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Accueil du site > Productions scientifiques > Séminaires à PHENIX > 2010 > Séminaire 20.09.2010 - 11h00

Séminaire 20.09.2010 - 11h00

par Guillaume Mériguet - 13 octobre 2010

William S. Price (Nanoscale Organisation and Dynamics Group - U. Western Sydney) présentera un séminaire le 20 septembre 2010 à 11h00 dans la bibliothèque du laboratoire PECSA (7e étage, batiment F, porte 754) intitulé :

Developments in NMR diffusion measurements and q-space imaging

Résumé

Translational diffusion is the most fundamental form of transport in the solution state and it is directly linked to the size and shape of a diffusing species. Consequently, diffusion is a natural probe for molecular association (e.g., protein-protein self-association or drug-protein binding). Further, if the timescale of the measurement is such that the diffusing molecules have time to interact with any boundaries (e.g., diffusion in a porous medium or biological cell), then the diffusion measurement will also provide information on the characteristic distances of the geometrical restrictions. Of the available methods for measuring diffusion, pulsed gradient spin-echo (PGSE) NMR diffusion measurements (also commonly referred to as DOSY or NMR diffusometry) are now widely used due to their wide applicability, efficacy, information content and non-invasive nature. When PGSE NMR is used to probe porous media it is sometimes referred to as q-space imaging.

To extract maximum information, the PGSE measurement must be accurate and precise as well as being robust. For example, it is important that the measurement technique is able to work on real samples and require minimal pre-treatment (e.g., containing protonated solvents, see Fig. 1). Further, flexibility in measurement timescales will allow more information to be obtained. For example, shorter measurement timescales will allow greater probing of the kinetics of protein association and folding. In restricted systems more information can also be gleaned if the timescale of the measurement can cover a greater range of possible times.

Fig. 1. The PGSTE-WATERGATE pulse sequence (left) and PGSE spectra (right) of 2 mM lysozyme in water (10:90 D2O/H2O). Note the almost complete removal of the water resonance.

Accurate PGSE measurements require high signal-to-noise ratios and, ideally, that the sample is subject to strictly defined (homogeneous) magnetic gradients. The reality is often thwarted by the effects of J-evolution and the presence of susceptibility-induced background gradients resulting from the magnetically heterogeneous nature of the sample and also limitations of the gradient generation system (including its calibration). At present, under ideal conditions the PGSE technique has a lower limit of 10-15 m2s-1.

Recent work in my laboratory has addressed many of the aspects mentioned above with a focus on developing versatile NMR pulse sequences for performing PGSE measurements (e.g., see Fig. 1 and Fig. 2)

Fig. 2. The MAG-PGSTE pulse sequence for determining diffusion in magnetically inhomogeneous samples.

This lecture will detail some recent advances in NMR diffusion measurements and the type of modelling needed to analyse the resulting data. The talk will be illustrated with data drawn from a number of experimental studies ranging from biological to chemical.

References

  1. W.S. Price, NMR Studies of Translational Motion : Principles and Applications, Cambridge University Press, Cambridge, 2009.
  2. G. Zheng, W.S. Price, MAG-PGSTE : A new STE-based PGSE NMR sequence for the determination of diffusion in magnetically inhomogeneous samples. J.Magn.Reson. 195 (2008) 40-44.
  3. G. Zheng, T. Stait-Gardner, P.G. Anil Kumar, A.M. Torres, W.S. Price, PGSTE-WATERGATE : An STE-based PGSE NMR sequence with excellent solvent suppression. J.Magn.Reson. 191 (2008) 159-163.
  4. N.N. Yadav, A.M. Torres, W.S. Price, NMR q-space imaging of macroscopic pores using singlet spin states. J.Magn.Reson. 204 (2010) 346-348.