Chemical-shift anisotropy motion effects

Most NMR measurements are carried out in solution, where rapid molecular motion removes the effects of chemical shift anisotropy and (generally) produces sharp lines. For the study of solids, techniques are used which combine rapid spinning about the magic angle axis with special pulses, which remove much of the anisotropy and produce pseudo-solution spectra. 

Equation (3) has several other important implications which can be directly confirmed by finite-frequency probes. One example is the motion-narrowing effect in NMR experiments which is expected to disappear when l/r is below the chemical-shift-anisotropy (CSA) width. Indeed the NMR results of Tycko et al. [16] indicate that for a CSA width of 18.2 kHz the line broadens below 190 K and develops a powder pattern at lower temperature. This is in fair agreement with the 200 K calculated from Eq. (3). They also concluded that the thermal activation energy is around 260 meV below TV, again close to the values we calculated. The glassy dynamics can be probed by other experiments such as sound attenuation, microwave absorption, and thermal conductivity. In particular the characteristic temperature will depend on probe frequency. Such studies are essential to fully understand the low-temperature orientational dynamics. 

There is no straightforward and completely rigorous procedure for determining the relative combinations of the various relaxation mechanisms, except where one mechanism clearly dominates (e.g., if the maximum possible nuclear Overhauser effect (NOE) for a resonance is obtained, dipolar relaxation must dominate its relaxation or an increase in relaxation rate in proportion to the square of the applied field must be due to chemical shift anisotropy). Hence, the study of molecular motion in proteins from relaxation data is performed most readily on nuclei directly bonded to H, and so principally relaxed via dipole-dipole interactions (see Section 4(e)(iii)). 

Fig. 2. The MAT experiment applied to poly(2-hydroxypropyl ether of bisphenol A)5 (top) to examine the 180° ring flips affecting 13C 4 and 5. (a) The complete two-dimensional MAT spectrum.5 The projection in f2 is effectively the lineshape that would be recorded for a powder sample. As this spectrum clearly shows, the chemical shift anisotropy powder patterns from the nine 13C sites in this polymer are extensively overlapped and would not be resolved without the aid of this MAT experiment, (b) The powder lineshapes for each 13C site taken from the two-dimensional spectrum in (a).5 Those for carbons 4 and 5 show distortions of the lineshape shoulders typical of motional averaging, in this case from 180° phenyl ring flips.

Phosphorus " P is as well as other nuclei, present in biological membranes and has special advantages. Phospholipid head groups contain an isolated 7=1/2 spin system which depends only on chemical-shift anisotropy and dipolar proton-phosphorus interactions. It is therefore a useful probe for structure and motion. The chemical shift of P changes with the orientation of the magnetic field with respect to the nucleus. The observed spectrum can therefore be measured over a wide range of about 100 ppm. As the chemical-shift difference for P is only 4 ppm the chemical-shift anisotropy, because of orientational effects, controls the spectrum. A typical P-NMR spectrum of polymorphic phases of phospholipid bilayers is depicted in Figure 11-6. For details the reader is referred to specific publications [63]. 

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