Improved Convection Compensating Pulsed Field Gradient Spin-Echo and Stimulated-Echo Methods

N-Methylacetamide (NMA) is known to self-associate in solution through the formation of hydrogen bonds. This behavior, and the presence in the molecule of an amide bond, make this an interesting model for study since the oligomer can be considered to be a protein analogue. The aims of the present study were to estimate the thermodynamic parameters of the self-association process, through the measurement of the diffusion coefficient of NMA in carbon tetrachloride (CCl 4 ), and to predict the population distribution of oligomers as a function of the concentration of NMA. Diffusion coefficients were measured using pulsed field gradient spin-echo (PGSE) NMR spectroscopy. A computer model based on the Kirkwood-Riseman theory of macromolecular diffusion and an attenuative model of self-association were used to fit the experimental data and to derive estimates of the thermodynamic association constants and bond length, and these values were, in turn, used to estimate the oligomer distribution. In addition, the temperature dependence of the diffusion coefficient was measured at three different concentrations in order to calculate the apparent Arrhenius activation energies so as to provide additional insight into the self-association process. Finally, the concentration dependence of the viscosity of NMA in CCl 4 was measured to characterize further the hydrodynamic behavior of the polydisperse system.

Section: Methodsmentioning

confidence: 99%

PGSE NMR diffusion study of the self‐association of N‐methylacetamide in carbon tetrachloride

Regan¹,

Chapman²,

Kuchel³

2002

“…As is seen when using bipolar pulsed field gradient pairs in the PFG experiment, 25 preparatory gradient pulses of opposite polarity to the applied gradient pulses will partially cancel out the magnetic field transients. 12,26 As shown in Fig. 4, this is an effective tool for improving the stability of the deuterium lock signal, which leads to better high-resolution NMR spectra.…”

Section: Transient Magnetic Fields Following Magnetic Flux Changesmentioning

confidence: 96%

Artefacts and pitfalls in diffusion measurements by NMR

Sørland

Aksnes

2002

Self Cite

When applying pulsed field gradient (PFG) NMR experiments to determine the molecular mobility characterized by the diffusion coefficient, it is crucial to have control over all experimental parameters that may affect the performance of the diffusion experiment. This could be diffusion measurement in the presence of magnetic field transients, internal magnetic field gradients, either constant or spatially varying, convection, mechanical vibrations, or in the presence of physical restrictions affecting the diffusion propagator. The effect of these parameters on the diffusion experiment is discussed and visualized. It is also outlined how to minimize their influence on the measured diffusivity that is extracted from the PFG-NMR experiment. For an expanded and more general treatment we refer to the excellent reviews by Dr William S. Price (Concepts Magn. Reson. 1997; 9: 299; 1998; 10: 197) and the references therein.

“…For this kind of investigations, in which longer storage periods are necessary, a last and reliable resort is convection elimination by double-stimulatedecho pulse sequences. [14,15] From these two methods, we chose the pulse sequence with bipolar gradients developed by Müller and Jerschow ( Fig. 1), [15] because our studies showed that for the gradient lengths and strengths required for our samples a bisection of the absolute gradient length is advantageous (see below).…”

Section: Introductionmentioning

confidence: 99%

“…In such low temperature studies also convectioncompensating pulsed field gradient spin echo methods [14] can often not be applied due to very short T 2 times. For this kind of investigations, in which longer storage periods are necessary, a last and reliable resort is convection elimination by double-stimulatedecho pulse sequences.…”

Section: Introductionmentioning

confidence: 99%

Improved applicability of DOSY experiments by high resolution probes combined with gradient amplifiers of diffusion units

Zhang¹,

Kerssebaum²,

Gschwind³

2009

Short relaxation times and small diffusion coefficients often impede reliable diffusion experiments due to insufficient signal to noise ratios, especially in low temperature studies, where in the case of small diffusion coefficients in combination with very short T(2) times and substantial convection, double-stimulated-echo experiments are required as last resort of convection-compensating DOSY pulse sequences. Therefore, the combination of a strong gradient amplifier of a diffusion unit with Z-gradient high resolution probes is tested for low temperature applications to combine the advantages of high (1)H resolution, flexible temperature, and multinuclear applications with short gradient durations and diffusion delay. The experimental spectra on phosphoramidite ligands and phosphoramidite-copper complexes show that signal to noise improvements up to 176% were achieved despite longer eddy current delays and increased noise levels. Calculational estimations of the enhancement factors predict special benefits for systems with short transversal relaxation times and small diffusion coefficients and promise even higher enhancement factors for noise level optimized combinations. In addition, an easy way is presented to find fast and effectively relaxation optimized DOSY parameters for the convection-compensated double-stimulated-echo pulse sequence of Jerschow and Müller.