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Aiwen Lei - One of the best experts on this subject based on the ideXlab platform.

  • homolytic cleavage of the o cu ii bond xafs and EPR Spectroscopy evidence for one electron reduction of cu ii to cu i
    Chemical Communications, 2016
    Co-Authors: Guanghui Zhang, Jie Xin, Yi Deng, Jeffrey T Miller, Arthur Jeremy Kropf, Emilio E Bunel, Yu Lan, Jyhfu Lee, Aiwen Lei
    Abstract:

    The investigation into the active copper(I) catalysts from copper(II) precursors has become a fundamental and important task in copper catalysis. In this work, we demonstrate that the tBuO− anion serves not only as a base but also as a mediator to promote the reduction of Cu(II) to Cu(I) in copper catalysis. XAFS and EPR Spectroscopy evidence the [Cu(OtBu)3]− ate complex as the key intermediate which undergoes homolytic-cleavage of the O–Cu(II) bond generating [Cu(OtBu)2]− ate complex.

  • direct observation of reduction of cu ii to cu i by p h compounds using xas and EPR Spectroscopy
    Organometallics, 2016
    Co-Authors: Dali Yang, Jyhfu Lee, Aiwen Lei, Yi Luo, Chihwen Pao
    Abstract:

    Recently, transition-metal-catalyzed radical phosphorylations have provided a direct and useful way to P-substituted compounds. Although a variety of methodologies involving phosphinoyl radicals have been developed, the mechanism of the P–H compounds in the redox process is still unclear. In this work, a mechanistic study on the reduction of Cu(II) by P–H compunds through XAS and EPR Spectroscopy has been demonstrated. Two commonly used P–H compounds, diphenylphosphine oxide and dialkyl phosphites, have been selected in this reduction system. The structure of formed Cu(I) species is evidenced through fitting results of the EXAFS spectrum. Furthermore, the halide ion can be a mediator to promote the reduction of Cu(II) by P–H compounds. These spectroscopic investigations provide useful insights into the reactions of P–H compounds, which would be helpful for an understanding of the mechanism and the future design of reactions.

  • evidence of cu i cu ii redox process by x ray absorption and EPR Spectroscopy direct synthesis of dihydrofurans from β ketocarbonyl derivatives and olefins
    Chemistry: A European Journal, 2015
    Co-Authors: Zhixiong Liao, Guanghui Zhang, Emilio E Bunel, Jyhfu Lee, Aiwen Lei, Chihwen Pao, Guoting Zhang, Chao Fan, Xu Zhang
    Abstract:

    The Cu(I)/Cu(II) and Cu(I)/Cu(III) catalytic cycles have been subject to intense debate in the field of copper-catalyzed oxidative coupling reactions. A mechanistic study on the Cu(I)/Cu(II) redox process, by X-ray absorption (XAS) and electron paramagnetic resonance (EPR) spectroscopies, has elucidated the reduction mechanism of Cu(II) to Cu(I) by 1,3-diketone and detailed investigation revealed that the halide ion is important for the reduction process. The oxidative nature of the thereby-formed Cu(I) has also been studied by XAS and EPR Spectroscopy. This mechanistic information is applicable to the copper-catalyzed oxidative cyclization of β-ketocarbonyl derivatives to dihydrofurans. This protocol provides an ideal route to highly substituted dihydrofuran rings from easily available 1,3-dicarbonyls and olefins.

Gareth R Eaton - One of the best experts on this subject based on the ideXlab platform.

  • a spirocyclohexyl nitroxide amino acid spin label for pulsed EPR Spectroscopy distance measurements
    Chemistry: A European Journal, 2010
    Co-Authors: Andrzej Rajca, Velavan Kathirvelu, Sandip K Roy, Maren Pink, Suchada Rajca, Santanu Sarkar, Sandra S Eaton, Gareth R Eaton
    Abstract:

    Site-directed spin labeling and EPR Spectroscopy offer accurate, sensitive tools for the characterization of structure and function of macromolecules and their assemblies. A new rigid spin label, spirocyclohexyl nitroxide alpha-amino acid and its N-(9-fluorenylmethoxycarbonyl) derivative, have been synthesized, which exhibit slow enough spin-echo dephasing to permit accurate distance measurements by pulsed EPR Spectroscopy at temperatures up to 125 K in 1:1 water/glycerol and at higher temperatures in matrices with higher glass transition temperatures. Distance measurements in the liquid nitrogen temperature range are less expensive than those that require liquid helium, which will greatly facilitate applications of pulsed EPR Spectroscopy to the study of structure and conformation of peptides and proteins.

  • a spirocyclohexyl nitroxide amino acid spin label for pulsed EPR Spectroscopy distance measurements
    Chemistry: A European Journal, 2010
    Co-Authors: Andrzej Rajca, Velavan Kathirvelu, Sandip K Roy, Maren Pink, Suchada Rajca, Santanu Sarkar, Sandra S Eaton, Gareth R Eaton
    Abstract:

    Site-directed spin labeling (SDSL) and electron paramagnetic resonance (EPR) Spectroscopy offer accurate, sensitive tools for the characterization of structure and function of macromolecules and their assemblies. A new rigid spin label, spirocyclohexyl nitroxide α-amino acid and its N-(9-fluorenylmethoxycarbonyl) (Fmoc) derivative, has been synthesized that exhibit slow enough spin echo dephasing to permit accurate distance measurements by pulse EPR at temperatures up to 125 K in 1:1 water:glycerol and at higher temperatures in matrices with higher glass transition temperatures. Distance measurements in the liquid nitrogen temperature range are less expensive than those that require liquid helium, which will greatly facilitate applications of pulsed EPR to the study of structure and conformation for peptides and proteins.

Thomas F Prisner - One of the best experts on this subject based on the ideXlab platform.

  • perspectives of shaped pulses for EPR Spectroscopy
    Journal of Magnetic Resonance, 2017
    Co-Authors: Philipp E Spindler, Philipp Schops, Wolfgang Kallies, Steffen J Glaser, Thomas F Prisner
    Abstract:

    This article describes current uses of shaped pulses, generated by an arbitrary waveform generator, in the field of EPR Spectroscopy. We show applications of sech/tanh and WURST pulses to dipolar Spectroscopy, including new pulse schemes and procedures, and discuss the more general concept of optimum-control-based pulses for applications in EPR Spectroscopy. The article also describes a procedure to correct for experimental imperfections, mostly introduced by the microwave resonator, and discusses further potential applications and limitations of such pulses.

  • distance measurement on an endogenous membrane transporter in e coli cells and native membranes using EPR Spectroscopy
    Angewandte Chemie, 2015
    Co-Authors: Benesh Joseph, Gunnar Jeschke, Arthur Sikora, Enrica Bordignon, David S Cafiso, Thomas F Prisner
    Abstract:

    Membrane proteins may be influenced by the environment, and they may be unstable in detergents or fail to crystallize. As a result, approaches to characterize structures in a native environment are highly desirable. Here, we report a novel general strategy for precise distance measurements on outer membrane proteins in whole Escherichia coli cells and isolated outer membranes. The cobalamin transporter BtuB was overexpressed and spin-labeled in whole cells and outer membranes and interspin distances were measured to a spin-labeled cobalamin using pulse EPR Spectroscopy. A comparative analysis of the data reveals a similar interspin distance between whole cells, outer membranes, and synthetic vesicles. This approach provides an elegant way to study conformational changes or protein-protein/ligand interactions at surface-exposed sites of membrane protein complexes in whole cells and native membranes, and provides a method to validate outer membrane protein structures in their native environment.

  • sterically shielded spin labels for in cell EPR Spectroscopy analysis of stability in reducing environment
    Free Radical Research, 2015
    Co-Authors: Anil P. Jagtap, Thomas F Prisner, Nitin C. Kunjir, Robert Hänsel, Ivan Krstic, Th S Sigurdsson
    Abstract:

    Electron paramagnetic resonance (EPR) Spectroscopy is a powerful and widely used technique for studying structure and dynamics of biomolecules under bio-orthogonal conditions. In-cell EPR is an emerging area in this field; however, it is hampered by the reducing environment present in cells, which reduces most nitroxide spin labels to their corresponding diamagnetic N-hydroxyl derivatives. To determine which radicals are best suited for in-cell EPR studies, we systematically studied the effects of substitution on radical stability using five different classes of radicals, specifically piperidine-, imidazolidine-, pyrrolidine-, and isoindoline-based nitroxides as well as the Finland trityl radical. Thermodynamic parameters of nitroxide reduction were determined by cyclic voltammetry; the rate of reduction in the presence of ascorbate, cellular extracts, and after injection into oocytes was measured by continuous-wave EPR Spectroscopy. Our study revealed that tetraethyl-substituted nitroxides are good candidates for in-cell EPR studies, in particular pyrrolidine derivatives, which are slightly more stable than the trityl radical.

  • Sterically shielded spin labels for in-cell EPR Spectroscopy: Analysis of stability in reducing environment
    Free radical research, 2014
    Co-Authors: Anil P. Jagtap, Thomas F Prisner, Ivan Krstić, Nitin C. Kunjir, Robert Hänsel, S. Th. Sigurdsson
    Abstract:

    AbstractElectron paramagnetic resonance (EPR) Spectroscopy is a powerful and widely used technique for studying structure and dynamics of biomolecules under bio-orthogonal conditions. In-cell EPR is an emerging area in this field; however, it is hampered by the reducing environment present in cells, which reduces most nitroxide spin labels to their corresponding diamagnetic N-hydroxyl derivatives. To determine which radicals are best suited for in-cell EPR studies, we systematically studied the effects of substitution on radical stability using five different classes of radicals, specifically piperidine-, imidazolidine-, pyrrolidine-, and isoindoline-based nitroxides as well as the Finland trityl radical. Thermodynamic parameters of nitroxide reduction were determined by cyclic voltammetry; the rate of reduction in the presence of ascorbate, cellular extracts, and after injection into oocytes was measured by continuous-wave EPR Spectroscopy. Our study revealed that tetraethyl-substituted nitroxides are go...

  • long range distance determinations in biomacromolecules by EPR Spectroscopy
    Quarterly Reviews of Biophysics, 2007
    Co-Authors: Olav Schiemann, Thomas F Prisner
    Abstract:

    Electron paramagnetic resonance (EPR) Spectroscopy provides a variety of tools to study structures and structural changes of large biomolecules or complexes thereof. In order to unravel secondary structure elements, domain arrangements or complex formation, continuous wave and pulsed EPR methods capable of measuring the magnetic dipole coupling between two unpaired electrons can be used to obtain long-range distance constraints on the nanometer scale. Such methods yield reliably and precisely distances of up to 80 A, can be applied to biomolecules in aqueous buffer solutions or membranes, and are not size limited. They can be applied either at cryogenic or physiological temperatures and down to amounts of a few nanomoles. Spin centers may be metal ions, metal clusters, cofactor radicals, amino acid radicals, or spin labels. In this review, we discuss the advantages and limitations of the different EPR spectroscopic methods, briefly describe their theoretical background, and summarize important biological applications. The main focus of this article will be on pulsed EPR methods like pulsed electron-electron double resonance (PELDOR) and their applications to spin-labeled biosystems.

Dariush Hinderberger - One of the best experts on this subject based on the ideXlab platform.

  • characterizing active pharmaceutical ingredient binding to human serum albumin by spin labeling and EPR Spectroscopy
    Chemistry: A European Journal, 2016
    Co-Authors: Till Hauenschild, Jörg Reichenwallner, Volker Enkelmann, Dariush Hinderberger
    Abstract:

    Drug binding to human serum albumin (HSA) has been characterized by a spin-labeling and continuous-wave (CW) EPR spectroscopic approach. Specifically, the contribution of functional groups (FGs) in a compound on its albumin-binding capabilities is quantitatively described. Molecules from different drug classes are labeled with EPR-active nitroxide radicals (spin-labeled pharmaceuticals (SLPs)) and in a screening approach CW-EPR Spectroscopy is used to investigate HSA binding under physiological conditions and at varying ratios of SLP to protein. Spectral simulations of the CW-EPR spectra allow extraction of association constants (KA ) and the maximum number (n) of binding sites per protein. By comparison of data from 23 SLPs, the mechanisms of drug-protein association and the impact of chemical modifications at individual positions on drug uptake can be rationalized. Furthermore, new drug modifications with predictable protein binding tendency may be envisaged.

  • EPR Spectroscopy provides a molecular view on thermoresponsive dendronized polymers below the critical temperature
    Macromolecular Chemistry and Physics, 2011
    Co-Authors: Matthias J N Junk, Dieter A Schluter, Gerhard Wegner, Hans Wolfgang Spiess, Afang Zhang, Dariush Hinderberger
    Abstract:

    The nanoscopic structure of thermoresponsive dendronized polymers below the critical aggregation temperature (TC) is revealed by CW EPR Spectroscopy. At temperatures far below TC, the water-swollen polymers start to dehydrate and hydrophobic cavities are formed. Two different dehydration processes can be discerned, the more effective of which is observed within 4K below TC. The dehydration predominantly takes place at the peripheral dendritic shell, rendering it increasingly hydrophobic and eventually triggering an interchain aggregation and the formation of mesoglobules at the critical temperature. While the polymer aggregation is mainly dependent on the dendron periphery, the efficiency of the dehydration below TC is closely related to the hydrophobicity of the dendritic core.

  • the solvation of nitroxide radicals in ionic liquids studied by high field EPR Spectroscopy
    Physical Chemistry Chemical Physics, 2010
    Co-Authors: Yasar Akdogan, Jeannine Heller, Herbert Zimmermann, Dariush Hinderberger
    Abstract:

    Ionic liquids (ILs) feature a variety of properties that make them a unique class of solvents. To gain a better understanding of how ILs solvate compounds of different chemical structure, we used pulsed high-field electron paramagnetic resonance (EPR) Spectroscopy at W-band (∼94 GHz) and continuous wave EPR at X-band (∼9.4 GHz) on three TEMPO-based spin probes with different substitutions at the 4-position: 4-R-2,2,6,6-tetramethylpiperidine-1-oxyl, with R = N(CH3)3+, Cat-1, R = COO−, TEMPO-4-carboxylate, and R = OH, TEMPOL. The spin probes are dissolved in imidazolium based ILs with different alkyl chain lengths (–C2H5, –C4H9, –C6H13) and anions (BF4−, PF6−) and also in molecular solvents (methanol, water–glycerol). X-Band EPR at RT shows that the reorientational motion of the charged spin probes in ILs is about fivefold slower than that of the TEMPOL. Moreover, anion variation from BF4− to PF6− in ILs most strongly slows down the rotational motion (as measured by the rotational correlation time τr) of Cat-1, followed by TEMPOL, while τr of TEMPO-4-carboxylate is least affected. The EPR parameters gxx and Azz (tensor elements of the g- and hyperfine tensor) are sensitive to environmental effects and are only fully resolved at the high field used in this study. Changes of gxx and Azz values of the Cat-1 in ILs and methanol are very small especially compared to that of TEMPO-4-carboxylate, indicating that Cat-1 is located in a polar region of the ILs resembling the situation in methanol. On the other hand, the gxx value of TEMPO-4-carboxylate is sensitive to the length of alkyl group which shows that TEMPO-4-carboxylate is close to the nonpolar region of ILs. The small differences in the chemical substitution of the spin probes used here are sufficient for the molecules to reside in different domains of different dielectric properties in ILs. Our combined results are in good agreement with a picture of a nanophase separation, in which the charged cations and anions form polar regions and the hydrophobic alkyl chains of the IL cations form non-polar regions.

  • Two sub-states of the red2 state of methyl-coenzyme M reductase revealed by high-field EPR Spectroscopy
    JBIC Journal of Biological Inorganic Chemistry, 2007
    Co-Authors: Denise I. Kern, Bernhard Jaun, Meike Goenrich, Jeffrey Harmer, Rudolf K Thauer, Dariush Hinderberger
    Abstract:

    Methyl-coenzyme M reductase (MCR) catalyzes the formation of methane from methyl-coenzyme M and coenzyme B in methanogenic archaea. The enzyme has two structurally interlinked active sites embedded in an α_2β_2γ_2 subunit structure. Each active site has the nickel porphyrinoid F_430 as a prosthetic group. In the active state, F_430 contains the transition metal in the Ni(I) oxidation state. The active enzyme exhibits an axial Ni(I)-based continuous wave (CW) electron paramagnetic resonance (EPR) signal, called red1a in the absence of substrates or red1c in the presence of coenzyme M. Addition of coenzyme B to the MCR-red1 state can partially and reversibly convert it into the MCR-red2 form, which shows a rhombic Ni(I)-based EPR signal (at X-band microwave frequencies of approximately 9.4 GHz). In this report we present evidence from high-field/high-frequency CW EPR Spectroscopy (W-band, microwave frequency of approximately 94 GHz) that the red2 state consists of two substates that could not be resolved by EPR Spectroscopy at X-band frequencies. At W-band it becomes apparent that upon addition of coenzyme B to MCR in the red1c state, two red2 EPR signals are induced, not one as was previously believed. The first signal is the well-characterized (ortho)rhombic EPR signal, thus far called red2, while the second previously unidentified signal is axial. We have named the two substates MCR-red2r and MCR-red2a after their rhombic and axial signals, respectively.

Tingyun Kuang - One of the best experts on this subject based on the ideXlab platform.

  • High-light induced superoxide radical formation in cytochrome b6f complex from Bryopsis corticulans as detected by EPR Spectroscopy
    Journal of Photochemistry and Photobiology B-biology, 2010
    Co-Authors: Min Sang, Xiao-bo Chen, Kebin Wang, Liangbi Li, Jian-ping Zhang, Wenda Wang, Tingyun Kuang
    Abstract:

    Abstract The generation of superoxide radical ( O 2 - ) in Cyt b6f of Bryopsis corticulans under high light illumination was studied using electron paramagnetic resonance (EPR) Spectroscopy. This could be evidenced by the addition of SOD which specifically reacted with O 2 - . The generation of O 2 - was lost in the absence of oxygen and was found to be suppressed in the presence of NaN3 and be scavenged by extraneous antioxidants such as ascorbate, β-carotene and glutathione which could also scavenged 1 O 2 ∗ . These results indicated that O 2 - which produced under high light illumination in Cyt b6f of B. corticulans might rise from a reaction which 1 O 2 ∗ could participated in. Also the photo-protection mechanism to Cyt b6f complex by antioxidants which might contain in thylakoid was speculated.

  • high light induced singlet oxygen formation in cytochrome b6f complex from bryopsis corticulans as detected by EPR Spectroscopy
    Biophysical Chemistry, 2010
    Co-Authors: Min Sang, Xiao-bo Chen, Kebin Wang, Jian-ping Zhang, Wenda Wang, Jie Xie, Xiaochun Qin, Jingquan Zhao, Tingyun Kuang
    Abstract:

    Abstract Electron paramagnetic resonance (EPR) Spectroscopy was used to detect the light-induced formation of singlet oxygen (1O2*) in the intact and the Rieske-depleted cytochrome b6f complexes (Cyt b6f) from Bryopsis corticulans, as well as in the isolated Rieske Fe–S protein. It is shown that, under white-light illumination and aerobic conditions, chlorophyll a (Chl a) bound in the intact Cyt b6f can be bleached by light-induced 1O2*, and that the 1O2* production can be promoted by D2O or scavenged by extraneous antioxidants such as l -histidine, ascorbate, β-carotene and glutathione. Under similar experimental conditions, 1O2* was also detected in the Rieske-depleted Cyt b6f complex, but not in the isolated Rieske Fe–S protein. The results prove that Chl a cofactor, rather than Rieske Fe–S protein, is the specific site of 1O2* formation, a conclusion which draws further support from the generation of 1O2* with selective excitation of Chl a using monocolor red light.