Cross-polarization experiment resonance assignments

Signal enhancements were obtained in H — O cross-polarization experiments without spinning, and reliable second-order quadrupolar powder patterns were observed [1291. Relaxation parameters involved in cross-polarization transfer were shown to be characteristic of the various sites, so that they can be used for signal assignment. In addition, in some cases the differences in crosspolarization rates were used to edit spectra by a selective enhancement of protonated oxygen resonances, such as those from surface hydroxyl groups in amorphous silica. The latter method can be applied to complicated systems, provided dipolar H — O interactions for the various sites are different. We illustrate this procedure by using a static H — O CP spectrum [126] of talc. 

In addition to sample rotation, a particular solid state NMR experiment is further characterized by the pulse sequence used. As in solution NMR, a multitude of such sequences exist for solids many exploit through-space dipolar couplings for either signal enhancement, spectral assignment, interauclear distance determination or full correlation of the spectra of different nuclei. The most commonly applied solid state NMR experiments are concerned with the measurement of spectra in which intensities relate to the numbers of spins in different environments and the resonance frequencies are dominated by isotropic chemical shifts, much like NMR spectra of solutions. Even so, there is considerable room for useful elaboration the observed signal may be obtained by direct excitation, cross polarization from other nuclei or other means, and irradiation may be applied during observation or in echo periods prior to... 

MQ NMR spectroscopy combined with heteronuclear NMR correlation experiments further increase spectral resolution and simplify the assignment of the detected NMR resonances. Moreover, information on interactions with neighboring atoms can be obtained along with the measurements of the interatomic distances. One of the first combined MQ experiments was conducted with the cross-polarization (CP) technique, which is normally used 1... 

Cross polarization can also be used for magnetization transfer between low-gamma nuclei like and as a mixing sequence in a multidimensional NMR experiment. Due to the substantial shift separation of about 120 ppm between carbonyl and Ca carbon atoms in proteins, the CP transfer can be tailored specifically to NCa- or NCO-transfer (SPECIFIC-CP) [81]. In protein NMR spectroscopy such a heteronuclear correlation experiment may be incorporated into multidimensional experiments and thus be used to residue-specific resonance assignment in NMR spectroscopy of immobilized peptides or proteins [82]. 

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