Long Distance Measurements up to 160 Å in the GroEL Tetradecamer Using Q‐Band DEER EPR Spectroscopy

Current distance measurements between spin‐labels on multimeric protonated proteins using double electron–electron resonance (DEER) EPR spectroscopy are generally limited to the 15–60 Å range. Here we show how DEER experiments can be extended to dipolar evolution times of ca. 80 μs, permitting distances up to 170 Å to be accessed in multimeric proteins. The method relies on sparse spin‐labeling, supplemented by deuteration of protein and solvent, to minimize the deleterious impact of multispin effects and substantially increase the apparent spin‐label phase memory relaxation time, complemented by high sensitivity afforded by measurements at Q‐band. We demonstrate the approach using the tetradecameric molecular machine GroEL as an example. Two engineered surface‐exposed mutants, R268C and E315C, are used to measure pairwise distance distributions with mean values ranging from 20 to 100 Å and from 30 to 160 Å, respectively, both within and between the two heptameric rings of GroEL. The measured distance distributions are consistent with the known crystal structure of apo GroEL. The methodology presented here should significantly expand the use of DEER for the structural characterization of conformational changes in higher order oligomers.     

Non‐Thermal Plasma in Contact with Water: The Origin of Species

Non‐thermal atmospheric pressure plasma has attracted considerable attention in recent years due to its potential for biomedical applications. Determining the mechanism of the formation of reactive species in liquid treated with plasma is thus of paramount importance for both fundamental and applied research. In this work, the origin of reactive species in plasma‐treated aqueous solutions was investigated by using spin‐trapping, hydrogen and oxygen isotopic labelling and electron paramagnetic resonance (EPR) spectroscopy. The species originating from molecules in the liquid phase and those introduced with the feed gas were differentiated by EPR and ¹H NMR analysis of liquid samples. The effects of water vapour and oxygen admixtures in the feed gas were investigated. All the reactive species detected in the liquid samples were shown to be formed largely in the plasma gas phase. It is suggested that hydrogen peroxide (determined by UV/Vis analysis) is formed primarily in the plasma tube, whereas the radical species ?OOH, ?OH and ?H are proposed to originate from the region between the plasma nozzle and the liquid sample.     

Reduction of Sulfur Dioxide to Sulfur Monoxide by Ferrous Porphyrin**

The reduction of SO₂ to fixed forms of sulfur can address the growing concerns regarding its detrimental effect on health and the environment as well as enable its valorization into valuable chemicals. The naturally occurring heme enzyme sulfite reductase (SiR) is known to reduce SO₂ to H₂S and is an integral part of the global sulfur cycle. However, its action has not yet been mimicked in artificial systems outside of the protein matrix even after several decades of structural elucidation of the enzyme. While the coordination of SO₂ to transition metals is documented, its reduction using molecular catalysts has remained elusive. Herein reduction of SO₂ by iron(II) tetraphenylporphyrin is demonstrated. A combination of spectroscopic data backed up by theoretical calculations indicate that Fe^IITPP reduces SO₂ by 2e⁻/2H⁺ to form an intermediate [Fe^III−SO]⁺ species, also proposed for SiR, which releases SO. The SO obtained from the chemical reduction of SO₂ could be evidenced in the form of a cheletropic adduct of butadiene resulting in an organic sulfoxide.     

Zero-Field Splittings and Redox Potentials in an Isostructural Series of Dinuclear FeIITiIV,FeIIITiIV, and MnIITiIV Complexes with a Face-Sharing Bridging Motive

The ligand H₆ioan has been used to synthesize the three dinuclear complexes [(ioan)Mn^IITi^IV], [(ioan)Fe^IITi^IV], and [(ioan)Fe^IIITi^IV]⁺. The face-sharing bridging mode of the three phenolates provides short M-Ti^IV distances of ≈3.0 Å. Mössbauer spectra of [(ioan)Fe^IIITi^IV]⁺ show a magnetically split six-line spectrum at 3 K in zero magnetic field demonstrating a slow magnetic relaxation. Magnetic measurements provide a zero-field splitting of |D|=5 cm⁻¹ in [(ioan)Fe^IITi^IV]. EPR spectroscopy demonstrates sizable zero-field splittings of the S=5/2 spin systems of [(ioan)Mn^IITi^IV] (D=0.246 cm⁻¹) and [(ioan)Fe^IIITi^IV]⁺ (D<−1 cm⁻¹) that can be related to enforced covalency of the M-O^ph bonds. [(ioan)Fe^IIITi^IV]⁺ exhibits a reversible reduction at −0.26 V vs. Fc⁺/Fc demonstrating the facile accessibility of Fe^III and Fe^II. In contrast to an irreversible oxidation in [(ioan)Ni^IITi^IV] at 0.78 V vs. Fc⁺/Fc, the reversible oxidation at 0.25 V vs. Fc⁺/Fc in [(ioan)Mn^IITi^IV] indicates even the access of Mn^III. These results indicate that pentanuclear complexes [(ioan)FeM¹M²M¹Fe(ioan)]ⁿ⁺ are meaningful targets to access electron delocalization in mixed-valence systems over five ions due to the facile accessibility of both Fe^II and Fe^III in the terminal positions. This study provides important local spin-Hamiltonian and Mössbauer parameters that will be essential for the understanding of the potentially complicated electronic structure in the anticipated pentanuclear complexes.     

Car-Parrinello molecular dynamics simulations and EPR property calculations on aqueous ubisemiquinone radical anion

Car-Parrinello molecular dynamics (CPMD) simulations have been performed on ubisemiquinone radical anion in aqueous solution. The different types of hydrogen bonding formed between the semiquinone and the solvent were studied in terms of frequency and directionality, in comparison with the parent benzosemiquinone radical anion. The EPR parameters (g-tensors and hyperfine coupling constants) were obtained from quantum chemical property calculations performed on snapshots along the MD trajectory, and the contributions of different solvation effects to the EPR parameters have been evaluated. The influence of the anion’s conformational behaviour was examined, including the orientation-dependent effects of hyperconjugation on side-chain hyperfine coupling.     

Characterisation of Nanohybrids of Porphyrins with Metallic and Semiconducting Carbon Nanotubes by EPR and Optical Spectroscopy

Single‐walled carbon nanotubes (SWCNTs) are noncovalently functionalised with octaethylporphyrins (OEPs) and the resulting nanohybrids are isolated from the free OEPs. Electron paramagnetic resonance (EPR) spectroscopy of cobalt(II)OEP, adsorbed on the nanotube walls by π–π‐stacking, demonstrates that the CNTs act as electron acceptors. EPR is shown to be very effective in resolving the different interactions for metallic and semiconducting tubes. Moreover, molecular oxygen is shown to bind selectively to nanohybrids with semiconducting tubes. Water solubilisation of the porphyrin/CNT nanohybrids using bile salts, after applying a thorough washing procedure, yields solutions in which at least 99 % of the porphyrins are interacting with the CNTs. Due to this purification, we observe, for the first time, the isolated absorption spectrum of the interacting porphyrins, which is strongly red‐shifted compared to the free porphyrin absorption. In addition a quasi‐complete quenching of the porphyrin fluorescence is also observed.     

The extended view on the empty C2(3)-C82 fullerene: isolation, spectroscopic, electrochemical, and spectroelectrochemical characterization and DFT calculations

An extended study of the spectroscopic and redox properties of the C(82) fullerene is presented. Among the nine isolated-pentagon-rule (IPR) isomers of the C(82) fullerene the C(82)(3) isomer with C(2) symmetry is the only stable, empty fullerene structure formed in the arc burning process that can be isolated in an isomerically pure form. Here, its formation and isolation are described and its structure is confirmed by experimental spectroscopic studies as well as time-dependent DFT calculations. The electrochemistry of the C(82)(3) isomer is studied in detail by cyclic voltammetry and spectroelectrochemistry. The anionic species of C(82) with the charge ranging from C(82) (-) to C(82) (4-) were successively generated in o-dichlorobenzene solution at room temperature and characterized by in situ ESR and visible/near-infrared (Vis/NIR) spectroscopy. The data give new insights into the charged states of the C(82)(3) fullerene.