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Natalia Stein, Natalie Gumataotao, Natalia Hajnas, Rui Wu, K P Wasantha Lankathilaka, Uwe T Bornscheuer, Dali Liu, Adam T Fiedler, Richard C Holz, Brian Bennett
Iron-type nitrile hydratases (NHases) contain an Fe(III) ion coordinated in a characteristic "claw setting" by an axial cysteine thiolate; two equatorial peptide nitrogens; the sulfur atoms of equatorial cysteine-sulfenic and cysteine-sulfinic acids, respectively; and an axial water/hydroxyl moiety. The cysteine-sulfenic acid is susceptible to oxidation and the enzyme is traditionally prepared using butyric acid as an oxidative protectant. The as-prepared enzyme exhibits a complex EPR spectrum due to multiple low-spin (S = 1/2) Fe(III) species...
May 18, 2017: Biochemistry
Steven Y Reece, Mohammad R Seyedsayamdost
Escherichia coli class Ia ribonucleotide reductase (RNR) catalyzes the conversion of nucleotides to 2'-deoxynucleotides using a radical mechanism. Each turnover requires radical transfer from an assembled diferric tyrosyl radical (Y•) cofactor to the enzyme active site over 35 Å away. This unprecedented reaction occurs via an amino acid radical hopping pathway spanning two protein subunits. To study the mechanism of radical transport in RNR, a suite of biochemical approaches have been developed, such as site-directed incorporation of unnatural amino acids with altered electronic properties and photochemical generation of radical intermediates...
May 9, 2017: Essays in Biochemistry
Sabine Richert, Jonathan Cremers, Harry L Anderson, Christiane R Timmel
Electron paramagnetic resonance (EPR) spectroscopy has been used to study the molecular geometry as well as metal-ligand interactions in ten-membered porphyrin nanorings ( c-P10Cu2 ) containing two copper and eight zinc centers. The presence of copper in the structures allows intramolecular interactions, including dipolar interactions between electron spins and hyperfine interactions to be quantified. Results obtained for c-P10Cu2 samples bound to two molecular templates with four or five binding sites, respectively, are compared to those obtained for a sample of the porphyrin ring in the absence of any templates...
December 1, 2016: Chemical Science
Dariya Savchenko, Vladimir Vorliček, Ekaterina Kalabukhova, Aleksandr Sitnikov, Andrii Vasin, Dmytro Kysil, Stanislav Sevostianov, Valentyn Tertykh, Alexei Nazarov
Optical and magnetic properties of SiO2:C nanopowders obtained by chemical and thermal modification of fumed silica were studied by Fourier transform infrared spectroscopy, Raman, continuous wave (CW) electron paramagnetic resonance (EPR), echo-detected EPR and pulsed electron nuclear double resonance (ENDOR) spectroscopy. Two overlapping signals of Lorentzian lineshape were detected in CW EPR spectra of the initial SiO2:C. The EPR signal at g = 2.0055(3) is due to the silicon dangling bonds, which vanishes after thermal annealing, and the second EPR signal at g = 2...
December 2017: Nanoscale Research Letters
Minh T Nguyen, Matthew D Krzyaniak, Magdalena Owczarek, Daniel P Ferris, Michael R Wasielewski, J Fraser Stoddart
We report the synthesis of a tetracationic macrocycle which contains two N,N'-bis(methylene)naphthalenediimide units inserted in between the pyridinium rings of the bipyridinium units in cyclobis(paraquat-p-phenylene) (CBPQT(4+) or "blue box") and describe the investigation of its potential use in materials for organic electronics. The incorporation of the two naphthalenediimide (NDI) units into the constitution of CBPQT(4+) , not only changes the supramolecular properties of the tetracation in the solid state, but also has a profound influence on the electrochemical and electronic behavior of the resulting tetracationic macrocycle...
April 21, 2017: Angewandte Chemie
Min Dong, Masaki Horitani, Boris Dzikovski, Jack H Freed, Steven E Ealick, Brian M Hoffman, Hening Lin
S-Adenosylmethionine (SAM) has a sulfonium ion with three distinct C-S bonds. Conventional radical SAM enzymes use a [4Fe-4S] cluster to cleave homolytically the C5',adenosine-S bond of SAM to generate a 5'-deoxyadenosyl radical, which catalyzes various downstream chemical reactions. Radical SAM enzymes involved in diphthamide biosynthesis, such as Pyrococcus horikoshii Dph2 (PhDph2) and yeast Dph1-Dph2 instead cleave the Cγ,Met-S bond of methionine to generate a 3-amino-3-carboxylpropyl radical. We here show radical SAM enzymes can be tuned to cleave the third C-S bond to the sulfonium sulfur by changing the structure of SAM...
April 13, 2017: Journal of the American Chemical Society
Sabine Richert, George Bullard, Jeff Rawson, Paul J Angiolillo, Michael J Therien, Christiane R Timmel
The influence of electronic symmetry on triplet state delocalization in linear zinc porphyrin oligomers is explored by electron paramagnetic resonance techniques. Using a combination of transient continuous wave and pulse electron nuclear double resonance spectroscopies, it is demonstrated experimentally that complete triplet state delocalization requires the chemical equivalence of all porphyrin units. These results are supported by density functional theory calculations, showing uneven delocalization in a porphyrin dimer in which a terminal ethynyl group renders the two porphyrin units inequivalent...
April 19, 2017: Journal of the American Chemical Society
Claudia E Tait, Stefan Stoll
Electron Nuclear DOuble Resonance (ENDOR) is based on the measurement of nuclear transition frequencies through detection of changes in the polarization of electron transitions. In Davies ENDOR, the initial polarization is generated by a selective microwave inversion pulse. The rectangular inversion pulses typically used are characterized by a relatively low selectivity, with full inversion achieved only for a limited number of spin packets with small resonance offsets. With the introduction of pulse shaping to EPR, the rectangular inversion pulses can be replaced with shaped pulses with increased selectivity...
April 2017: Journal of Magnetic Resonance
Masaki Horitani, Adam R Offenbacher, Cody A Marcus Carr, Tao Yu, Veronika Hoeke, George E Cutsail, Sharon Hammes-Schiffer, Judith P Klinman, Brian M Hoffman
In enzymatic C-H activation by hydrogen tunneling, reduced barrier width is important for efficient hydrogen wave function overlap during catalysis. For native enzymes displaying nonadiabatic tunneling, the dominant reactive hydrogen donor-acceptor distance (DAD) is typically ca. 2.7 Å, considerably shorter than normal van der Waals distances. Without a ground state substrate-bound structure for the prototypical nonadiabatic tunneling system, soybean lipoxygenase (SLO), it has remained unclear whether the requisite close tunneling distance occurs through an unusual ground state active site arrangement or by thermally sampling conformational substates...
February 8, 2017: Journal of the American Chemical Society
Sebastian Sinnecker, Wolfgang Lubitz
In reaction centers (RCs) of photosynthesis a light-induced charge separation takes place creating radical cations and anions of the participating cofactors. In photosynthetic bacteria different bacteriochlorophylls (BChl) are involved in this process. Information about the electronic structure of the BChl radical cations and anions can be obtained by measuring the electron spin density distribution via the electron-nuclear hyperfine interaction using EPR and ENDOR techniques. In this communication we report isotropic hyperfine coupling constants (hfcs) of the BChl b and g radical cations and anions, calculated by density functional theory, and compare them with the more common radical ions of BChl a and with available experimental data...
January 25, 2017: Photochemistry and Photobiology
Christian Hintze, Patrick Korf, Frank Degen, Friederike Schütze, Stefan Mecking, Ulrich E Steiner, Malte Drescher
In this work, the triplet state delocalization in a series of monodisperse oligo(p-phenyleneethynylene)s (OPEs) is studied by pulsed electron paramagnetic resonance (EPR) and pulsed electron nuclear double resonance (ENDOR) determining zero-field splitting, optical spin polarization, and proton hyperfine couplings. Neither the zero-field splitting parameters nor the optical spin polarization change significantly with OPE chain length, in contrast to the hyperfine coupling constants, which showed a systematic decrease with chain length n according to a 2/(1 + n) decay law...
January 24, 2017: Journal of Physical Chemistry Letters
Hiroki Nagashima, Hiroyuki Mino
Proton matrix electron nuclear double resonance (ENDOR) spectroscopy was performed to specify the location of the methanol molecule near the manganese cluster in photosystem II. Comparison of the ENDOR spectra in the presence of CH3OH and CD3OH revealed two pairs of hyperfine couplings, 1.2 MHz for A⊥ and 2.5 MHz for A//, arising from the methyl group in methanol. On the basis of the crystal structure, the possible location of methanol close to the manganese cluster was discussed.
January 20, 2017: Journal of Physical Chemistry Letters
Eelco C Brand, Vincent K Dik, Martijn G H van Oijen, Peter D Siersema
BACKGROUND AND AIMS: The Third Eye Retroscope, Full Spectrum Endoscope (FUSE), and EndoRings devices have been shown to reduce overall adenoma miss rates. We evaluated the characteristics of adenomas and patient subgroups for which these behind-folds visualizing technologies mostly reduce adenoma miss rates. METHODS: Data of 3 multicenter randomized trials (NCT01044732, NCT01549535, NCT01955122) were combined. Patients underwent same-day, back-to-back tandem examinations with standard colonoscopy and Third Eye Retroscope, FUSE, or EndoRings colonoscopy, respectively...
January 6, 2017: Gastrointestinal Endoscopy
Sung Jae Shin
Early detection and removal of adenomatous polyps can prevent the development of colorectal cancer. However, it is fairly common-up to 20%-for polyps to be undetected in a colonoscopy due to poor visualization of the proximal aspect of colonic folds and anatomical flexures. To overcome these limitations, many new endoscopes and accessories have been developed. A wide-angle colonoscopy did not improve the detection of adenoma compared with the standard colonoscopy. An extra-wide angle and Retroview colonoscopies showed a significantly lower miss rate of polyps in the colon model...
December 25, 2016: Korean Journal of Gastroenterology, Taehan Sohwagi Hakhoe Chi
Amy R Marts, Joshua C Kaine, Robert R Baum, Vivien L Clayton, Jami R Bennett, Laura J Cordonnier, Robert McCarrick, Abed Hasheminasab, Laura A Crandall, Christopher J Ziegler, David L Tierney
Paramagnetic resonance studies (EPR, ESEEM, ENDOR, and NMR) of a series of cobalt(II) bis-trispyrazolylmethane tetrafluoroborates are presented. The complexes studied include the parent, unsubstituted ligand (Tpm), two pyrazole-substituted derivatives (4Me and 3,5-diMe), and tris(1-pyrazolyl)ethane (Tpe), which includes a methyl group on the apical carbon atom. NMR and ENDOR establish the magnitude of (1)H hyperfine couplings, while ESEEM provides information on the coordinated (14)N. The data show that the pyrazole 3-position is more electron rich in the Tpm analogues, that the geometry about the apical atom influences the magnetic resonance, and that apical atom geometry appears more fixed in Tpm than in Tp...
January 3, 2017: Inorganic Chemistry
Stefan Pelties, Emma Carter, Andrea Folli, Mary F Mahon, Damien M Murphy, Michael K Whittlesey, Robert Wolf
Potassium graphite reduction of the half-sandwich Ni(II) ring-expanded diamino/diamidocarbene complexes CpNi(RE-NHC)Br gave the Ni(I) derivatives CpNi(RE-NHC) (where RE-NHC = 6-Mes (1), 7-Mes (2), 6-MesDAC (3)) in yields of 40%-50%. The electronic structures of paramagnetic 1-3 were investigated by CW X-/Q-band electron paramagnetic resonance (EPR) and Q-band (1)H electron nuclear double resonance (ENDOR) spectroscopy. While small variations in the g-values were observed between the diaminocarbene complexes 1 and 2, pronounced changes in the g-values were detected between the almost isostructural species (1) and diamidocarbene species (3)...
November 7, 2016: Inorganic Chemistry
Katharina Keller, Michal Zalibera, Mian Qi, Vanessa Koch, Julia Wegner, Henrik Hintz, Adelheid Godt, Gunnar Jeschke, Anton Savitsky, Maxim Yulikov
The four Mn(ii) complexes Mn-DOTA, Mn-TAHA, Mn-PyMTA, and Mn-NO3Py were characterized by electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR), and relaxation measurements, to predict their relative performance in the EPR pulse dipolar spectroscopy (PDS) experiments. High spin density localization on the metal ions was proven by ENDOR on (1)H, D, (14)N, and (55)Mn nuclei. The transverse relaxation of the Mn(ii) complexes appears to be slow enough for PDS-based spin-spin distance determination...
September 14, 2016: Physical Chemistry Chemical Physics: PCCP
Marilena Di Valentin, Maria Giulia Dal Farra, Laura Galazzo, Marco Albertini, Tim Schulte, Eckhard Hofmann, Donatella Carbonera
Triplet-triplet energy transfer from chlorophylls to carotenoids is the mechanism underlying the photoprotective role played by carotenoids in many light harvesting complexes, during photosynthesis. The peridinin-chlorophyll-a protein (PCP) is a water-soluble light harvesting protein of the dinoflagellate Amphidinium carterae, employing peridinin as the main carotenoid to fulfil this function. The dipolar coupling of the triplet state of peridinin, populated under light excitation in isolated PCP, to the MTSSL nitroxide, introduced in the protein by site-directed mutagenesis followed by spin labeling, has been measured by Pulse ELectron-electron DOuble Resonance (PELDOR) spectroscopy...
December 2016: Biochimica et Biophysica Acta
Chang Sun, Alexander T Taguchi, Josh V Vermaas, Nathan J Beal, Patrick J O'Malley, Emad Tajkhorshid, Robert B Gennis, Sergei A Dikanov
The respiratory cytochrome bo3 ubiquinol oxidase from Escherichia coli has a high-affinity ubiquinone binding site that stabilizes the one-electron reduced ubisemiquinone (SQH), which is a transient intermediate during the electron-mediated reduction of O2 to water. It is known that SQH is stabilized by two strong hydrogen bonds from R71 and D75 to ubiquinone carbonyl oxygen O1 and weak hydrogen bonds from H98 and Q101 to O4. In this work, SQH was investigated with orientation-selective Q-band (∼34 GHz) pulsed (1)H electron-nuclear double resonance (ENDOR) spectroscopy on fully deuterated cytochrome (cyt) bo3 in a H2O solvent so that only exchangeable protons contribute to the observed ENDOR spectra...
October 11, 2016: Biochemistry
Hazel M Girvan, Justin M Bradley, Myles R Cheesman, James R Kincaid, Yilin Liu, Kazimierz Czarnecki, Karl Fisher, David Leys, Stephen E J Rigby, Andrew W Munro
DGCR8 is the RNA-binding partner of the nuclease Drosha. Their complex (the "Microprocessor") is essential for processing of long, primary microRNAs (pri-miRNAs) in the nucleus. Binding of heme to DGCR8 is essential for pri-miRNA processing. On the basis of the split Soret ultraviolet-visible (UV-vis) spectrum of ferric DGCR8, bis-thiolate sulfur (cysteinate, Cys(-)) heme iron coordination of DGCR8 heme iron was proposed. We have characterized DGCR8 heme ligation using the Δ276 DGCR8 variant and combined electron paramagnetic resonance (EPR), magnetic circular dichroism (MCD), electron nuclear double resonance, resonance Raman, and electronic absorption spectroscopy...
September 13, 2016: Biochemistry
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