Double cross polarization for the indirect detection of nitrogen-14 nuclei in magic angle spinning NMR spectroscopy

Abstract

Nitrogen-14 NMR spectra at fast magic-angle spinning rates can be acquired indirectly by means of two-dimensional techniques based on double cross polarization transfer H →N →H. Experimental evidence is given for polycrystalline samples of glycine, l-histidine, and the dipeptide Ala-Gly. Either one-bond or long-range correlations can be favored by choosing the length of the cross polarization contact pulses. Longer contact pulses allow the detection of unprotonated nitrogen sites. In contrast to earlier methods that exploited second-order quadrupolar/dipolar cross-terms, cross polarization operates in the manner of the method of Hartmann and Hahn, even for N quadrupolar couplings up to 4 MHz. Simulations explain why amorphous samples tend to give rise to featureless spectra because theN quadrupolar interactions may vary dramatically with the lattice environment. The experiments are straightforward to set up and are shown to be effective for different nitrogen environments and robust with respect to the rf-field strengths and to the N carrier frequency during cross polarization. The efficiency of indirect detection ofN nuclei by double cross polarization is shown to be similar to that of isotopically enriched C nuclei.

Dipolar couplings in solid polypeptides probed by 14N NMR spectroscopy

“…The experimental vertical cross sections do not allow one to resolve any second-order features of the 14 N lineshapes. This is consistent with our earlier work 20 and can be attributed to (i) the non-uniform efficiency of the CP transfer over the inhomogeneous lineshapes, and (ii) homogeneous signal decay in the evolution interval.…”

“…However, the acquisition of 14 N spectra of large biomolecules such as the GB3 protein (6 kDa) has required the use of dynamic nuclear polarization (DNP) techniques 19 . We have recently introduced a double cross-polarization (DCP) transfer 1 H → 14 N → 1 H for the indirect detection of 14 N spectra 20 . We have shown the method to be straightforward to implement and robust with respect to sample-specific parameters, and efficient for C Q values up to about 3 MHz.…”

Evaluation of excitation schemes for indirect detection of 14N via solid-state HMQC NMR experiments

“…As previously noted, HMQC experiments using DANTE trains are less robust to offset than those using SLP, although DANTE benefits from a slight increase in overall on-resonance excitation efficiency [40]. It must be noted that the robustness to 14 N SQ offset of 1 H-{ 14 N} D-HMQC 2D experiment with SLP excitation (SFWHM/nR » 0.6-1.0) is much larger than that of the corresponding method with double CP MAS transfer (SFWHM/nR » 0.25) ( Fig.4d-AM in [31]). It has very recently been demonstrated that optimizing the carrier frequency is recommended when using SLP and this is even more the case for D !…”

Self‐Assembly of DNA and RNA Building Blocks Explored by Nitrogen‐14 NMR Crystallography: Structure and Dynamics

“…However, NMR studies exploiting the 14 N isotope are far from routine. We have recently investigated the performance and efficiency of 1 H→ 14 N→ 1 H double cross‐polarization (DCP) schemes for the indirect detection of 14 N spectra, and found these methods to be remarkably straightforward compared to earlier techniques . We have illustrated the power of the DCP technique by investigating networks of ( 1 H, 14 N) dipole‐dipole couplings in powders of the cyclic undecapeptide cyclosporin, an important immunosuppressant drug …”