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G Jeschke

Publications and source records attributed to G Jeschke.

15 recordsLinked to original sources
PubMed2006

Spin pair geometry revealed by high-field DEER in the presence of conformational distributions.

Orientation selection on two nitroxide-labelled shape-persistent molecules is demonstrated by high-field pulsed electron-electron double resonance experiments at a frequency of 95 GHz with a commercial spectrometer. The experiments are performed with fixed observer and pump frequencies by variation of the magnetic field, so that the variation of both the dipolar frequencies and the modulation depths can be analyzed. By applying the deadtime-free four-pulse double electron-electron resonance (DEER) sequence, the lineshapes of the dipolar spectra are obtained. In the investigated linear biradical and equilateral triradical the nitroxide labels undergo restricted dynamics, so that their relative orientations are not fixed, but are correlated to some extent. In this situation, the general dependence of the dipolar spectra on the observer field can be satisfyingly modelled by simple geometrical models that involve only one rotational degree of freedom for the biradical and two rotational degrees of freedom for the triradical. A somewhat better agreement of the dipolar lineshapes for the biradical is obtained by simulations based on a molecular dynamics trajectory. For the triradical, small but significant deviations of the lineshape are observed with both models, indicating that the technique can reveal deficiencies in modelling of the conformational ensemble of a macromolecule.

Algorithms↗
PubMed2006

Isotope selection in distance measurements between nitroxides.

Self-assembly of spin-labeled synthetic macromolecules or biomacromolecules can lead to structures that contain more than two nitroxide radicals. Label-to-label distance distributions are then poorly resolved since established electron paramagnetic resonance techniques for distance measurements cannot select between the different pairs of nitroxides. A separation into different contributions can be achieved by partially labeling the nitroxide radicals by (15)N or by deuterium and applying pulse electron electron double resonance techniques. With (15)N labeling, strong suppression of either the (14)N or the (15)N contribution can be achieved by suitable choices of the excitation bandwidths and frequencies of the observer subsequence and pump pulse and linear combination of data sets. With deuterium labeling, interactions between only the isotope-labeled nitroxides can be selected by a two-dimensional version of the four-pulse double electron electron resonance experiment. This selection is based on the deep electron spin echo envelope modulation of deuterated nitroxides.

Electron Spin Resonance Spectroscopy↗
PubMed2005

EPR techniques for studying radical enzymes.

EPR studies on radical enzymes are reviewed under the aspects of the information that they can provide and of the techniques that are used. An overview of organic radicals derived from amino acids, modified amino acids, and cofactors is given and g tensor data are compiled. The information accessible from a spectroscopic point of view is contrasted with the information required to understand enzyme structure and function, and some precautions are discussed that must be taken to derive the latter kind of information from the former. Structural dynamics is identified as an aspect that has rarely been addressed in the past although it is highly relevant for enzyme function. It is proposed that techniques introduced recently on other classes of proteins could help to close this gap.

Amino Acids↗
PubMed2005

Separation of motional processes in a [2]catenane by combining synthetic, dual-frequency EPR and molecular modelling approaches.

Continuous-wave EPR of nitroxide spin labels at conventional (9.4 GHz) and high (94.2 GHz) frequencies is applied to characterize molecular dynamics in [2]catenanes composed of macrocycles with rigid phenyleneethynylene and flexible alkyl chain building blocks. By using a set of compounds with increasing complexity, which were all labelled at the centre of a rigid building block, it was possible to find regimes where spectral lineshapes were dominated by local motion of the spin label or those that contained information on tumbling of the building blocks. In chloroform, the macrocycles do not move as rigid objects, rather the rigid building block can reorient with some ease, with respect to the rest of the molecule. Furthermore, in that solvent the [2]catenane samples the co-conformational space on a timescale of microseconds or shorter. In a mechanical picture, chloroform can thus be considered as an effective lubricant that prevents the macrocycles from sticking together.

Journal Article↗
PubMed2004

2D TRIPLE in orientationally disordered samples--a means to resolve and determine relative orientation of hyperfine tensors.

The two-dimensional (2D) TRIPLE experiment provides correlations between electron-nuclear double resonance (ENDOR) frequencies that belong to the same electron-spin manifold, M(S), and therefore allows to assign ENDOR lines to their specific paramagnetic centers and M(S) manifolds. This, in turn, also provides the relative signs of the hyperfine couplings. So far this experiment has been applied only to single crystals, where the cross-peaks in the 2D spectrum are well resolved with regular shapes. Here we introduce the application of the 2D TRIPLE experiment to orientationally disordered systems, where it can resolve overlapping powder patterns. Moreover, analysis of the shape of the cross-peaks shows that it is highly dependent on the relative orientation of the hyperfine tensors of the two nuclei contributing to this particular peak. This is done initially through a series of simulations and then demonstrated experimentally at a high field (W-band, 95 GHz). The first example concerned the (1)H hyperfine tensors of the stable radical alpha,gamma-bisdiphenylene-beta-phenylallyl (BDPA) immobilized in a polystyrene matrix. Then, the experiment was applied to a more complex system, a frozen solution of Cu(II)-bis(2,2':6',2'' terpyridine) complex. There, the 2D TRIPLE experiment was combined with the variable mixing time (VMT) ENDOR experiment, which determined the absolute sign of the hyperfine couplings involved, and orientation selective ENDOR experiments. Analysis of the three experiments gave the hyperfine tensors of a few coupled protons.

Journal Article↗
PubMed2004

Sensitivity enhancement in pulse EPR distance measurements.

Established pulse EPR approaches to the measurement of small dipole-dipole couplings between electron spins rely on constant-time echo experiments to separate relaxational contributions from dipolar time evolution. This requires a compromise between sensitivity and resolution to be made prior to the measurement, so that optimum data are only obtained if the magnitude of the dipole-dipole coupling is known beforehand to a good approximation. Moreover, the whole dipolar evolution function is measured with relatively low sensitivity. These problems are overcome by a variable-time experiment that achieves suppression of the relaxation contribution by reference deconvolution. Theoretical and experimental results show that this approach leads to significant sensitivity improvements for typical systems and experimental conditions. Further sensitivity improvements or, equivalently, an extension of the accessible distance range can be obtained by matrix deuteration or digital long-pass filtering of the time-domain data. Advantages and limitations of the new variable-time experiment are discussed by comparing it to the established analogous constant-time experiment for measurements of end-to-end distances of 5 and 7.5 nm on rod-like shape-persistent biradicals and for the measurement of a broadly distributed transmembrane distance in a doubly spin-labeled mutant of plant light harvesting complex II.

Algorithms↗
PubMed2000

EPR probes with well-defined, long distances between two or three unpaired electrons

The synthesis of rod- and star-shaped compounds carrying two or three spin labels as end groups is described. The unpaired electrons are 2.8-5.1 nm apart from each other. The shape-persistent scaffolds were obtained through Pd-Cu-catalyzed alkynyl-aryl coupling and Pd-Cu-catalyzed alkyne dimerization in the presence of oxygen using p-phenyleneethynylene as the basic shape-persistent building block. The spin label 1-oxyl-2,2,5,5-tetramethylpyrroline-3-carboxylic acid (4) was attached through esterification of the terminal phenolic OH groups of the scaffold.

Journal Article↗
PubMed2000

Dead-time free measurement of dipole-dipole interactions between electron spins.

A four-pulse version of the pulse double electron-electron resonance (DEER) experiment is presented, which is designed for the determination of interradical distances on a nanoscopic length-scale. With the new pulse sequence electron-electron couplings can be studied without dead-time artifacts, so that even broad distributions of electron-electron distances can be characterized. A version of the experiment that uses a pulse train in the detection period exhibits improved signal-to-noise ratio. Tests on two nitroxide biradicals with known length indicate that the accessible range of distances extends from about 1.5 to 8 nm. The four-pulse DEER spectra of an ionic spin probe in an ionomer exhibit features due to probe molecules situated both on the same and on different ion clusters. The former feature provides information on the cluster size and is inaccessible with previous methods.

Electron Spin Resonance Spectroscopy↗
PubMed2000

Magic-angle sample spinning electron paramagnetic resonance--instrumentation, performance, and limitations.

An electron paramagnetic resonance (EPR) setup for line narrowing experiments with fast sample spinning at variable angles between the rotation axis and the static magnetic field is described and applied in the magic-angle sample spinning (MAS) EPR experiment at X-band frequencies (9.5 GHz). Sample spinning speeds up to 17 kHz at temperatures down to 200 K can be achieved with rotors of 4-mm outer and 2.5-mm inner diameter without severe losses in microwave amplitude compared to standard pulse EPR probeheads. A phase cycle is introduced that provides pure absorption MAS EPR spectra and allows one to distinguish between positive and negative frequency offsets (pseudo-quadrature detection). Possible broadening mechanisms in MAS EPR spectra are discussed. It is demonstrated both by theory and by experiment that the MAS EPR experiment requires excitation bandwidths that are comparable to the total spectral width, since otherwise destructive interference between contributions of spins with similar resonance offsets suppresses the signal. Experimental observations on the E(1) center in gamma-irradiated silica glass and on the SO(-)(3) radical in gamma-irradiated sulfamic acid are reported.

Electron Spin Resonance Spectroscopy↗
PubMed1999

Pure absorption electron spin echo envelope modulation spectra by using the filter-diagonalization method for harmonic inversion.

Harmonic inversion of electron spin echo envelope (ESEEM) time-domain signals by filter diagonalization is investigated as an alternative to Fourier transformation. It is demonstrated that this method features enhanced resolution compared to Fourier-transform magnitude spectra, since it can eliminate dispersive contributions to the line shape, even if no linear phase correction is possible. Furthermore, instrumental artifacts can be easily removed from the spectra if they are narrow either in time or frequency domain. This applies to echo crossings that are only incompletely eliminated by phase cycling and to spurious spectrometer frequencies, respectively. The method is computationally efficient and numerically stable and does not require extensive parameter adjustments or advance knowledge of the number of spectral lines. Experiments on gamma-irradiated methyl-alpha-d-glucopyranoside show that more information can be obtained from typical ESEEM time-domain signals by filter-diagonalization than by Fourier transformation.

Absorption↗
PubMed1998

Nutation-Frequency Correlated EPR Spectroscopy: The PEANUT Experiment

The phase-inverted echo-amplitude detected nutation (PEANUT) experiment for the measurement of transient electron spin nutation frequencies is introduced. In this new pulse sequence, nutation refocused to a rotary echo is detected via the amplitude modulation of a primary electron spin echo. Since detection and excitation of the nutation are fully separated in time, experiments at very high nutation frequencies become feasible. The nutation frequencies which are proportional to the transition moment of an EPR transition can be used to label individual EPR lines by an additional parameter. Using a two-dimensional PEANUT experiment which correlates the nutation frequencies with the resonance fields, the interpretation of complicated field-swept EPR spectra can considerably be simplified. A theoretical description of the experiment is given and the inner working of the approach is described. The predicted features of PEANUT spectra are verified experimentally and examples of applications to both ordered and disordered systems are given. Copyright 1998 Academic Press. Copyright 1998 Academic Press

Journal Article↗
PubMed1998

Sensitivity Enhancement by Matched Microwave Pulses in One- and Two-Dimensional Electron Spin Echo Envelope Modulation Spectroscopy

The concept of microwave pulse matching is applied to three-pulse electron spin echo envelope modulation and sublevel correlation (HYSCORE) spectroscopy. Matched pulses enhance the efficiency of forbidden transfers and may drastically increase the signal intensity of basic frequency and combination frequency transitions in these conventional pulse EPR experiments. The theory of matched pulses is extended to the case of strong and largely isotropic hyperfine interactions, and numerical simulations are presented to gain a deeper insight into the inner working of the matched-pulse approach. It is shown that the enhancement of combination frequencies in matched HYSCORE can be used to determine the relative sign of hyperfine coupling constants as well as the number of equivalent nuclei. The enormous capacity of the approach is demonstrated on ordered and disordered systems. In particular, it is shown that in HYSCORE experiments the signal-to-noise ratio improvement for strongly coupled nitrogens and for proton combination peaks may be considerably larger than one order of magnitude, corresponding to a reduction in measuring time of more than a factor of 100. Copyright 1998 Academic Press.

Journal Article↗
PubMed1998

A comprehensive NMR study of cubic and hexagonal boron nitride.

A variety of techniques and measurements on all NMR accessible nuclei allow one to obtain a complete and precise set of chemical shift and quadrupole coupling parameters for both boron and nitrogen in cubic and hexagonal boron nitride. For hexagonal boron nitride, 11B to 15N cross polarization under magic angle sample spinning conditions is demonstrated at natural isotope abundance. The presented approach for NMR characterization of the crystalline boron nitrides should also be applicable to structurally related composite materials, nanotubes, and amorphous ceramics.

Boron Compounds↗
PubMed1988

[Quantitative serial computed tomography. Kidney and tumors].

Density-time-functions after intravenous contrast medium injections are simultaneously taken by a computer tomograph in the region of the aorta and the cortex of the kidney or the region of the aorta and a tumor respectively. The differential equation system resulting from this is solved explicitly on the basis of a compartment model. The resulting functions are related to the density-time functions. These functions include the constants of distribution velocity and discharge velocity. They are modified by application of an iteration procedure (computer programme) to ensure the adequate correspondence between the curves and the density-time-trends. Between the constants of inversion velocity and eversion velocity of the cortex of the kidney, there is a linear as well as allometric relationship, the correlation amounts to r = 0.99. In the case of all tumors, only an allometric relationship (r = 0.92) could be found as regards the conversion constants.

Adult↗