Daniel Abergel

Neutralization of radiation damping by selective feedback on a 400 MHz NMR spectrometer

SummaryRadiation damping is a phenomenon well known among NMR spectroscopists of proteins as a source of undesirable features, especially in high-field and high-Q probe NMR. In this paper, we present an electronic neutralization network which dramatically reduces radiation damping. It detects the radiation field profile and feeds back into the probe an rf field with identical amplitude and opposite phase. Experimental results of a practical implementation carried out on a 400 MHz Bruker spectrometer are shown.

Comment on: Is multiple quantum nuclear magnetic resonance spectroscopy of liquid water real?

Recently, two‐dimensional NMR experiments have been performed, which were likely to show that a coupling between otherwise uncoupled or equivalent spins was observable, yielding signals originating from multiple‐spin multiple‐quantum operators in the density matrix. According to the authors, such surprising results could be interpreted as a consequence of the radiation damping phenomenon. For this purpose, a fully quantum‐mechanical frame was used, thus treating the radiation field in a quantized manner. Our analysis, based on the same theoretical grounds, as well as on new experimental arguments, lead us to diverging conclusions.

Amplification of radiation damping in a 600-MHz NMR spectrometer: Application to the study of water-protein interactions.

SummaryA new application of a recently developed electronic radiation-damping (RD) control system is presented. It is possible to amplify radiation damping so as to make the water magnetization return back to its equilibrium direction in a time shorter than the characteristic RD time. Certain types of experiments involving radiation damping as a selective inversion pulse can be significantly improved by this new method. Moreover, amplification of RD is shown to improve water suppression and consequently the dynamics of 2D NOESY experiments on proteins.

Self-sustained Maser oscillations of a large magnetization driven by a radiation damping-based electronic feedback

In this paper, the dynamics of a magnetization undergoing a radiation-damping based feedback radio-frequency field is investigated both theoretically and experimentally. It is shown that due to the presence of T1 relaxation the evolution equations predict the existence of self-sustained maser pulses. This phenomenon is a consequence of the competition between two different processes, namely, T1 relaxation and a precession about a magnetization-dependent radio-frequency field. Experiments show the existence of periodic revivals of the free induction decay over unusually long periods of time, on the order of tens of seconds.

A NEW CONCEPT FOR SELECTIVE EXCITATION IN NMR

Abstract The new NMR concept of selective excitation generated by pulses whose envelope is slaved to that of a signal originating from the sample is described. It is shown that these kinds of pulses exhibit interesting features such as a sort of auto-calibration of the pulse length and phase. These particular features are discussed and experimental evidence is given. Illustration of the use of slaved pulses is demonstrated in the case of 1D transient NOE by selective inversion.

Enhancement of water suppression by radiation damping-based manipulation of residual water in Jump and Return NMR experiments

Abstract It is shown in this Letter that amplification of radiation damping obtained by electronic feedback between consecutive transients of a Jump–Return (JR) experiment, residual in-plane water magnetization is suppressed, leading to substantial enhancement of water suppression. Application of this approach to RNA spectroscopy, where JR is particularly useful, is shown.

Experimental time saving in NMR measurement of time dependent diffusion coefficients

Abstract In this Letter, we describe a simplification of traditional NMR diffusion experiments suitable for the case of time dependent diffusion coefficients. It is shown that by setting the experimental attenuation parameter b=γ2g2δ2td, where td is the diffusion time, to a fixed value, it is possible to observe the characteristic variation of the normalized echo amplitude in restricted diffusion conditions and to construct the experimental curve D(td)/D0 without actually performing all the measurements of the diffusion coefficient at the various times. This yields dramatic experimental time reduction.

On the possibility of performing self-calibrated selective π/2 pulses in nuclear-magnetic resonance

In this paper, a generalization of the concept of selective “slaved” pulses in NMR spectroscopy introduced recently by the authors is presented. The shape of these pulses is slaved to that of a signal produced by the sample by the action of an electronic feedback loop. The theoretical analysis is based on results of the theory of nonlinear dynamical systems, which predicts the possibility of performing 90° self-calibrated radiofrequency pulses, through the simultaneous action of a constant radio-frequency field and of a magnetization dependent field of the kind above. Experimental demonstrations show that rotation of the magnetization onto the xy-plane is achieved, regardless of its initial position. Moreover, it is shown to remain stable for more than 50 ms, without significant loss of intensity.

THE STOCHASTIC BLOCH EQUATIONS DRIVEN BY A COLOURED NOISE

Abstract In this Letter, the lagged cross-moments and cross-spectra of the Bloch equations driven by a radiofrequency field whose amplitude is fluctuating randomly with Markovian statistics are calculated. Exact expressions for the lineshape are derived in the special case of a Markovian binary noise. The limiting case of white noise is investigated and compared with results found in the literature.

Automatic extraction of connected subspectra from homonuclear 3D NMR data sets

Abstract Multidimensional NMR spectroscopy is a particularly interesting tool for the structural study of macromolecules. However, the assignment of crowded spectra obtained from large molecules is not an easy task, owing to superpositions of signals. We present here a method based on a mathematical analysis of ω2 planes extracted from symmetrical 3D experiments that permits a removal of these degeneracies and an automatic identification of the spin systems.