Biomedical subjects

E Kupce

Publications and source records attributed to E Kupce.

11 recordsLinked to original sources

PubMed2001

Adiabatic TOCSY MAS in liquids.

The effect of magic angle spinning (MAS) of liquids upon the performance of various isotropic mixing sequences is investigated. Although the mathematical formalism for isotropic mixing under MAS conditions is similar for both liquids and solids, the mechanism through which the coherence transfer is disturbed is different. In liquids, the use of sample spinning in the presence of both RF and magnetic-field inhomogeneities introduces a modulation of the effective field, which compromises the performance of the conventional mixing sequences. This effect is further amplified by supercycles, which normally improve the performance of the mixing and decoupling experiments. It is demonstrated that adiabatic mixing sequences are less susceptible to such modulations and perform considerably better in TOCSY MAS experiments. The best performance of TOCSY MAS is observed under the rotational resonance condition when the sample appears static in the nutation reference frame.

Journal Article↗

PubMed2000

Strange effects of pulse shaping in water presaturation experiments

During the course of some water presaturation experiments with a shaped pulse envelope we observed inverted responses from certain signals flanking the water response. This phenomenon did not occur when a rectangular presaturation envelope was used. Apparently the leading and trailing edges of the shaped pulse act as adiabatic sweeps, causing the coupled magnetizations in question to be spin-locked. This gives rise to Hartmann-Hahn coherence transfer, and when the spin lock duration is equal to 1/(2J) the trajectories are such as to carry these magnetization vectors to the -z axis, leading to inverted signals in the final spectrum. Copyright 2000 Academic Press.

Journal Article↗

PubMed1999

Line narrowing in spectra of proteins dissolved in a dilute liquid crystalline phase by band-selective adiabatic decoupling: application to 1HN-15N residual dipolar coupling measurements.

Residual heteronuclear dipolar couplings obtained from partially oriented protein samples can provide unique NMR constraints for protein structure determination. However, partial orientation of protein samples also causes severe 1H line broadening resulting from residual 1H-1H dipolar couplings. In this communication we show that band-selective 1H homonuclear decoupling during data acquisition is an efficient way to suppress residual 1H-1H dipolar couplings, resulting in spectra that are still amenable to solution NMR analysis, even with high degrees of alignment. As an example, we present a novel experiment with improved sensitivity for the measurement of one-bond 1HN-15N residual dipolar couplings in a protein sample dissolved in magnetically aligned liquid crystalline bicelles.

Anisotropy↗

PubMed1998

Adiabatic mixing in the liquid state.

Adiabatic spin inversion has been used in the liquid state very efficiently for decoupling purposes. Here we show that it can also be adapted for spin mixing experiments, such as the TOCSY and clean TOCSY experiment, and is superior to previously employed mixing sequences. The main advantage of adiabatic mixing sequences over the conventional mixing schemes used in liquid state experiments is an extremely low sensitivity to RF field inhomogeneity and miscalibration of the B1 field strength. The method is evaluated experimentally by comparing results obtained with different mixing schemes in the basic 2D TOCSY experiment. In addition to higher reliability, adiabatic mixing provides a sensitivity improvement of ca. 20% as compared to conventional mixing schemes. This is explained by higher signal losses due to RF inhomogeneity in the experiments employing traditional mixing schemes. More significant sensitivity improvements can be expected in situations where RF homogeneity is traditionally poor, for example, in large volume probes and magnetic resonance imaging experiments.

Algorithms↗

PubMed1997

Decoupling: theory and practice. I. Current methods and recent concepts.

Current methods for broadband heteronuclear decoupling are reviewed from a historical perspective. The principal concern is that decoupling should be effective over a wide range of chemical shifts without undue radiofrequency heating of the sample, particularly when human patients are involved. Continuous-wave methods are the least efficient in this respect, followed by noise decoupling. Composite pulse schemes offer a more effective use of radiofrequency power, while adiabatic passage methods are the most efficient of all. Bi-level decoupling employs a low level of radiofrequency irradiation during the relaxation delay to maintain the nuclear Overhauser effect, with a higher level during signal acquisition in order to decouple over a wide frequency band. All decoupling sequences introduce cycling sidebands into the observed spectrum, and schemes are described to minimize the intensity of these artifacts. In part II, practical applications of decoupling methods are examined in the context of in vivo spectroscopy, where the improvements in sensitivity and resolution through broadband decoupling can be critical for solving clinical problems. Attention is focused on the regulatory limits on power deposition in these experiments. A tabulation of the existing work on decoupling in biological tissue is presented, mainly involving 31P and 13C spectroscopy in vivo or in vitro.

Electromagnetic Fields↗

PubMed1997

Effect of sweep direction on sidebands in adiabatic decoupling

The appearance of sidebands in adiabatic decoupling can be substantially reduced simply by matching the sweep rate and direction of adiabatic pulses with the evolution of different J couplings. Alternatively, a matched adiabatic defocusing pulse is applied just before the decoupling is turned on, providing an efficient means for complete suppression of sidebands. Copyright 1997 Academic Press. Copyright 1997Academic Press

Journal Article↗

PubMed1996