PFGSE

We finish this section by comparing our results with NMR and incoherent neutron scattering experiments on water dynamics. Self-diffusion constants on the millisecond time scale have been measured by NMR with the pulsed field gradient spin echo (PFGSE) method. Applying this technique to oriented egg phosphatidylcholine bilayers, Wassail [68] demonstrated that the water motion was highly anisotropic, with diffusion in the plane of the bilayers hundreds of times greater than out of the plane. The anisotropy of... 

Abbreviations D, self-diffusion coefficient ge, gradient-echo IR, inversion recovery IRFT, inversion recovery fourier transform MRS, magnetic resonance spectroscopy PD, proton density PFGSE, pulsed field gradient spin echo se, spin-echo. 

To date, D coefficients of carbohydrates established with the PFGSE approactf - " have been undertaken to (1) validate the theoretical self-diffusion coefficients calculated from MD trajectories, (2) demonstrate the complexation of lanthanide cations by sugars,(3) probe the geometry of a molecular capsule formed by electrostatic interactions between oppositely charged P-cyclodextrins, (4) study the influence of concentration and temperature dependence on the hydrodynamic properties of disaccharides, and (5) discriminate between extended and folded conformations of nucleotide-sugars. ... 

The Pulsed Field Gradient Spin Echo (PFGSE)... 

The same is true for the opposite side of the spectral window (—l/2r) as a 180° rotation gives the same result whether it is cw or ccw. If we put this 6-pulse sequence at the center of our PFGSE, it will reverse the sense of the coherence helix for the resonance l/2r away from the center, and it will maintain the sense of the coherence helix for the on-resonance water peak and for peaks 1/r away from the center. The resonance l/2r away will be unwound under the influence of the second gradient, whereas the on-resonance (water) peak will be wound twice as tightly, leading to zero net magnetization when summed over the whole sample (Fig. 8.21). 

We saw in Chapter 8 how a selective 180° pulse can be placed between two gradients of the same sign and duration to give a pulsed field gradient spin echo (PFGSE) that not only selects the desired coherence but also destroys any other coherences. First, we use a hard 90° pulse to create coherence on all spins, and then the first gradient twists the coherence into a helix (Fig. 8.21). The selective 180° pulse reverses the direction of twist in... 

The first term is just like 12C-bound H coherence It has not experienced a 180° pulse, and it will be destroyed by the second gradient. Only the last term will survive because it has experienced the equivalent of a 180°), pulse. In the complete G-BIRD there are two PFGSE elements, so the term becomes sin4(jr/r). In the example of Figure 11.4 (center), r was set to 2.94 ms, so the effect on various ch values can be predicted, as well as on long-range 2/ and 3J values ... 

The d values in PFGSE experiments are much larger than the network size considered here but much smaller than the size of a gel.71,87-89/98... 

TABLE 4 Determined diffusion coefficients and fractions for the fast and slow diffusion components of PSs in PMMA gels by PFGSE H NMR method... 

Figure 4. PFGSE H self diffusion in PolyPOMSOs Based Polymers and Monomers.

Figure 8.2 PFGSE 1H NMR spectrum of a 0.1 M solution of 3-encfc>-bromo-4-chlorocamphor (59) in CDCIg at 400 MHz. (a) Normal spectrum, (b) After irradiation of the C(8) methyl protons at 8 0.966. This results in enhancement of the H36 0 signal at 8 4.641. Signals for the methylene protons are absent, (c) After irradiation of the C(9) methyl protons at 8 1.128. NOE signals for the C(5) and C(6) exo methylene protons are observed. Signals for the endo methylene protons and the H3exo proton are absent, (d) After irradiation of H3 3 0 at 8 4.461 a NOE signal for the C(8) methyl proton is observed at 8 0.966...

In the gradient methods discussed above, a pulse train (S) is placed within two gradients G of equal intensity and duration such a sequence is a special case of a PFGSE, defined by the sequence G-t-G. The effect of a PFGSE on a frequency-selective sequence (G-S-G) as in the above examples is to refocus all frequencies experiencing a net inversion (180° rotation) and to dephase stationary components. A drawback of such a PFGSE (apart from... 

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