Slow MAS

Wlien Al MAS NMR spectra were first collected at moderate Bq with relatively slow MAS rates (<4 kHz) there was much confiision about the quantitative integrity of such spectra. In fact, provided the correct excitations are employed (i.e. and 1) all that is necessary to make MAS NMR spectra... 

Fig. 8 13C and 29Si MAS-NMR of two SiC polytypes. Left panel-, see text for assignments. Right panel-. Experimental and calculated ultra-slow MAS-NMR spectra at ultra-high field of 21.1 T. Modified and reprinted with permission from [133]. Copyright 2009 by the American Chemical Society... 

While the (heteronuclear and homonuclear) residual dipolar couplings have to be eliminated in such J-resolved experiments, they are the main source of information in the WISE experiment described in the next section. For the investigation of viscoelastic materials the experiment thus must be performed under static conditions or under slow MAS. 

The dilute or medium-y spin- 2 nuclei have relatively small dipolar coupling (especially homonuclear coupling) because of spatial dilution or the small magnetic moment. Thus, even slow MAS is often sufficient to narrow these resonances. The factors which reduce the dipolar coupling unfortunately also lie behind the low... 

MAT) to resolve powder lineshapes arising from chemical shift anisotropy.5 The MAT experiment was developed by Gan6 and is a two-dimensional experiment, which resolves chemical shift anisotropy powder patterns in f2 according to their isotropic chemical shifts in /j. The whole experiment is conducted under very slow MAS. Under very slow MAS, the spectrum approximates to that of a static (non-spinning) experiment, and it is this feature that produces static-like powder patterns in the f2 dimension of the MAT experiment. 

The slow-MAS spin-diffusion experiment (S-MAS) [21] is a modified version of the proton-driven spin-diffusion experiment described above. It is applicable to situations where the chemical-shift differences in the spectrum are mainly due to the orientational dependence of the CSA tensor. In these... 

Fig. 4.4. Spectral factor/ fj(0) in the spin-diffusion rate constant calculated from the experimental separated-local-field spectrum of amorphous polystyrene [21, 30] for (a) a static sample, and (b) for a spectrum obtained under slow-MAS conditions. (Adapted from Ref. [21], with permission).

Fig. 33. Directions of the principal axes of the l3C chemical shift tensor of the C=0 group, helical axis, and static magnetic field, B0, and 13C NMR spectral patterns of the C=0 carbons corresponding to the orientation of the a-helix with respect to the surface of the magnetically oriented lipid bilayers. Simulated spectra were calculated using 5, =241 ppm, 22 =189 ppm, and < 33 =96 ppm for the rigid case (a), rotation about the helical axis (slow MAS) (b), fast MAS (c), magnetic orientation parallel to the magnetic field (d), an angle 8 with the magnetic field (e), and the direction perpendicular to the magnetic field (f).11 Reproduced with permission from the Biophysical Society.

MAS NOESY experiments have been widely used in SSNMR to study peptide-lipid interactions because of the fast axially rotation and segmental motion of membrane lipids in the liquid crystalline phase which average out efficiently the H- H dipolar couplings, resulting in a high resolution spectrum of membrane lipids under the slow MAS frequencies [166], which leads to the rapid applications of the NOESY-type [167] of solution NMR methods to study peptide-membrane interactions in MAS SSNMR [168-170]. 

The static (or slow MAS) spectra can be simulated assuming modes and frequencies for independent mechanisms of re-orientation, and simulations proceed iteratively to match the spectra. Several computer codes are available to perform the simulation, while some groups calculate spectra directly from analytical functions. wideline NMR studies have been applied to many adsorbate-microporous solid systems, including both physisorbed and chemisorbed species. The lineshape-matching process can sometimes be ambiguous, so that additional constraints on the possible mechanisms of motion, such as those provided by molecular dynamics or (time-averaged) by crystallography, are very helpful. 

It is worth noting that the slow-MAS methodology is a new area of research, and that several further improvements are necessary to make PASS and PHORMAT more viable methods for biochemical and biomedical research. Particularly, specific combinations of RF and pulsed-field-gradient sequences need to be developed so that PASS or PHORMAT spectra can be obtained of a select volume in the sample or the animal rather than the whole sample or... 

For cocrystal systems containing phosphorous (triphenylphosphine oxide 6-chloro-2-pyridone), P MAS NMR at slow MAS rates offers a more rapid way to confirm H-bonding and cocrystal formation fitting the CSA profile of the P spectrum. The anisotropy of the CST of a phosphine oxide group is strongly related to the... 

Heterogeneous binding of lipoteichoic acid to the surface of titanium dioxide was determined with MAS NMR spectroscopy. Using relatively slow MAS the structure of the lipoteichoic acid could be determined when it had bound to the Ti02 surface. ... 

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