. When L is PMe3, the complex is the first example of an isolated monomeric MnII hydride complex. In contrast, when L is C2H4 or CO, the complexes are only stable at low temperatures; upon warming to room temperature, the former decomposed to afford [Mn(dmpe)3]+, accompanied by ethane and ethylene, whereas the latter eliminated H2, generating [Mn(MeCN)(CO)(dmpe)2]+ or a mixture of products including [Mn(κ1-PF6)(CO)(dmpe)2], depending on the reaction conditions. All PMHs were characterized by low-temperature electron paramagnetic resonance (EPR) spectroscopy, and stable [MnH(PMe3)(dmpe)2]+ was further characterized by UV-vis and IR spectroscopy, Superconducting Quantum Interference Device magnetometry, and single-crystal X-ray diffraction. Noteworthy spectral properties are the significant EPR superhyperfine coupling to the hydride (∼85 MHz) and an increase (+33 cm-1) in the Mn-H IR stretch upon oxidation. Density functional theory calculations were also employed to gain insights into the acidity and bond strengths of the complexes. MnII-H bond dissociation free energies are estimated to decrease in the series of complexes from 60 (L = PMe3) to 47 kcal/mol (L = CO).

Experimental and computational studies of photoelectrochemical degradation of atrazine by modified nanoporous titanium dioxide

The presence of herbicides like Atrazine (ATZ) in groundwater from non-target runoff of the agriculture industry becomes a big concern due to its potential negative impacts on the environment and human health. The use of advanced oxidative processes (AOP) to remove harmful contaminants has been shown to be effective for wastewater treatment. Herein, we report on an advanced photoelectrochemical (PEC) approach based on electrochemically modified nanoporous TiO2 electrode for efficient degradation of ATZ. The electrochemical treated TiO2 electrodes were shown to have a six-fold increase in the photo-current density over the untreated ones. This increase in PEC activity was attributed to the increase in Ti3+ sites after the electrochemical modification, which was corroborated by low-temperature electron paramagnetic resonance (EPR) studies. The removal of ATZ by the PEC process resulted in a rate constant of 1.91 × 10−3 s−1, compared to 3.12 × 10−4 s−1 obtained by a strictly photocatalytic process. Liquid-Chromatography Mass-Spectrometric measurements showed the modified TiO2 electrodes highly effective at removing ATZ, with 96.1% removed after 10 h. Monitoring of the common degradation products desethyl atrazine (DEA), desisopropyl atrazine (DIA) and desethyl desisopropyl atrazine (DDA) revealed very low concentrations throughout the degradation process, indicating that further degradation was achieved. Quantum mechanical-based test for overall free radical scavenging activity (QM-ORSA) computational studies were performed and a mechanism for the N-dealkylation processes of ATZ has been proposed.

Оптические и спиновые свойства вакансионных кремниевых центров, созданных облучением протонами в гетероструктуре карбида кремния 6H/15R

Optically active silicon vacancy defects (VSi) with an electron spin S = 3/2 in a 6H-SiC/15R-SiC silicon carbide heterostructure grown by high-temperature sublimation technique have been studied. By means of low-temperature micro-photoluminescence (µ-PL) and electron paramagnetic resonance (EPR) techniques, we demonstrate the potential to generate five disparate types of VSi centers with distinct spectral properties in the aforementioned heterostructure using proton irradiation with E = 15 MeV. Wherein each type of VSi center is defined by its zero-phonon line (ZPL) and a distinct value of spin sublevel splitting in a zero magnetic field. As a result, we have demonstrated the scalability of the number of optically active spin centers that can be enclosed within a single crystalline matrix.

Optical and spin properties of silicon vacancy centers created by proton irradiation in a 6H/15R silicon carbide heterostructure

Optically active silicon vacancy defects (VSi) with an electron spin S=3/2 in a 6H-SiC/15R-SiC silicon carbide heterostructure grown by high-temperature sublimation technique have been studied. By means of low-temperature micro-photoluminescence (μ-PL) and electron paramagnetic resonance (EPR) techniques, we demonstrate the potential to generate five disparate types of VSi centers with distinct spectral properties in the aforementioned heterostructure using proton irradiation with E=15 MeV. Wherein each type of VSi center is defined by its zero-phonon line (ZPL) and a distinct value of spin sublevel splitting in a zero magnetic field. As a result, we have demonstrated the scalability of the number of optically active spin centers that can be enclosed within a single crystalline matrix. Keywords: silicon carbide, heterostructures, photoluminescence, electron paramagnetic resonance, proton irradiation, spin centers.

Electron spin resonance in P-doped Si nanocrystals/SiC stacked structures with various dot sizes

Doping in semiconductor nanocrystals have a significant impact on electronic structures and the relative studies are needed in order to deeply understand the doping effect in nanoscale. Here, we fabricate un-doped and P-doped Si nanocrystals (Si NCs)/Silicon carbide (SiC) multilayers with various dot sizes (8, 4 and 2 nm). Low temperature (77 K) electron spin resonance spectra are measured to study the electronic structures before and after P doping. It is found that the g value is kept at 2.005 for un-doped Si NCs samples, which is independent of the dots size, indicating the existence of dangling bonds at the surface of Si NCs. After P doping, for samples with dot size of 8 nm, the g value is 1.998, which is assumed to the conduction electrons. It suggests that P impurities are introduced into the inner of Si NCs substitutionally to provide the conduction electrons. However, with the reduction of dot size, the signals with g = 2.004, which is associated with the Si vacancies, are identified in P-doped samples, while the peak-to-peak linewidth ΔBpp is narrowed with the dots size decreasing. It shows the possibility to apply P-doped Si NCs/SiC multilayers with ultra-small dot sizes into Si-based spintronics.

Preparation of La-doped Bi2WO6 with rich oxygen vacancies and enhanced photocatalytic performance for removal of Rhodamine B

• > La doped Bi 2 WO 6 with rich oxygen vacancies was prepared. • > The separation rate of photo-generated carriers is largely enhanced. • > The level of •O 2 - is significantly boosted. • > La-Bi 2 WO 6 exhibits higher activity than the reference Bi 2 WO 6 . To ameliorate the photocatalytic performance of Bi 2 WO 6 , La-doped Bi 2 WO 6 photocatalysts were prepared by a hydrothermal method using glacial acetic acid as solvent. The samples were carefully characterized by serval analytic methods. X ray photoelectron spectroscopy (XPS) and low-temperature electron spin-resonance (ESR) spectroscopy demonstrate that La-doped Bi 2 WO 6 photocatalysts have rich oxygen vacancies (OVs). Elemental mapping and XPS confirm the existence of La in Bi 2 WO 6 . Doping La into Bi 2 WO 6 significantly boosts the level of OVs, separation of photoinduced carriers and the generation of superoxide radical (•O 2 - ). Photocatalytic performance of the samples was evaluated by removal of Rhodamine B (RhB) under simulated sunlight irradiation. The results indicate that the activity of La-Bi 2 WO 6 is higher than the reference Bi 2 WO 6 . When the content of La is 0.6%, the degradation rate constant of RhB on the 0.6% La-Bi 2 WO 6 is 1.83 times of that of the blank Bi 2 WO 6 . Doping La into Bi 2 WO 6 is a reliable approach to promote the photocatalytic activity of Bi 2 WO 6 .

Microscopic Spin Hamiltonian for a Dipolar Heisenberg Magnet LiGdF4 from EPR Measurements

Low-temperature electron paramagnetic resonance measurements are performed on single crystals of LiY1 ‒xGdxF4 with low x = 0.005 and moderate x = 0.05 concentration of Gd ions. Modeling of the experimental spectra allows us to precisely determine microscopic parameters of the spin Hamiltonian of the parent LiGdF4 material, including the nearest-neighbor exchange constant. The obtained parameters are further tested by comparing a strongly anisotropic Curie–Weiss temperature obtained for LiGdF4 in our static magnetization measurements with theoretically computed values. We find a fine balance between principal magnetic interactions in LiGdF4, which results in a hidden magnetic frustration presumably leading to a delayed magnetic ordering and an enhanced magnetocaloric effect at low temperatures.

Biological Assessment of the NO-Dependent Endothelial Function.

Nitric oxide (NO) is implicated in numerous physiological processes, including vascular homeostasis. Reduced NO bioavailability is a hallmark of endothelial dysfunction, a prequel to many cardiovascular diseases. Biomarkers of an early NO-dependent endothelial dysfunction obtained from routine venous blood sampling would be of great interest but are currently lacking. The direct measurement of circulating NO remains a challenge due by its high reactivity and short half-life. The current techniques measure stable products from the NO signaling pathway or metabolic end products of NO that do not accurately represent its bioavailability and, therefore, endothelial function per se. In this review, we will concentrate on an original technique of low temperature electron paramagnetic resonance spectroscopy capable to directly measure the 5-α-coordinated heme nitrosyl-hemoglobin in the T (tense) state (5-α-nitrosyl-hemoglobin or HbNO) obtained from fresh venous human erythrocytes. In humans, HbNO reflects the bioavailability of NO formed in the vasculature from vascular endothelial NOS or exogenous NO donors with minor contribution from erythrocyte NOS. The HbNO signal is directly correlated with the vascular endothelial function and inversely correlated with vascular oxidative stress. Pilot studies support the validity of HbNO measurements both for the detection of endothelial dysfunction in asymptomatic subjects and for the monitoring of such dysfunction in patients with known cardiovascular disease. The impact of therapies or the severity of diseases such as COVID-19 infection involving the endothelium could also be monitored and their incumbent risk of complications better predicted through serial measurements of HbNO.

Laser-assisted rapid synthesis of anatase/rutile TiO2 heterojunction with Function-specified micro-zones for the effective photo-oxidation of sulfamethoxazole

A solid-state laser scanning treatment (SLST) was adopted to synthesize anatase/rutile TiO2 heterojunction with effective charge carrier separation and high surface adsorption performances for the photodegradation of sulfamethoxazole (SMX). The 1064 nm pulsed fiber laser could in-situ generate rutile zones on the pristine sheet-like anatase crystal enclosed by (001) facets, which could form the well-defined anatase/rutile interface to promote the formation of dominating active species (h+ and ∙OH) on the anatase zones. X-ray absorption fine structure (XAFS) measurements, low temperature electron paramagnetic resonance (LT-EPR) as well as the density functional theory (DFT) simulation proved that the oxygen vacancies (VO) could both introduce defective energy states to promote the light absorption and enhance the SMX adsorption on anatase zones. Based on the delicately designed micro-zones, the photodegradation efficiency of the optimized catalyst could reach 99.3 %, which demonstrated ∼7 times higher performance than that of pristine TiO2 and exhibited robust stability under environmental conditions. Gaussian and Vienna Ab Initio Package (VASP) programs were used to provide detailed degradation pathways and mechanism for SMX. This SLST could be potentially employed to create function-specified zones on various catalysts for different applicational aims.

Crystal-field Stark effect on the upconversion light emission spectrum of α−NaYF4 nanoparticles doped with Dy3+, Er3+ , or Yb3+

${\mathrm{NaYF}}_{4}$ nanoparticles (NPs) form in two crystal structures, cubic ($\ensuremath{\alpha}$-phase) and hexagonal ($\ensuremath{\beta}$-phase), each one presenting a different crystal electric field (CEF) Stark effect, that affects the upconversion (UC) light emission of the NPs when doped with rare-earth elements. Therefore, the knowledge of the CEF parameters, the wave functions, and energy levels of the rare earth (RE) $J$-multiplet is expected to be of great help for the understanding and improvement of the UC light emission. In this work, $\ensuremath{\alpha}$-phase ${\mathrm{NaYF}}_{4}$ NPs doped with ${\mathrm{Dy}}^{3+}, {\mathrm{Er}}^{3+}$, or ${\mathrm{Yb}}^{3+}$ were investigated by means of magnetization, electron spin resonance (ESR), and optical spectroscopy techniques. Fittings of the temperature- and magnetic-field-dependent magnetization were performed to determine the fourth- and sixth-order cubic CEF parameters, ${B}_{4}$ and ${B}_{6}$. The ground state of ${\mathrm{Er}}^{3+}, {\mathrm{Yb}}^{3+}$, and ${\mathrm{Dy}}^{3+}$ in these $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{NaYF}}_{4}$ NPs was confirmed by low-temperature ESR experiments. The obtained CEF parameters were used to write down a total Hamiltonian that allows to determine the CEF Stark splitting for all energy levels of the REs $4f$ unfilled shell. We give details of how the Stark effect affects the overall energy splitting of the various $J$-multiplets and may explain the fine structure of the UC light emission in these cubic ${\mathrm{NaY}}_{1\ensuremath{-}\ensuremath{\delta}}{\mathrm{RE}}_{\ensuremath{\delta}}{\mathrm{F}}_{4}$ NPs.

Experimental study on oxygen adsorption capacity and oxidation characteristics of coal samples with different particle sizes

This study is aimed at investigating the effect of particle size on the oxygen adsorption capacity and spontaneous combustion and oxidation of coal, so as to provide guidance for the prevention of spontaneous combustion of residual coal in goafs. First, gas adsorption experiments, low-temperature oxidation (LTO) experiments and electron spin resonance (ESR) experiments were carried out to obtain the oxygen adsorption capacities of coal samples with different particle sizes (less than0.075 mm, 0.075–0.096 mm, 0.096–0.125 mm, 0.125–0.180 mm and 0.180–0.425 mm) and the variations of macroscopic and microscopic characteristic parameters (CO concentration, crossing-point temperature, g-factor, linewidth △H, free radical concentration Ng) during LTO. The results show that the cumulative amount of oxygen adsorption on coal surface presents grows linearly with the rise of adsorption equilibrium pressure. A smaller particle size corresponds to a higher growth rate and larger cumulative amount of oxygen adsorption. At the same temperature, coal samples with smaller particle size generate higher concentrations of CO at higher rates, and the crossing-point temperature decreases with the decrease in particle size. Such a result indicates that coal samples with smaller particle sizes are more capable of oxidizing and self-heating. During LTO, the free radical concentrations of different-sized coal samples increase continuously with the rise of temperature, and the increase differs in different stages. Specifically, after the temperature reaches 150 °C, the rate of coal-oxygen reaction accelerates, and the free radical concentration surges. For the coal sample with the particles size of less than 0.075 mm, the growth of free radical concentration in the temperature range of 150-250 °C accounts for 90.23 % of the total growth. In summary, a smaller particle size and a higher temperature conduce to accelerating the spontaneous combustion and oxidation of coal.

Excited State Magneto-Structural Correlations Related to Photoinduced Electron Spin Polarization

Photoinduced electron spin polarization (ESP) is reported in the ground state of a series of complexes consisting of an organic radical (nitronylnitroxide, NN) covalently attached to a donor-acceptor chromophore either directly or via para-phenylene bridges substituted with 0-4 methyl groups. These molecules represent a class of chromophores that undergo visible light excitation to produce an initial exchange-coupled, three-spin [bpy•-, CAT•+ (= semiquinone, SQ) and NN•], charge-separated doublet 2S1 (S = chromophore spin singlet configuration) excited state that rapidly decays by magnetic exchange-enhanced internal conversion to a 2T1 (T = chromophore excited spin triplet configuration) state. The 2T1 state equilibrates with chromophoric and NN radical-derived excited states, resulting in absorptive ESP of the recovered ground state, which persists for greater than a millisecond and can be measured by low-temperature time-resolved electron paramagnetic resonance spectroscopy. The magnitude of the ground state ESP is found to correlate with the excited state magnetic exchange interaction between the CAT+• and NN• radicals, which in turn is controlled by the structure of the bridge fragment.

Crystal structure of Trypanosoma cruzi heme peroxidase and characterization of its substrate specificity and compound I intermediate

The protozoan parasite Trypanosoma cruzi is the causative agent of American trypanosomiasis, otherwise known as Chagas disease. To survive in the host, the T. cruzi parasite needs antioxidant defense systems. One of these is a hybrid heme peroxidase, the T. cruzi ascorbate peroxidase-cytochrome c peroxidase enzyme (TcAPx-CcP). TcAPx-CcP has high sequence identity to members of the class I peroxidase family, notably ascorbate peroxidase (APX) and cytochrome c peroxidase (CcP), as well as a mitochondrial peroxidase from Leishmania major (LmP). The aim of this work was to solve the structure and examine the reactivity of the TcAPx-CcP enzyme. Low temperature electron paramagnetic resonance spectra support the formation of an exchange-coupled [Fe(IV)=O Trp233•+] compound I radical species, analogous to that used in CcP and LmP. We demonstrate that TcAPx-CcP is similar in overall structure to APX and CcP, but there are differences in the substrate-binding regions. Furthermore, the electron transfer pathway from cytochrome c to the heme in CcP and LmP is preserved in the TcAPx-CcP structure. Integration of steady state kinetic experiments, molecular dynamic simulations, and bioinformatic analyses indicates that TcAPx-CcP preferentially oxidizes cytochrome c but is still competent for oxidization of ascorbate. The results reveal that TcAPx-CcP is a credible cytochrome c peroxidase, which can also bind and use ascorbate in host cells, where concentrations are in the millimolar range. Thus, kinetically and functionally TcAPx-CcP can be considered a hybrid peroxidase.