Probing structural and dynamic properties of MAPbCl3 hybrid perovskite using Mn2+ EPR

Hybrid methylammonium (MA) lead halide perovskites have emerged as materials exhibiting excellent photovoltaic performance related to their rich structural and dynamic properties. Here, we use multifrequency (X-, Q-, and W-band) electron paramagnetic resonance (EPR) spectroscopy of Mn2+ impurities in MAPbCl3 to probe the structural and dynamic properties of both the organic and inorganic sublattices of this compound. The temperature dependent continuous-wave (CW) EPR experiments reveal a sudden change of the Mn2+ spin Hamiltonian parameters at the phase transition to the ordered orthorhombic phase indicating its first-order character and significant slowing down of the MA cation reorientation. Pulsed EPR experiments are employed to measure the temperature dependences of the spin-lattice relaxation T1 and decoherence T2 times of the Mn2+ ions in the orthorhombic phase of MAPbCl3 revealing a coupling between the spin center and vibrations of the inorganic framework. Low-temperature electron spin echo envelope modulation (ESEEM) experiments of the protonated and deuterated MAPbCl3 analogues show the presence of quantum rotational tunneling of the ammonium groups, allowing to accurately probe their rotational energy landscape.

Geometry and electronic structure of Yb(III)[CH(SiMe3)2]3 from EPR and solid-state NMR augmented by computations

Characterization of paramagnetic compounds, in particular regarding the detailed conformation and electronic structure, remains a challenge, and - still today it often relies solely on the use of X-ray crystallography, thus limiting the access to electronic structure information. This is particularly true for lanthanide elements that are often associated with peculiar structural and electronic features in relation to their partially filled f-shell. Here, we develop a methodology based on the combined use of state-of-the-art magnetic resonance spectroscopies (EPR and solid-state NMR) and computational approaches as well as magnetic susceptibility measurements to determine the electronic structure and geometry of a paramagnetic Yb(III) alkyl complex, Yb(III)[CH(SiMe3)2]3, a prototypical example, which contains notable structural features according to X-ray crystallography. Each of these techniques revealed specific information about the geometry and electronic structure of the complex. Taken together, both EPR and NMR, augmented by quantum chemical calculations, provide a detailed and complementary understanding of such paramagnetic compounds. In particular, the EPR and NMR signatures point to the presence of three-centre-two-electron Yb-γ-Me-β-Si secondary metal-ligand interactions in this otherwise tri-coordinate metal complex, similarly to its diamagnetic Lu analogues. The electronic structure of Yb(III) can be described as a single 4f13 configuration, while an unusually large crystal-field splitting results in a thermally isolated ground Kramers doublet. Furthermore, the computational data indicate that the Yb-carbon bond contains some π-character, reminiscent of the so-called α-H agostic interaction.

Active Sites in Cr(III)-Based Ethylene Polymerization Catalysts from Machine-Learning-Supported XAS and EPR Spectroscopy

The ethylene polymerization Phillips catalyst has been employed for decades and is central to the polymer industry. While Cr(III) alkyl species are proposed to be the propagating sites, there is so far no direct experimental evidence for such proposal. In this work, by coupling Surface organometallic chemistry, EPR spectroscopy, and machine learning-supported XAS studies, we have studied the electronic structure of well-defined silica-supported Cr(III) alkyls and identified the presence of several surface species in high and low-spin states, associated with different coordination environments. Notably, low-spin Cr(III) sites are shown to participate in ethylene polymerization, indicating that similar Cr(III) alkyl species could be involved in the related Phillips catalyst.

Methane Oxidation over Cu2+ /[CuOH]+ Pairs and Site-Specific Kinetics in Copper Mordenite Revealed by Operando Electron Paramagnetic Resonance and UV/Visible Spectroscopy

Cu-exchanged mordenite (MOR) is a promising material for partial CH4 oxidation. The structural diversity of Cu species within MOR makes it difficult to identify the active Cu sites and to determine their redox and kinetic properties. In this study, the Cu speciation in Cu-MOR materials with different Cu loadings has been determined using operando electron paramagnetic resonance (EPR) and operando ultraviolet-visible (UV/Vis) spectroscopy as well as in situ photoluminescence (PL) and Fourier-transform infrared (FTIR) spectroscopy. A novel pathway for CH4 oxidation involving paired [CuOH]+ and bare Cu2+ species has been identified. The reduction of bare Cu2+ ions facilitated by adjacent [CuOH]+ demonstrates that the frequently reported assumption of redox-inert Cu2+ centers does not generally apply. The measured site-specific reaction kinetics show that dimeric Cu species exhibit a faster reaction rate and a higher apparent activation energy than monomeric Cu2+ active sites highlighting their difference in the CH4 oxidation potential.

Ti-Doping in Silica-Supported PtZn Propane Dehydrogenation Catalysts: From Improved Stability to the Nature of the Pt-Ti Interaction

Propane dehydrogenation is an important industrial reaction to access propene, the world's second most used polymer precursor. Catalysts for this transformation are required to be long living at high temperature and robust toward harsh oxidative regeneration conditions. In this work, combining surface organometallic chemistry and thermolytic molecular precursor approach, we prepared well-defined silica-supported Pt and alloyed PtZn materials to investigate the effect of Ti-doping on catalytic performances. Chemisorption experiments and density functional calculations reveal a significant change in the electronic structure of the nanoparticles (NPs) due to the Ti-doping. Evaluation of the resulting materials PtZn/SiO₂ and PtZnTi/SiO₂ during long deactivation phases reveal a stabilizing effect of Ti in PtZnTi/SiO₂ with a k_d of 0.015 h^-1 compared to PtZn/SiO₂ with a k_d of 0.022 h^-1 over 108 h on stream. Such a stabilizing effect is also present during a second deactivation phase after applying a regeneration protocol to the materials under O₂ and H₂ at high temperatures. A combined scanning transmission electron microscopy, in situ X-ray absorption spectroscopy, electron paramagnetic resonance, and density functional theory study reveals that this effect is related to a sintering prevention of the alloyed PtZn NPs in PtZnTi/SiO₂ due to a strong interaction of the NPs with Ti sites. However, in contrast to classical strong metal-support interaction, we show that the coverage of the Pt NPs with TiO_x species is not needed to explain the changes in adsorption and reactivity properties. Indeed, the interaction of the Pt NPs with Ti^III sites is enough to decrease CO adsorption and to induce a red-shift of the CO band because of electron transfer from the Ti^III sites to Pt⁰.

Multiscale Multimodal Investigation of the Intratissural Biodistribution of Iron Nanotherapeutics with Single Cell Resolution Reveals Co-Localization with Endogenous Iron in Splenic Macrophages

Imaging of iron-based nanoparticles (NPs) remains challenging because of the presence of endogenous iron in tissues that is difficult to distinguish from exogenous iron originating from the NPs. Here, an analytical cascade for characterizing the biodistribution of biomedically relevant iron-based NPs from the organ scale to the cellular and subcellular scales is introduced. The biodistribution on an organ level is assessed by elemental analysis and quantification of magnetic iron by electron paramagnetic resonance, which allowed differentiation of exogenous and endogenous iron. Complementary to these bulk analysis techniques, correlative whole-slide optical and electron microscopy provided spatially resolved insight into the biodistribution of endo- and exogenous iron accumulation in macrophages, with single-cell and single-particle resolution, revealing coaccumulation of iron NPs with endogenous iron in splenic macrophages. Subsequent transmission electron microscopy revealed two types of morphologically distinct iron-containing structures (exogenous nanoparticles and endogenous ferritin) within membrane-bound vesicles in the cytoplasm, hinting at an attempt of splenic macrophages to extract and recycle iron from exogenous nanoparticles. Overall, this strategy enables the distinction of endo- and exogenous iron across scales (from cm to nm, based on the analysis of thousands of cells) and illustrates distribution on organ, cell, and organelle levels.

Probing Methyl Group Tunneling in [(CH3)2NH2][Zn(HCOO)3] Hybrid Perovskite Using Co2+ EPR

At low temperature, methyl groups act as hindered quantum rotors exhibiting rotational quantum tunneling, which is highly sensitive to a local methyl group environment. Recently, we observed this effect using pulsed electron paramagnetic resonance (EPR) in two dimethylammonium-containing hybrid perovskites doped with paramagnetic Mn²⁺ ions. Here, we investigate the feasibility of using an alternative fast-relaxing Co²⁺ paramagnetic center to study the methyl group tunneling, and, as a model compound, we use dimethylammonium zinc formate [(CH₃)₂NH₂][Zn(HCOO)₃] hybrid perovskite. Our multifrequency (X-, Q- and W-band) EPR experiments reveal a high-spin state of the incorporated Co²⁺ center, which exhibits fast spin-lattice relaxation and electron spin decoherence. Our pulsed EPR experiments reveal magnetic field independent electron spin echo envelope modulation (ESEEM) signals, which are assigned to the methyl group tunneling. We use density operator simulations to extract the tunnel frequency of 1.84 MHz from the experimental data, which is then used to calculate the rotational barrier of the methyl groups. We compare our results with the previously reported Mn²⁺ case showing that our approach can detect very small changes in the local methyl group environment in hybrid perovskites and related materials.

Intermolecular contributions, filtration effects and signal composition of SIFTER (single-frequency technique for refocusing)

To characterize structure and molecular order in the nanometre range, distances between electron spins and their distributions can be measured via dipolar spin-spin interactions by different pulsed electron paramagnetic resonance experiments. Here, for the single-frequency technique for refocusing dipolar couplings (SIFTER), the buildup of dipolar modulation signal and intermolecular contributions is analysed for a uniform random distribution of monoradicals and biradicals in frozen glassy solvent by using the product operator formalism for electron spin S = 1 / 2 . A dipolar oscillation artefact appearing at both ends of the SIFTER time trace is predicted, which originates from the weak coherence transfer between biradicals. The relative intensity of this artefact is predicted to be temperature independent but to increase with the spin concentration in the sample. Different compositions of the intermolecular background are predicted in the case of biradicals and in the case of monoradicals. Our theoretical account suggests that the appropriate procedure of extracting the intramolecular dipolar contribution (form factor) requires fitting and subtracting the unmodulated part, followed by division by an intermolecular background function that is different in shape. This scheme differs from the previously used heuristic background division approach. We compare our theoretical derivations to experimental SIFTER traces for nitroxide and trityl monoradicals and biradicals. Our analysis demonstrates a good qualitative match with the proposed theoretical description. The resulting perspectives for a quantitative analysis of SIFTER data are discussed.

Quantifying methyl tunneling induced (de)coherence of nitroxides in glassy ortho-terphenyl at low temperatures

The low-temperature Hahn echo decay signal of the pyrroline-based nitroxide H-mNOHex in ortho- terphenyl (OTP) shows two contributions on distinct time scales. Tunneling of the nitroxide’s methyl groups cause electron...

Bis(imidazolium)-1,3-diphosphete-diide: A Building Block for FeC2 P2 Complexes and Clusters

Reaction of the 6π-electron aromatic four-membered heterocycle (IPr)₂ C₂ P₂ (1) (IPr=1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene) with [Fe₂ CO₉ ] gives the neutral iron tricarbonyl complex [Fe(CO)₃ -η³ -{(IPr)₂ C₂ P₂ }] (2). Oxidation with two equivalents of the ferrocenium salt, [Fe(Cp)₂ ](BArF₂₄ ), affords the dicationic tricarbonyl complex [Fe(CO)₃ -η⁴ -{(IPr)₂ C₂ P₂ }](BArF₂₄ )₂ (4). The one-electron oxidation proceeds under concomitant loss of one CO ligand to give the paramagnetic dicarbonyl radical cation complex [Fe(CO)₂ -η⁴ -{(IPr)₂ C₂ P₂ }](BArF₂₄ ) (5). Reduction of 5 allows the preparation of the neutral dicarbonyl complex [Fe(CO)₂ -η⁴ -{(IPr)₂ C₂ P₂ }] (6). An analysis by various spectroscopic techniques (⁵⁷ Fe Mössbauer, EPR) combined with DFT calculations gives insight into differences of the electronic structure within the members of this unique series of iron carbonyl complexes, which can be either described as electron precise or Wade-Mingos clusters.

A Robust and Efficient Propane Dehydrogenation Catalyst from Unexpectedly Segregated Pt2Mn Nanoparticles

The increasing demand for short chain olefins like propene for plastics production and the availability of shale gas make the development of highly performing propane dehydrogenation (PDH) catalysts, robust toward industrially applied harsh regeneration conditions, a highly important field of research. A combination of surface organometallic chemistry and thermolytic molecular precursor approach was used to prepare a nanometric, bimetallic Pt-Mn material (3 wt % Pt, 1.3 wt % Mn) supported on silica via consecutive grafting of a Mn and Pt precursor on surface OH groups present on the support surface, followed by a treatment under a H₂ flow at high temperature. The material exhibits a 70% fraction of the overall Mn as Mn^II single sites on the support surface; the remaining Mn is incorporated in segregated Pt₂Mn nanoparticles. The material shows great performance in PDH reaction with a low deactivation rate. In particular, it shows outstanding robustness during repeated regeneration cycles, with conversion and selectivity stabilizing at ca. 37 and 98%, respectively. Notably, a material with a lower Pt loading of only 0.05 wt % shows an outstanding catalytic performance─initial productivity of 4523 g_C3H6/g_Pt h and an extremely low k_d of 0.003 h^-1 under a partial pressure of H₂, which are among the highest reported productivities. A combined in situ X-ray absorption spectroscopy, scanning transmission electron microscopy, electron paramagnetic resonance, and metadynamics at the density functional theory level study could show that the strong interaction between the Mn^II-decorated support and the unexpectedly segregated Pt₂Mn particles is most likely responsible for the outstanding performance of the investigated materials.

Cu2+-Induced self-assembly and amyloid formation of a cyclic D,L-α-peptide: structure and function

In a wide spectrum of neurodegenerative diseases, self-assembly of pathogenic proteins to cytotoxic intermediates is accelerated by the presence of metal ions such as Cu²⁺. Only low concentrations of these early transient oligomeric intermediates are present in a mixture of species during fibril formation, and hence information on the extent of structuring of these oligomers is still largely unknown. Here, we investigate dimers as the first intermediates in the Cu²⁺-driven aggregation of a cyclic D,L-α-peptide architecture. The unique structural and functional properties of this model system recapitulate the self-assembling properties of amyloidogenic proteins including β-sheet conformation and cross-interaction with pathogenic amyloids. We show that a histidine-rich cyclic D,L-α-octapeptide binds Cu²⁺ with high affinity and selectivity to generate amyloid-like cross-β-sheet structures. By taking advantage of backbone amide methylation to arrest the self-assembly at the dimeric stage, we obtain structural information and characterize the degree of local order for the dimer. We found that, while catalytic amounts of Cu²⁺ promote aggregation of the peptide to fibrillar structures, higher concentrations dose-dependently reduce fibrillization and lead to formation of spherical particles, showing self-assembly to different polymorphs. For the initial self-assembly step to the dimers, we found that Cu²⁺ is coordinated on average by two histidines, similar to self-assembled peptides, indicating that a similar binding interface is perpetuated during Cu²⁺-driven oligomerization. The dimer itself is found in heterogeneous conformations that undergo dynamic exchange, leading to the formation of different polymorphs at the initial stage of the aggregation process.

Union carbide polymerization catalysts: from uncovering active site structures to designing molecularly-defined analogs

The Union Carbide (UC) ethylene polymerization catalysts, based on chromocene dispersed on silica, show distinct features from the Phillips catalysts, but share the same heated debate regarding the structure of their active sites. Based on a combination of IR, EPR spectroscopies, labeling experiments, and DFT modeling, we identified monomeric surface-supported Cr(iii) hydrides, ( Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> SiO)Cr(Cp)–H, as the active sites of the UC catalyst. These sites are formed in the presence of grafted and adsorbed chromocene as well as residual surface OH groups, only possible at high Cr loading, and involve a C–H activation of the Cp ring. These Cr-hydrides initiate polymerization, yielding Cr(iii) alkyl species that insert ethylene through a Cossee–Arlman-type mechanism, as evidenced by spectroscopic studies. These insights inspired the design of a well-defined analog, CpCr(CH(SiMe₃)₂)₂ grafted on partially dehydroxylated silica, that shows similar spectroscopic and polymer structure to the UC catalyst, further supporting the proposed active site structure.

A combined SOMC and DFT studies on the Union Carbide catalyst enable to propose that Cr(iii)-H, formed at high-loading, are active in ethylene polymerization. We therefore design a well-defined supported CpCr(iii)-R ethylene polymerization catalyst.

Resolving distance variations by single-molecule FRET and EPR spectroscopy using rotamer libraries

Förster resonance energy transfer (FRET) and electron paramagnetic resonance (EPR) spectroscopy are complementary techniques for quantifying distances in the nanometer range. Both approaches are commonly employed for probing the conformations and conformational changes of biological macromolecules based on site-directed fluorescent or paramagnetic labeling. FRET can be applied in solution at ambient temperature and thus provides direct access to dynamics, especially if used at the single-molecule level, whereas EPR requires immobilization or work at cryogenic temperatures but provides data that can be more reliably used to extract distance distributions. However, a combined analysis of the complementary data from the two techniques has been complicated by the lack of a common modeling framework. Here, we demonstrate a systematic analysis approach based on rotamer libraries for both FRET and EPR labels to predict distance distributions between two labels from a structural model. Dynamics of the fluorophores within these distance distributions are taken into account by diffusional averaging, which improves the agreement with experiment. Benchmarking this methodology with a series of surface-exposed pairs of sites in a structured protein domain reveals that the lowest resolved distance differences can be as small as ∼0.25 nm for both techniques, with quantitative agreement between experimental and simulated transfer efficiencies within a range of ±0.045. Rotamer library analysis thus establishes a coherent way of treating experimental data from EPR and FRET and provides a basis for integrative structural modeling, including studies of conformational distributions and dynamics of biological macromolecules using both techniques.

Spectroscopic glimpses of the transition state of ATP hydrolysis trapped in a bacterial DnaB helicase

The ATP hydrolysis transition state of motor proteins is a weakly populated protein state that can be stabilized and investigated by replacing ATP with chemical mimics. We present atomic-level structural and dynamic insights on a state created by ADP aluminum fluoride binding to the bacterial DnaB helicase from Helicobacter pylori. We determined the positioning of the metal ion cofactor within the active site using electron paramagnetic resonance, and identified the protein protons coordinating to the phosphate groups of ADP and DNA using proton-detected 31P,1H solid-state nuclear magnetic resonance spectroscopy at fast magic-angle spinning > 100 kHz, as well as temperature-dependent proton chemical-shift values to prove their engagements in hydrogen bonds. 19F and 27Al MAS NMR spectra reveal a highly mobile, fast-rotating aluminum fluoride unit pointing to the capture of a late ATP hydrolysis transition state in which the phosphoryl unit is already detached from the arginine and lysine fingers.

Spectroscopic Signature and Structure of the Active Sites in Ziegler-Natta Polymerization Catalysts Revealed by Electron Paramagnetic Resonance

Despite decades of extensive studies, the atomic-scale structure of the active sites in heterogeneous Ziegler-Natta (ZN) catalysts, one of the most important processes of the chemical industry, remains elusive and a matter of debate. In the present work, the structure of active sites of ZN catalysts in the absence of ethylene, referred to as dormant active sites, is elucidated from magnetic resonance experiments carried out on samples reacted with increasing amounts of BCl₃ so as to enhance the concentration of active sites and observe clear spectroscopic signatures. Using electron paramagnetic resonance (EPR) and NMR spectroscopies, in particular 2D HYSCORE experiments complemented by density functional theory (DFT) calculations, we show that the activated ZN catalysts contain bimetallic alkyl-Ti(III),Al species whose amount is directly linked to the polymerization activity of MgCl₂-supported Ziegler-Natta catalysts. This connects those spectroscopic signatures to the active species formed in the presence of ethylene and enables us to propose an ethylene polymerization mechanism on the observed bimetallic alkyl-Ti(III),Al species based on DFT computations.

Identification of Kinetic and Spectroscopic Signatures of Copper Sites for Direct Oxidation of Methane to Methanol

Copper-exchanged zeolites of different topologies possess high activity in the direct conversion of methane to methanol via the chemical looping approach. Despite a large number of studies, identification of the active sites, and especially their intrinsic kinetic characteristics remain incomplete and ambiguous. In the present work, we collate the kinetic behavior of different copper species with their spectroscopic identities and track the evolution of various copper motifs during the reaction. Using time-resolved UV/Vis and in situ EPR, XAS, and FTIR spectroscopies, two types of copper monomers were identified, one of which is active in the reaction with methane, in addition to a copper dimeric species with the mono-μ-oxo structure. Kinetic measurements showed that the reaction rate of the copper monomers is somewhat slower than that of the dicopper mono-μ-oxo species, while the activation energy is two times lower.

Methane-to-Methanol on Mononuclear Copper(II) Sites Supported on Al2 O3 : Structure of Active Sites from Electron Paramagnetic Resonance*

The selective conversion of methane to methanol remains one of the holy grails of chemistry, where Cu‐exchanged zeolites have been shown promote this reaction under stepwise conditions. Over the years, several active sites have been proposed, ranging from mono‐, di‐ to trimeric Cu^II. Herein, we report the formation of well‐dispersed monomeric Cu^II species supported on alumina using surface organometallic chemistry and their reactivity towards the selective and stepwise conversion of methane to methanol. Extensive studies using various transition alumina supports combined with spectroscopic characterization, in particular electron paramagnetic resonance (EPR), show that the active sites are associated with specific facets, which are typically found in γ‐ and η‐alumina phase, and that their EPR signature can be attributed to species having a tri‐coordinated [(Al₂O)CuIIO(OH)]⁻ T‐shape geometry. Overall, the selective conversion of methane to methanol, a two‐electron process, involves two monomeric Cu^II sites that play in concert.

Potential-Induced Spin Changes in Fe/N/C Electrocatalysts Assessed by In Situ X-ray Emission Spectroscopy

The commercial success of the electrochemical energy conversion technologies required for the decarbonization of the energy sector requires the replacement of the noble metal-based electrocatalysts currently used in (co-)electrolyzers and fuel cells with inexpensive, platinum-group metal-free analogs. Among these, Fe/N/C-type catalysts display promising performances for the reduction of O₂ or CO₂ , but their insufficient activity and stability jeopardize their implementation in such devices. To circumvent these issues, a better understanding of the local geometric and electronic structure of their catalytic active sites under reaction conditions is needed. Herein we shed light on the electronic structure of the molecular sites in two Fe/N/C catalysts by probing their average spin state with X-ray emission spectroscopy (XES). Chiefly, our in situ XES measurements reveal for the first time the existence of reversible, potential-induced spin state changes in these materials.

Spatiotemporal Resolution of Conformational Changes in Biomolecules by Combining Pulsed Electron-Electron Double Resonance Spectroscopy with Microsecond Freeze-Hyperquenching

The function of proteins is linked to their conformations that can be resolved with several high-resolution methods. However, only a few methods can provide the temporal order of intermediates and conformational changes, with each having its limitations. Here, we combine pulsed electron-electron double resonance spectroscopy with a microsecond freeze-hyperquenching setup to achieve spatiotemporal resolution in the angstrom range and lower microsecond time scale. We show that the conformational change of the C_α-helix in the cyclic nucleotide-binding domain of the Mesorhizobium loti potassium channel occurs within about 150 μs and can be resolved with angstrom precision. Thus, this approach holds great promise for obtaining 4D landscapes of conformational changes in biomolecules.

Molecular and supported Ti(iii)-alkyls: efficient ethylene polymerization driven by the π-character of metal-carbon bonds and back donation from a singly occupied molecular orbital

While Ti(iii) alkyl species are the proposed active sites in Ziegler–Natta ethylene polymerization catalysts, the corresponding well-defined homogeneous catalysts are not known. We report that well-defined neutral β-diiminato Ti(iii) alkyl species, namely [Ti(nacnac)(CH₂^tBu)₂] and its alumina-grafted derivative [(Al_sO)Ti(nacnac)(CH₂^tBu)], are active towards ethylene polymerization at moderate pressures and temperatures and possess an electron configuration well-adapted to insertion of ethylene. Advanced EPR spectroscopy showed that ethylene insertion into a Ti(iii)–C bond takes place during polymerization from Ti(nacnac)(CH₂^tBu)₂. A combination of pulsed EPR spectroscopy and DFT calculations, based on a crystal structure of [Ti(nacnac)(CH₂^tBu)₂], enabled us to reveal details about the structure and electronic configurations of both molecular and surface-grafted species. For both compounds, the α-agostic C–H interaction, which involves the singly occupied molecular orbital, indicates a π character of the metal–carbon bond; this π character is enhanced upon ethylene coordination, leading to a nearly barrier-less C₂H₄ insertion into Ti(iii)–C bonds after this first step. During coordination, back donation from the SOMO to the π*(C₂H₄) occurs, leading to stabilization of π-ethylene complexes and to a significant lowering of the overall energy of the C₂H₄ insertion transition state. In d¹ alkyl complexes, ethylene insertion follows an original “augmented” Cossee–Arlman mechanism that involves the delocalization of unpaired electrons between the SOMO, π*(C₂H₄) and σ*(Ti–C) in the transition state, which further favors ethylene insertion. All these factors facilitate ethylene polymerization on Ti(iii) neutral alkyl species and make d¹ alkyl complexes potentially more effective polymerization catalysts than their d⁰ analogues.

Ti(iii) alkyl species polymerize ethylene via an original mechanism, which involves back donation to the π*(C₂H₄) and a delocalization of the unpaired electron in the transition state of C₂H₄ insertion into the partially alkylidenic Ti(iii)–C bond.

Protein NMR Spectroscopy at 150 kHz Magic-Angle Spinning Continues To Improve Resolution and Mass Sensitivity

Spectral resolution is the key to unleashing the structural and dynamic information contained in NMR spectra. Fast magic-angle spinning (MAS) has recently revolutionized the spectroscopy of biomolecular solids. Herein, we report a further remarkable improvement in the resolution of the spectra of four fully protonated proteins and a small drug molecule by pushing the MAS rotation frequency higher (150 kHz) than the more routinely used 100 kHz. We observed a reduction in the average homogeneous linewidth by a factor of 1.5 and a decrease in the observed linewidth by a factor 1.25. We conclude that even faster MAS is highly attractive and increases mass sensitivity at a moderate price in overall sensitivity.

Magnetic excitation and readout of methyl group tunnel coherence

Methyl groups are ubiquitous in synthetic materials and biomolecules. At sufficiently low temperature, they behave as quantum rotors and populate only the rotational ground state. In a symmetric potential, the three localized substates are degenerate and become mixed by the tunnel overlap to delocalized states separated by the tunnel splitting ν _t . Although ν _t can be inferred by several techniques, coherent superposition of the tunnel-split states and direct measurement of ν _t have proven elusive. Here, we show that a nearby electron spin provides a handle on the tunnel transition, allowing for its excitation and readout. Unlike existing dynamical nuclear polarization techniques, our experiment transfers polarization from the electron spin to methyl proton spins with an efficiency that is independent of the magnetic field and does not rely on an unusually large tunnel splitting. Our results also demonstrate control of quantum states despite the lack of an associated transition dipole moment.

In cell Gd3+-based site-directed spin labeling and EPR spectroscopy of eGFP

A newly synthesized Gd³⁺ chelate complex allows in cell spin labeling and detection of eGFP by EPR spectroscopy.

Alignment for the first precision measurements at Belle II

On March 25th 2019, the Belle II detector recorded the first collisions delivered by the SuperKEKB accelerator. This marked the beginning of the physics run with vertex detector.

The vertex detector was aligned initially with cosmic ray tracks without magnetic field simultaneously with the drift chamber. The alignment method is based on Millepede II and the General Broken Lines track model and includes also the muon system or primary vertex position alignment. To control weak modes, we employ sensitive validation tools and various track samples can be used as alignment input, from straight cosmic tracks to mass-constrained decays.

With increasing luminosity and experience, the alignment is approaching the target performance, crucial for the first physics analyses in the era of Super-BFactories. We will present the software framework for the detector calibration and alignment, the results from the first physics run and the prospects in view of the experience with the first data.

Data quality monitors of vertex detectors at the start of the Belle II experiment

The Belle II experiment features a substantial upgrade of the Belle detector and will operate at the SuperKEKB energy-asymmetric e+e− collider at KEK in Tsukuba, Japan. The accelerator completed its first phase of commissioning in 2016, and the Belle II detector saw its first electron-positron collisions in April 2018. Belle II features a newly designed silicon vertex detector based on double-sided strip layers and DEPFET pixel layers. A subset of the vertex detector was operated in 2018 to determine background conditions (Phase 2 operation). The collaboration completed full detector installation in January 2019, and the experiment started full data taking.

This paper will report on the final arrangement of the silicon vertex detector part of Belle II with a focus on online monitoring of detector conditions and data quality, on the design and use of diagnostic and reference plots, and on integration with the software framework of Belle II. Data quality monitoring plots will be discussed with a focus on simulation and acquired cosmic and collision data.

Accessing distributions of exchange and dipolar couplings in stiff molecular rulers with Cu(ii) centres

Novel approaches to quantitatively analyse distributed exchange couplings are described and tested on experimental data sets for stiff synthetic molecules.

UWB DEER and RIDME distance measurements in Cu(II)-Cu(II) spin pairs

Distance determination by Electron Paramagnetic Resonance (EPR) based on measurements of the dipolar coupling are technically challenging for electron spin systems with broad spectra due to comparatively narrow microwave pulse excitation bandwidths. With Na₄[{Cu^II(PyMTA)}-(stiff spacer)-{Cu^II(PyMTA)}] as a model compound, we compared DEER and RIDME measurements and investigated the use of frequency-swept pulses. We found very large improvements in sensitivity when substituting the monochromatic pump pulse by a frequency-swept one in DEER experiments with monochromatic observer pulses. This effect was especially strong in X band, where nearly the whole spectrum can be included in the experiment. The RIDME experiment is characterised by a trade-off in signal intensity and modulation depth. Optimal parameters are further influenced by varying steepness of the background decay. A simple 2-point optimization experiment was found to serve as good estimate to identify the mixing time of highest sensitivity. Using frequency-swept pulses in the observer sequences resulted in lower SNR in both the RIDME and the DEER experiment. Orientation selectivity was found to vary in both experiments with the detection position as well as with the settings of the pump pulse in DEER. In RIDME, orientation selection by relaxation anisotropy of the inverted spin appeared to be negligible as form factors remain relatively constant with varying mixing time. This reduces the overall observed orientation selection to the one given by the detection position. Field-averaged data from RIDME and DEER with a shaped pump pulse resulted in the same dipolar spectrum. We found that both methods have their advantages and disadvantages for given instrumental limitations and sample properties. Thus the choice of method depends on the situation at hand and we discuss which parameters should be considered for optimization.

Including Protons in Solid-State NMR Resonance Assignment and Secondary Structure Analysis: The Example of RNA Polymerase II Subunits Rpo4/7

¹H-detected solid-state NMR experiments feasible at fast magic-angle spinning (MAS) frequencies allow accessing ¹H chemical shifts of proteins in solids, which enables their interpretation in terms of secondary structure. Here we present ¹H and ¹³C-detected NMR spectra of the RNA polymerase subunit Rpo7 in complex with unlabeled Rpo4 and use the ¹³C, ¹⁵N, and ¹H chemical-shift values deduced from them to study the secondary structure of the protein in comparison to a known crystal structure. We applied the automated resonance assignment approach FLYA including ¹H-detected solid-state NMR spectra and show its success in comparison to manual spectral assignment. Our results show that reasonably reliable secondary-structure information can be obtained from ¹H secondary chemical shifts (SCS) alone by using the sum of ¹H^α and ¹H^N SCS rather than by TALOS. The confidence, especially at the boundaries of the observed secondary structure elements, is found to increase when evaluating ¹³C chemical shifts, here either by using TALOS or in terms of ¹³C SCS.

Non-uniform HYSCORE: Measurement, processing and analysis with Hyscorean

Non-uniform sampling (NUS) provides a considerable reduction of measurement time especially for multi-dimensional experiments. This comes at the cost of additional signal processing steps to reconstruct the complete signal from the experimental data points. Despite being routinely employed in NMR for many experiments, EPR applications have not benefited from NUS due to the lack of a straightforward implementation to perform NUS in common commercial spectrometers. In this work we present a novel method to perform NUS HYSCORE experiments on commercial Bruker EPR spectrometers, along with a benchmark of modern reconstruction methods, and new processing software tools for NUS HYSCORE signals. All of this comes in the form of a free-software package: Hyscorean. Experimental NUS spectra are measured and processed with this package using different reconstruction methods and compared to their uniform sampled counterparts, thereby showcasing the method's potential for EPR spectroscopy.

Structural basis and mechanism for metallochaperone-assisted assembly of the CuA center in cytochrome oxidase

The mechanisms underlying the biogenesis of the structurally unique, binuclear Cu^1.5+•Cu^1.5+ redox center (Cu_A) on subunit II (CoxB) of cytochrome oxidases have been a long-standing mystery. Here, we reconstituted the CoxB•Cu_A center in vitro from apo-CoxB and the holo-forms of the copper transfer chaperones ScoI and PcuC. A previously unknown, highly stable ScoI•Cu²⁺•CoxB complex was shown to be rapidly formed as the first intermediate in the pathway. Moreover, our structural data revealed that PcuC has two copper-binding sites, one each for Cu¹⁺ and Cu²⁺, and that only PcuC•Cu¹⁺•Cu²⁺ can release CoxB•Cu²⁺ from the ScoI•Cu²⁺•CoxB complex. The CoxB•Cu_A center was then formed quantitatively by transfer of Cu¹⁺ from a second equivalent of PcuC•Cu¹⁺•Cu²⁺ to CoxB•Cu²⁺. This metalation pathway is consistent with all available in vivo data and identifies the sources of the Cu ions required for Cu_A center formation and the order of their delivery to CoxB.

Conformational Dynamics of Sensory Rhodopsin II in Nanolipoprotein and Styrene-Maleic Acid Lipid Particles

Styrene-maleic acid lipid particles (SMALPs) provide stable water-soluble nanocontainers for lipid-encased membrane proteins. Possible effects of the SMA-stabilized lipid environment on the interaction dynamics between functionally coupled membrane proteins remain to be elucidated. The photoreceptor sensory rhodopsin II, NpSRII and its cognate transducer, NpHtrII, of Natronomonas pharaonis form a transmembrane complex, NpSRII₂ /NpHtrII₂ that plays a key role in negative phototaxis and provides a unique model system to study the light-induced transfer of a conformational signal between two integral membrane proteins. Photon absorption induces transient structural changes in NpSRII comprising an outward movement of helix F that cause further conformational alterations in NpHtrII. We applied site-directed spin labeling and time-resolved optical and EPR spectroscopy to compare the conformational dynamics of NpSRII₂ /NpHtrII₂ reconstituted in SMALPs with that of nanolipoprotein particle and liposome preparations. NpSRII and NpSRII₂ /NpHtrII₂ show similar photocycles in liposomes and nanolipoprotein particles. An accelerated decay of the M photointermediate found for SMALPs can be explained by a high local proton concentration provided by the carboxylic groups of the SMA polymer. Light-induced large-scale conformational changes of NpSRII₂ /NpHtrII₂ observed in liposomes and nanolipoprotein particles are affected in SMALPs, indicating restrictions of the protein's conformational freedom.

Scalable Biosynthesis of Melanin by the Basidiomycete Armillaria cepistipes

Natural melanin features many interesting properties, including the ability to shield electromagnetic radiation, the ability to act as scavenger for radical and reactive oxygen species and the capacity to chelate different metal ions. For these reasons, melanin is becoming increasingly relevant for the development of functional materials with potential applications in cosmetics, drug delivery, and water purification. However, the extraction and purification of melanin from conventional sources (e.g., sepia ink, hair, and wool) is inefficient and not easily scalable, hence diverting its technological applications. Some fungal species, especially wood-decay basidiomycetes, can be regarded as promising sources of melanin. In the present study, we screened different fungi in regard to their melanin-biosynthesis abilities using l-tyrosine as a precursor, and we found that an Armillaria cepistipes strain (Empa 655) produced the highest yield of melanin (27.98 g L^-1). Physicochemical characterization of the obtained fungal melanin revealed a typical eumelanin structure. The method for the biosynthesis of fungal melanin we propose is efficient, scalable, and sustainable and has the potential to provide support for further technological exploitation.

Electron paramagnetic resonance of a copper doped [(CH3)2NH2][Zn(HCOO)3] hybrid perovskite framework

We present an EPR study of the structural phase transition in a copped doped dimethylammonium zinc formate hybrid perovskite framework.

Highly Efficient UV Protection of the Biomaterial Wood by A Transparent TiO2/Ce Xerogel

Titanium dioxide is widely used in sunscreens because of its strong ultraviolet (UV) light absorbing capabilities and its resistance to discoloration under UV exposure. However, when deposited as a thin film, the high refractive index of titanium dioxide typically results in whiteness and opacity, which limits the use of titanium dioxide for material surfaces, for which long-term natural appearance is of high relevance. Since the whitish appearance is due to the strong light scattering and reflection on the interface of oxide particles and air, one can increase the transparency of TiO₂ coatings by forming a continuous TiO₂ layer. The purpose of the present article is 2-fold. First, we show that, in the presence of cerium ammonium nitrate, titanium dioxide can be turned from a white powder into a TiO₂/Ce xerogel via a facile bottom-up fabrication process. Second, we demonstrate that the transparent TiO₂/Ce xerogel can diminish surface deterioration induced by UV light and preserve the natural appearance of the highly abundant biomaterial wood. Furthermore, EPR spectroscopy revealed that the TiO₂/Ce xerogel coating suppresses free radical generation on wood surfaces upon UV irradiation. Our research expands the applicability of the protective effect of titanium dioxide to coatings for natural engineering materials, which will become increasingly important in future bioeconomies.

Orthogonal Tyrosine and Cysteine Site-Directed Spin Labeling for Dipolar Pulse EPR Spectroscopy on Proteins

Site-directed spin labeling of native tyrosine residues in isolated domains of the protein PTBP1, using a Mannich-type reaction, was combined with conventional spin labeling of cysteine residues. Double electron-electron resonance (DEER) EPR measurements were performed for both the nitroxide-nitroxide and Gd(III)-nitroxide label combinations within the same protein molecule. For the prediction of distance distributions from a structure model, rotamer libraries were generated for the two linker forms of the tyrosine-reactive isoindoline-based nitroxide radical Nox. Only moderate differences exist between the spatial spin distributions for the two linker forms of Nox. This strongly simplifies DEER data analysis, in particular, if only mean distances need to be predicted.

Orthogonal spin labeling using click chemistry for in vitro and in vivo applications

Site-directed spin labeling for EPR- and NMR spectroscopy has mainly been achieved exploiting the specific reactivity of cysteines. For proteins with native cysteines or for in vivo applications, an alternative coupling strategy is required. In these cases click chemistry offers major benefits by providing a fast and highly selective, biocompatible reaction between azide and alkyne groups. Here, we establish click chemistry as a tool to target unnatural amino acids in vitro and in vivo using azide- and alkyne-functionalized spin labels. The approach is compatible with a variety of labels including reduction-sensitive nitroxides. Comparing spin labeling efficiencies from the copper-free with the strongly reducing copper(I)-catalyzed azide-alkyne click reaction, we find that the faster kinetics for the catalyzed reaction outrun reduction of the labile nitroxide spin labels and allow quantitative labeling yields within short reaction times. Inter-spin distance measurements demonstrate that the novel side chain is suitable for paramagnetic NMR- or EPR-based conformational studies of macromolecular complexes.