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https://www.readbyqxmd.com/read/28715026/exchangeable-proton-endor-as-a-probe-of-the-redox-active-iron-center-in-activated-bleomycin-and-ferric-bleomycin
#1
Richard M Burger, Vladimir M Grigoryants, Charles P Scholes
Activated bleomycin (ABLM) is a drug-Fe(iii)-hydroperoxide complex kinetically competent in DNA attack (via H4' abstraction). This intermediate is relatively stable, but its spontaneous conversion to ferric bleomycin (Fe(iii)·BLM) is poorly characterized because no observable intermediate product accumulates. The Fe(iii)·BLM formed cryophotolytically from ABLM and kept at 77 K was remarkably similar by EPR and ENDOR criteria to Fe(iii)·BLM formed from Fe(iii) + BLM solution. The notable ENDOR criteria were the ENDOR frequencies and features of orientation-selected, strongly hyperfine-coupled, exchangeable protons associated with the environs of the iron within <3...
July 17, 2017: Dalton Transactions: An International Journal of Inorganic Chemistry
https://www.readbyqxmd.com/read/28712805/direct-spectroscopic-detection-of-atp-turnover-reveals-mechanistic-divergence-of-abc-exporters
#2
Alberto Collauto, Smriti Mishra, Aleksei Litvinov, Hassane S Mchaourab, Daniella Goldfarb
We have applied high-field (W-band) pulse electron-nuclear double resonance (ENDOR) and electron-electron double resonance (ELDOR)-detected nuclear magnetic resonance (EDNMR) to characterize the coordination sphere of the Mn(2+) co-factor in the nucleotide binding sites (NBSs) of ABC transporters. MsbA and BmrCD are two efflux transporters hypothesized to represent divergent catalytic mechanisms. Our results reveal distinct coordination of Mn(2+) to ATP and transporter residues in the consensus and degenerate NBSs of BmrCD...
July 13, 2017: Structure
https://www.readbyqxmd.com/read/28678489/electronic-delocalization-in-the-radical-cations-of-porphyrin-oligomer-molecular-wires
#3
Martin D Peeks, Claudia E Tait, Patrik Neuhaus, Georg M Fischer, Markus Hoffmann, Renee Haver, Arjen Cnossen, Jeffrey R Harmer, Christiane R Timmel, Harry L Anderson
The radical cations of a family of π-conjugated porphyrin arrays have been investigated: linear chains of N = 1-6 porphyrins, a 6-porphyrin nanoring and a 12-porphyrin nanotube. The radical cations were generated in solution by chemical and electrochemical oxidation, and probed by vis-NIR-IR and EPR spectroscopies. The cations exhibit strong NIR bands at ~1000 nm and 2000-5000 nm which shift to longer wavelength with increasing oligomer length. Analysis of the NIR and IR spectra indicates that the polaron is delocalized over 2-3 porphyrin units in the linear oligomers...
July 5, 2017: Journal of the American Chemical Society
https://www.readbyqxmd.com/read/28668640/spectroscopic-studies-of-the-cytochrome-p450-reaction-mechanisms
#4
REVIEW
Piotr J Mak, Ilia G Denisov
The cytochrome P450 monooxygenases (P450s) are thiolate heme proteins that can, often under physiological conditions, catalyze many distinct oxidative transformations on a wide variety of molecules, including relatively simple alkanes or fatty acids, as well as more complex compounds such as steroids and exogenous pollutants. They perform such impressive chemistry utilizing a sophisticated catalytic cycle that involves a series of consecutive chemical transformations of heme prosthetic group. Each of these steps provides a unique spectral signature that reflects changes in oxidation or spin states, deformation of the porphyrin ring or alteration of dioxygen moieties...
June 28, 2017: Biochimica et Biophysica Acta
https://www.readbyqxmd.com/read/28640584/spectroscopic-evidence-for-a-h-bond-network-at-y356-located-at-the-subunit-interface-of-active-e-coli-ribonucleotide-reductase
#5
Thomas U Nick, Kanchana R Ravichandran, JoAnne Stubbe, Müge Kasanmascheff, Marina Bennati
The reaction catalyzed by E. coli ribonucleotide reductase (RNR) composed of α and β subunits that form an active α2β2 complex is a paradigm for proton-coupled electron transfer (PCET) processes in biological transformations. β2 contains the diferric tyrosyl radical (Y122·) cofactor that initiates radical transfer (RT) over 35 Å via a specific pathway of amino acids (Y122· ⇆ [W48] ⇆ Y356 in β2 to Y731 ⇆ Y730 ⇆ C439 in α2). Experimental evidence exists for colinear and orthogonal PCET in α2 and β2, respectively...
July 7, 2017: Biochemistry
https://www.readbyqxmd.com/read/28628313/tunneling-kinetics-and-nonadiabatic-proton-coupled-electron-transfer-in-proteins-the-effect-of-electric-fields-and-anharmonic-donor-acceptor-interactions
#6
Bridget Salna, Abdelkrim Benabbas, Douglas Russo, Paul M Champion
A proper description of proton donor-acceptor (D-A) distance fluctuations is crucial for understanding tunneling in proton-coupled electron transport (PCET). The typical harmonic approximation for the D-A potential results in a Gaussian probability distribution, which does not appropriately reflect the electronic repulsion forces that increase the energetic cost of sampling shorter D-A distances. Because these shorter distances are the primary channel for thermally activated tunneling, the analysis of tunneling kinetics depends sensitively on the inherently anharmonic nature of the D-A interaction...
July 11, 2017: Journal of Physical Chemistry. B
https://www.readbyqxmd.com/read/28627547/probing-the-electronic-structure-of-a-copper-ii-complex-by-cw-and-pulse-epr-spectroscopy
#7
Zoi G Lada, Yiannis Sanakis, Catherine P Raptopoulou, Vassilis Psycharis, Spyros P Perlepes, George Mitrikas
Nucleophilic attack by the carbanion (-):CH2COCH3 at the carbonyl group of di-2-pyridyl ketone, (py)2CO, in the presence of Cu(II) under moderately basic conditions has yielded the cationic mononuclear complex [Cu{(py)2C(CH2COCH3)(OH)}2](NO3)2·2H2O (1·2H2O) in ∼40% yield, where (py)2C(CH2COCH3)(OH) is the ligand bis(2-pyridine-2-yl)butane-1-ol-3-one. The Cu(II) atom of the cation sits on a crystallographically imposed inversion center. The neutral molecule is coordinated to the metal ion as a tridentate fac chelating ligand through the hydroxyl oxygen atom and two 2-pyridyl nitrogen atoms...
June 19, 2017: Dalton Transactions: An International Journal of Inorganic Chemistry
https://www.readbyqxmd.com/read/28579103/eldor-detected-nmr-a-general-and-robust-method-for-electron-nuclear-hyperfine-spectroscopy
#8
Nicholas Cox, Anna Nalepa, Wolfgang Lubitz, Anton Savitsky
ELDOR-detected NMR (EDNMR) performed at higher magnetic fields is becoming an increasingly popular alternative to conventional ENDOR for the characterization of electron-nuclear hyperfine interactions owing to its enhanced sensitivity. However there are two key problems that limit its widespread adoption, with factors controlling: (i) lineshape distortions and; (ii) overall spectral resolution, still largely understood only at a qualitative level. Indeed highly anisotropic (dipolar) coupled species are particularly problematic in the EDNMR experiment...
July 2017: Journal of Magnetic Resonance
https://www.readbyqxmd.com/read/28571321/epr-endor-and-theoretical-study-of-the-jahn-teller-active-hiptn3n-mo-v-l-complexes-l-n-nh
#9
Ajay Sharma, Michael Roemelt, Michael Reithofer, Richard R Schrock, Brian Hoffman, Frank Neese
The molybdenum trisamidoamine (TAA) complex [Mo] {[3,5-(2,4,6-i-Pr3C6H2)2C6H3NCH2CH2N]Mo} carries out catalytic reduction of N2 to ammonia (NH3) by protons and electrons at room temperature. A key intermediate in the proposed [Mo] nitrogen reduction cycle is nitridomolybdenum(VI), [Mo(VI)]N. The addition of [e(-)/H(+)] to [Mo(VI)]N to generate [Mo(V)]NH might, in principle, follow one of three possible pathways: direct proton-coupled electron transfer; H(+) first and then e(-); e(-) and then H(+). In this study, the paramagnetic Mo(V) intermediate {[Mo]N}(-) and the [Mo]NH transfer product were generated by irradiating the diamagnetic [Mo]N and {[Mo]NH}(+) Mo(VI) complexes, respectively, with γ-rays at 77 K, and their electronic and geometric structures were characterized by electron paramagnetic resonance and electron nuclear double resonance spectroscopies, combined with quantum-chemical computations...
June 1, 2017: Inorganic Chemistry
https://www.readbyqxmd.com/read/28567262/a-magnetic-look-into-the-protecting-layer-of-au25-clusters
#10
Mikhail Agrachev, Sabrina Antonello, Tiziano Dainese, José A Gascón, Fangfang Pan, Kari Rissanen, Marco Ruzzi, Alfonso Venzo, Alfonso Zoleo, Flavio Maran
The field of molecular metal clusters protected by organothiolates is experiencing a very rapid growth. So far, however, a clear understanding of the fine interactions between the cluster core and the capping monolayer has remained elusive, despite the importance of the latter in interfacing the former to the surrounding medium. Here, we describe a very sensitive methodology that enables comprehensive assessment of these interactions. Pulse electron nuclear double resonance (ENDOR) was employed to study the interaction of the unpaired electron with the protons of the alkanethiolate ligands in four structurally related paramagnetic Au25(SR)018 clusters (R = ethyl, propyl, butyl, 2-methylpropyl)...
December 1, 2016: Chemical Science
https://www.readbyqxmd.com/read/28520398/multiple-states-of-nitrile-hydratase-from-rhodococcus-equi-tg328-2-structural-and-mechanistic-insights-from-electron-paramagnetic-resonance-and-density-functional-theory-studies
#11
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, 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 electron paramagnetic resonance (EPR) spectrum due to multiple low-spin (S = (1)/2) Fe(III) species...
June 2, 2017: Biochemistry
https://www.readbyqxmd.com/read/28487404/long-range-proton-coupled-electron-transfer-in-the-escherichia-coli-class-ia-ribonucleotide-reductase
#12
REVIEW
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
https://www.readbyqxmd.com/read/28451129/exploring-template-bound-dinuclear-copper-porphyrin-nanorings-by-epr-spectroscopy
#13
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
https://www.readbyqxmd.com/read/28445998/infrared-raman-and-magnetic-resonance-spectroscopic-study-of-sio2-c-nanopowders
#14
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
https://www.readbyqxmd.com/read/28429421/a-boat-shaped-tetracationic-macrocycle-with-a-semiconducting-organic-framework
#15
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
https://www.readbyqxmd.com/read/28383907/substrate-dependent-cleavage-site-selection-by-unconventional-radical-s-adenosylmethionine-enzymes-in-diphthamide-biosynthesis
#16
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
https://www.readbyqxmd.com/read/28353344/on-the-importance-of-electronic-symmetry-for-triplet-state-delocalization
#17
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
https://www.readbyqxmd.com/read/28222392/endor-with-band-selective-shaped-inversion-pulses
#18
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
https://www.readbyqxmd.com/read/28121140/-13-c-endor-spectroscopy-of-lipoxygenase-substrate-complexes-reveals-the-structural-basis-for-c-h-activation-by-tunneling
#19
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
https://www.readbyqxmd.com/read/28120345/probing-the-electronic-structure-of-bacteriochlorophyll-radical-ions-a-theoretical-study-of-the-effect-of-substituents-on-hyperfine-parameters
#20
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
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