Linebroadening

This angular dependence is different from the first-order perturbations so that the conventional teclmique of removing linebroadening in solids, MAS (see below), caimot completely remove this interaction at the same time as removing the first-order broadening. Flence, the resolution of MAS spectra from quadnipolar nuclei is usually worse than for spin-2 nuclei and often characteristic lineshapes are observed. If this is the case, it is... 

The complications that arise in solid state NMR spectra as compared to NMR spectra in solution are the consequence of the fixed orientation of the sample relative to the external magnetic field Bq. Mainly three interactions are responsible for the enormous linebroadening that can be observed for solid powder samples. These are (1) the shielding or chemical shift, including the chemical shift or shielding anisotropy CSA or Acr Hqsa) (2) homo- and/or heteronuclear dipole-dipole coupling (ffoo) and (3) in addition, for nuclei with spin >1/2, the quadrupolar interactions (Hq). 

What, then, is the end-group pattern that emerges from sequential addition of reactive components We calculated the expected line-broadening effects for randomly functionalized dendrimers and compared them to the experimentally observed linebroadening effects. Figure 13.18 compares the experimental results from the stackplot in Figure 13.16 to the calculated results. Because a wide body of hterature now indicates that dendrimer end groups can backfold into the interior of the dendrimer (Ballauff and Likos 2004), we included a volume shell in our calculations. 

Figure 8.2.4 Effect on the resonant frequencies of samples A to D of applying a z-gradient during data acquisition. The gradient strengths are in the order G3 > G2 > G > 0. The linebroadening effects on individual resonances are not shown for clarity...

From the simpler resonance line-shape and H/D-exchange analysis to the more complex studies of inherent dynamics, occurring on various time scale of motion, NMR remains a good choice to investigate protein flexibility and plasticity. If linebroadening due to exchange and inhomogeneity is minimized (or completely eliminated), then half-width, Aom, of a line becomes proportional to R, the transverse relaxation rate constant. 

Linebroadening resulting from paramagnetism, nuclear spin >1/2, or large molecular weight (MW) decreases resolution and may severely limit analysis. 

NMR analysis of a paramagnetic biomolecule may require modifications to the standard experiments. In some cases, this is to compensate for deleterious line broadening caused by unpaired electrons, which interferes with detection of resonances and NOEs. In other cases, modifications are made to take advantage of the effects of unpaired electrons to selectively observe nuclei in the vicinity of the paramagnetic metal. The effects of the metal on the NMR spectrum depend largely on the Xs value, which determines amount of linebroadening (equations 16 and 17). For systems with relatively short tg, few, if any, modifications need to be made, whereas study of systems with long tg require specially tailored experiments to obtain any information on nuclei in the vicinity of the metal site. ... 

Line shape analysis has been utilized to detect and elucidate dynamics of ligands bound to metals. For iron porphyrin models of metalloproteins, axial imidazole ligands may rotate relative to the heme axes linebroadening of heme substituents resulting from this process can be observed at low temperature. Such ligand motion is not commonly observed within metalloproteins because of constraints of the protein structure and metal-ligand bonding. NMR studies of... 

H. P. King andL. E. Alexander, X-Ray Diffraction Procedures , 2nd edition, John Whey Sons, New York, 1974, A very comprehensive treatment. X-ray powder methods including linebroadening analysis, texture determination, stress measurement and studies of amorphous materials. 

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