Q-Band Pulsed Electron Spin-Echo Spectrometer and Its Application to ENDOR and ESEEM

Davoust, Clark E.; Doan, Peter E.; Hoffman, Brian M.

doi:10.1006/jmra.1996.0049

Cited by 141 publications

(166 citation statements)

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“…Relaxation rates for vanadyl porphyrin doped into zinc porphyrin below about 50 K are somewhat faster at W-band than at X-band, which suggests a larger contribution from the direct process at the higher microwave frequency. Hoffman reported (Davoust et a!., 1996) that TJ is shorter at Q-band than at X-band at 2-4 K for ferredoxin and nitrile hydratase. Mizoguchi and coworkers (1984) found that relaxation in polyacetylene as a function of frequency from 5 to 450 MHz fit the square root of frequency relation,…”

Section: Tl For Immobilized Samplesmentioning

confidence: 98%

Relaxation Times of Organic Radicals and Transition Metal Ions

Eaton

2002

Distance Measurements in Biological Systems by EPR

152

165

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Abstract:Review of electron spin relaxation times for organic radicals and transition metal ions in magnetically dilute samples. Emphasis is placed on studies that have been performed as a function of temperature and that provide insight into the relaxation processes. INTRODUCTION Scope of this ChapterMost of the methods for determining distances between spins (ch.l) are either experimentally feasible or theoretically applicable for a limited range of electron spin relaxation times. For example, analysis of the dipolar contribution to the CW line shape to determine interspin distance requires that the relaxation rate for the interacting partner is slow enough that the dipolar splitting is not collapsed by electron spin relaxation. In addition, analysis of the line shapes assumes that spectra are obtained under nonsaturating conditions and are free from passage effects. Application of methods based on changes in relaxation times requires that the relaxation rate of the interacting partner fall within certain limits. The longest distance that can be obtained by pulsed techniques is limited, in principle, by the relaxation-determined width of an individual spin packet. Thus, knowledge of electron spin relaxation times is foundational to the methodologies presented in this book.

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Section: Tl For Immobilized Samplesmentioning

confidence: 98%

Relaxation Times of Organic Radicals and Transition Metal Ions

Eaton

2002

Distance Measurements in Biological Systems by EPR

152

165

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“…The first approach, used in the homemade spectrometers, consisted of constructing a stand-alone instrument, where the transmitter and the receiver operated completely in the K a band (4,10). A different design was implemented by Bruker in their E580-Q spectrometer (14), where the microwave pulses formed in X band were upconverted to the K a band, while the transients generated in the K a band were downconverted back to the X band and detected by the X-band receiver.…”

Section: Design Highlightsmentioning

confidence: 99%

“…This prejudice was mainly caused by the fact that, when thinking about ESEEM, one often had in mind the 1 H ESEEM, which is already somewhat underexcited even at X band and becomes increasingly difficult to excite with an increase of the operational frequency (1). As a result, the first pulsed K a -band spectrometers were simple in design and were used only for relaxation measurements (2,3) The first full-fledged pulsed K a -band (ϳ35 GHz) spectrometer was constructed, to the best of our knowledge, only about a decade ago by Davoust and colleagues (4). However, although that spectrometer could be used for ESEEM measurements, its main intended application was pulsed electron nuclear double resonance (ENDOR), where it performed impressively (4 -9).…”

Section: Introductionmentioning

confidence: 99%

26.5–40 GHz Ka-band pulsed EPR spectrometer

Astashkin

Enemark

Raitsimring

2006

Concepts Magn. Reson.

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ABSTRACT:The design and performance of a new homebuilt K a -band pulsed electron paramagnetic resonance (EPR) spectrometer is described. The spectrometer is unique in that it is broadband and allows experiments to be performed at any operational frequency within the range from 26.5 to 40 GHz. The high-power microwave amplifier employed in this spectrometer allowed us to use resonators with low quality factor and to achieve a minimum inversion microwave pulse duration of 11 ns. As a result, this spectrometer can be used for all types of pulsed EPR experiments, including electron spin echo envelope modulation (ESEEM), electron-nuclear double resonance (ENDOR), and electron-electron double resonance (ELDOR) with a difference between the pumping and observation frequencies of up to several hundred megahertz. The timing is controlled by an arbitrary waveform generator with 12 digital outputs and 1 ns time resolution. The spectrometer control/data acquisition software is home-written and allows an easy implementation of any one-or two-dimensional experiment using sweep definition tables.

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“…CW and Mims/ReMims pulsed 35 GHz ENDOR spectra were recorded at 2 K on spectrometers described previously (37). The ENDOR pattern for an I = ½ nucleus ( 1 H, 15 N) exhibits a ν(±) doublet that is split by the hyperfine coupling, A, and centered at the nuclear Larmor frequency.…”

Section: Ghz Epr/endor Spectroscopymentioning

confidence: 99%

Diazene (HNNH) Is a Substrate for Nitrogenase: Insights into the Pathway of N₂ Reduction

et al. 2007

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Nitrogenase catalyzes the sequential addition of six electrons and six protons to a N 2 that is bound to the active site metal cluster FeMo-cofactor, yielding two ammonia molecules. The nature of the intermediates bound to FeMo-cofactor along this reduction pathway remains unknown, although it has been suggested that there are intermediates at the level of reduction of diazene (HN=NH, also called diimide) and hydrazine (H 2 N-NH 2 ). Through in situ generation of diazene during nitrogenase turnover, we show that diazene is a substrate for the wild-type nitrogenase and is reduced to NH 3 . Diazene reduction, like N 2 reduction, is inhibited by H 2 . This contrasts with the lack of H 2 inhibition when nitrogenase reduces hydrazine. These results support the existence of an intermediate early in the N 2 reduction pathway at the level of reduction of diazene. Freeze-quenching a MoFe protein variant with α-195 His substituted by Gln and α-70 Val substituted by Ala during steady-state turnover with diazene resulted in conversion of the S = 3/2 resting state FeMo-cofactor to a novel S = 1/2 state with g 1 = 2.09, g 2 = 2.01, and g 3 ~ 1.98. 15 N-and 1 H-ENDOR establish that this state consists of a diazene-derived [-NH x ] moiety bound to FeMo-cofactor. This moiety is indistinguishable from the hydrazine-derived [-NH x ] moiety bound to FeMo-cofactor when the same MoFe protein is trapped during turnover with hydrazine. These observations suggest that diazene joins the normal N 2 -reduction pathway, and that the diazene-and hydrazine-trapped turnover states represent the same intermediate in the normal reduction of N 2 by nitrogenase. The implications of these findings for the mechanism of N 2 reduction by nitrogenase are discussed. KeywordsFeMo-cofactor; EPR; ENDOR; Active Site; Substrate Nitrogenase is the enzyme responsible for catalyzing biological reduction of N 2 to two NH 3 , an essential reaction in the global biogeochemical nitrogen cycle (1-3). The minimum stoichiometry for the nitrogenase catalyzed reduction of N 2 involves delivery of 8e − and 8H + (eqn 1). † This work was supported by grants from the National Institutes of Health (R01-GM59087 to LCS and DRD; HL13531 to BMH), the National Science Foundation (MCB-0316038 to BMH) and the United States Department of Agriculture Postdoctoral Fellowship program (2004-35318-14905 to BMB).*Address correspondence to these authors: LCS, phone (435) 797-3964, fax (435) email seefeldt@cc.usu.edu; DRD, phone (540) 231-5895, fax (540) email deandr@vt.edu; BMH, phone (847) (Figure 1).Relatively little is known at a molecular level about the nitrogenase N 2 -reduction mechanism beyond the fact that N 2 binds to and is reduced at one or more of the metal atoms of FeMocofactor ( Figure 1) Both the Chatt and Schrock cycles belong to one fundamental class of potential nitrogenase mechanismsin which the first three 'H-atoms' (e − /H + ) are sequentially added to a single N atom, in those instances the distal N of an end-on bound N 2 , followed by cleava...

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Q-Band Pulsed Electron Spin-Echo Spectrometer and Its Application to ENDOR and ESEEM

Cited by 141 publications

References 3 publications

Relaxation Times of Organic Radicals and Transition Metal Ions

Relaxation Times of Organic Radicals and Transition Metal Ions

26.5–40 GHz Ka-band pulsed EPR spectrometer

Diazene (HNNH) Is a Substrate for Nitrogenase: Insights into the Pathway of N₂ Reduction

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Q-Band Pulsed Electron Spin-Echo Spectrometer and Its Application to ENDOR and ESEEM

Cited by 141 publications

References 3 publications

Relaxation Times of Organic Radicals and Transition Metal Ions

Relaxation Times of Organic Radicals and Transition Metal Ions

26.5–40 GHz Ka-band pulsed EPR spectrometer

Diazene (HNNH) Is a Substrate for Nitrogenase: Insights into the Pathway of N2 Reduction

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Diazene (HNNH) Is a Substrate for Nitrogenase: Insights into the Pathway of N₂ Reduction