Spectroscopic and theoretical approaches for studying radical reactions in class I ribonucleotide reductase

Ribonucleotide reductases (RNRs) catalyze the production of deoxyribonucleotides, which are essential for DNA synthesis and repair in all organisms. The three currently known classes of RNRs are postulated to utilize a similar mechanism for ribonucleotide reduction via a transient thiyl radical, but they differ in the way this radical is generated. Class I RNR, found in all eukaryotic organisms and in some eubacteria and viruses, employs a diferric iron center and a stable tyrosyl radical in a second protein subunit, R2, to drive thiyl radical generation near the substrate binding site in subunit R1. From extensive experimental and theoretical research during the last decades, a general mechanistic model for class I RNR has emerged, showing three major mechanistic steps: generation of the tyrosyl radical by the diiron center in subunit R2, radical transfer to generate the proposed thiyl radical near the substrate bound in subunit R1, and finally catalytic reduction of the bound ribonucleotide. Amino acid- or substrate-derived radicals are involved in all three major reactions. This article summarizes the present mechanistic picture of class I RNR and highlights experimental and theoretical approaches that have contributed to our current understanding of this important class of radical enzymes

Temperature Dependence of the Proton Overhauser DNP Enhancements on Aqueous Solutions of Fremy's Salt Measured in a Magnetic Field of 9.2 T

The temperature dependence of the water-proton dynamic nuclear polarization (DNP) enhancement from Fremy's salt nitroxide radicals was measured in a magnetic field of 9.2 T (corresponding to 260 GHz microwave (MW) and 392 MHz NMR frequencies) in the temperature range of 15-65 °C. The temperature could be determined directly from the proton NMR line shift of the sample. Very high DNP enhancements of -38 (signal integral) or -81 (peak intensity) could be achieved with a high-power gyrotron MW source. The experimental findings are compared with classical Overhauser theory for liquids, which is based on the translational and rotational motion of the molecules and with molecular dynamics calculations of the coupling factor. © 2012 Springer-Verlag

Dynamic nuclear polarization at high magnetic fields in liquids

High field dynamic nuclear polarization spectrometer for liquid samples have been constructed. ► The field dependence of the Overhauser DNP efficiency has been measured for the first time up to 9.2 T. ► High DNP enhancements for liquid samples have been observed at high magnetic fields. ► The enhancements have been compared with results from NMRD, MD and theoretical models. ► Coherent and relaxation effects within fast magnetic field changes have been analyzed

High field dynamic nuclear polarization—the renaissance

Sensitivity is a critical issue in NMR spectroscopy, microscopy and imaging, and the factor that often limits the success of various applications. The origin of low sensitivity in NMR is well known to be due to the small magnetic moment of nuclear spins, which yields small Boltzmann polarizations and weak absorption signals. Historically, each advance in technology and methodology that has increased the signal-to-noise in NMR has shifted the boundary of what is achievable, often opening new areas of application and directions of research. The archetypal example of this phenomenon was the introduction of Fourier transform spectroscopy which led to increases of ~10[superscript 2]-fold in signal-to-noise, revolutionizing NMR and many other forms of spectroscopy. More recent technological developments of note include the continuing development of higher field superconducting magnets which increases polarization, and cryoprobes in which the excitation/detection coil is maintained at low temperatures increasing sensitivity through a higher probe Q and decreasing receiver noise. In addition, innovations in NMR methodology have improved sensitivity, classic examples being Hartmann–Hahn cross polarization, and J-coupling meditated transfer methods, and the introduction of 1H detection of [superscript 13]C/[superscript 15]N resonances. Furthermore, techniques for non-inductive detection of resonance, such as the AFM-based technique of magnetic resonance force microscopy (MRFM), have recently allowed observation of a single electron spin, and ~100 nuclear spins/√Hz[superscript 8]

Study of e+e−→ppˉe^+e^- \rightarrow p\bar{p} in the vicinity of ψ(3770)\psi(3770)

Using 2917 $\rm{pb}^{-1}$ of data accumulated at 3.773~$\rm{GeV}$, 44.5~$\rm{pb}^{-1}$ of data accumulated at 3.65~$\rm{GeV}$ and data accumulated during a $\psi(3770)$ line-shape scan with the BESIII detector, the reaction $e^+e^-\rightarrow p\bar{p}$ is studied considering a possible interference between resonant and continuum amplitudes. The cross section of $e^+e^-\rightarrow\psi(3770)\rightarrow p\bar{p}$, $\sigma(e^+e^-\rightarrow\psi(3770)\rightarrow p\bar{p})$, is found to have two solutions, determined to be ($0.059\pm0.032\pm0.012$) pb with the phase angle $\phi = (255.8\pm37.9\pm4.8)^\circ$ ($<$0.11 pb at the 90% confidence level), or $\sigma(e^+e^-\rightarrow\psi(3770)\rightarrow p\bar{p}) = (2.57\pm0.12\pm0.12$) pb with $\phi = (266.9\pm6.1\pm0.9)^\circ$ both of which agree with a destructive interference. Using the obtained cross section of $\psi(3770)\rightarrow p\bar{p}$, the cross section of $p\bar{p}\rightarrow \psi(3770)$, which is useful information for the future PANDA experiment, is estimated to be either ($9.8\pm5.7$) nb ($<17.2$ nb at 90% C.L.) or $(425.6\pm42.9)$ nb

Probing Flexibility in Porphyrin-Based Molecular Wires Using Double Electron Electron Resonance

A series of butadiyne-linked zinc porphyrin oligomers, with one, two, three, and four porphyrin units and lengths of up to 75 angstrom, have been spin-labeled at both ends with stable nitroxide TEMPO radicals. The pulsed EPR technique of double electron electron resonance (DEER) was used to probe the distribution of intramolecular end-to-end distances, under a range of conditions. DEER measurements were carried out at 50 K in two types of dilute solution glasses: deutero-toluene (with 10% deutero-pyridine) and deutero-o-terphenyl (with 5% 4-benzyl pyridine). The complexes of the porphyrin oligomers with monodentate ligands (pyridine or 4-benzyl pyridine) principally adopt linear conformations. Nonlinear conformations are less populated in the lower glass-transition temperature solvent. When the oligomers bind star-shaped multidentate ligands, they are forced to bend into nonlinear geometries, and the experimental end-to-end distances for these complexes match those from molecular mechanics calculations. Our results show that porphyrin-based molecular wires are shape-persistent, and yet that their shapes can deformed by binding to multivalent ligands. Self-assembled ladder-shaped 2:2 complexes were also investigated to illustrate the scope of DEER measurements for providing structural information on synthetic noncovalent nanostructures

Broadband Ferromagnetic Resonance Spectrometer : Instrument and Applications

This thesis compiles results of research in two mutuallydependent parts: 1) development of ferromagnetic resonance(FMR) spectrometer to study microwave properties offerromagnetic materials, and 2) characterization of new irongarnets: pulsed laser deposited Y3Fe5O12and Bi3Fe5O12films and Ce:Y3Fe5O12single crystal. First part describes a novelBroadbandFMRSpectrometerdesigned to characterize thin ferromagneticfilms. The spectrometer uses two probeheads: one is the X-bandmicrowave reflection cavity for room temperature measurementsand the in-cryostat microstrip line probe to perform FMRexperiments in the frequency range from 50 MHz to 40 GHz. Veryuniform and stable magnetic field up to 2.4 T, temperatures 4 Kto 420 K, and continuous frequency scan performed byHP8722Dvector network analyzer provide various modes ofoperation. Both probeheads are equipped with two-circlegoniometers to ensure accurate study of magneticanisotropy. The spectrometer was used to make express-analysis ofquality thus to optimize processing parameters of epitaxialiron garnet films grown by pulsed laser deposition (PLD).Comprehensive study of uniaxial and cubic magnetocrystallineanisotropy has been performed for Ce:Y3Fe5O12bulk crystal as well as for Y3Fe5O12and Bi3Fe5O12films grown on different substrates by PLD andreactive ion beam sputtering techniques. BroadbandFMR-spectroscopy revealed difference in spectra of domain wallresonances: instead of\u93soft\u94spin modes in filmsgrown by liquid phase epitaxy, PLD-made films show\u93diffuse\u94transformation of domains near thesaturation field. This effect indicates non-uniformity ofsaturation magnetization and field of uniaxial anisotropy inPLD-iron garnets. Spin wave resonances in comparison withuniform FMR have been studied to evaluate\u93localquality\u94of ferromagnetic films. The resonance field andFMR linewidth behavior were studied at various crystallographicdirections determined by X-ray diffraction. FMR was used to choose PLD-made YIG films with low losses atmicrowave frequencies and to build magnetostatic surface wavesmicrowave bandpass filter. The filter was designed as a planarfilm structure with a microstrip line for transducers. It is afirst demonstration of feasibility to introduce PLD processingtechnique to magnetostatic wave technology. Magneto-optical study of Ce:Y3Fe5O12single crystal complements results ofFMR-spectroscopy of new garnets. Keywords:ferrites, thin films, ferromagnetic resonance,microwaves, FMR spectrometer, magnetic anisotropy,magnetostatic waves.NR 20140805</p