Dipolar couplings in solid polypeptides probed by 14N NMR spectroscopy

Abstract

The acquisition of 14 N NMR spectra in solid samples is challenging due to quadrupolar couplings with magnitudes up to several MHz. This nucleus is nonetheless important as it is involved in the formation of essential secondary structures in biological systems. Here we report the structural study of the atomic environment of amide functions in polypeptides using magic-angle spinning NMR spectroscopy of the ubiquitous 14 N isotope. The cyclic undecapeptide cyclosporin, in which only four hydrogen atoms are directly bound to nitrogen atoms, is chosen for illustration. Structural details of different environments can be revealed without resorting to isotopic enrichment. The network of inter-and intra-residue dipolar couplings between amide 14 N nuclei and nearby protons can be probed and mapped out up to a tunable cutoff distance. Density functional theory calculations of NMR quadrupolar interaction tensors agree well with the experimental evidence and allow the unambiguous assignment of all four non-methylated NH nitrogen sites and neighboring proton nuclei.

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

“…The white empty arrows indicate the improvements obtained by including the Cl − anion in the structure considered in this study for data points associated with the N(9)H proton and 14 N environments. In Figure b, the centers of gravity (first moments M 1 ) of the experimental 14 N lineshapes were correlated with the isotropic shifts . The calculated frequencies for the N(9)H proton and 14 N result from averaging over the two N(9)H and N(9)H−Cl − environments given in Table .…”

“…The offset of the carrier frequency ν rf in these simulations matched the experimental conditions. The simulated 2D spectra of Figure (c,d) represent pure adsorptive correlations between ideal 14 N and 1 H lineshapes, and results from the sum of individual 2D spectra associated with different ( 1 H, 14 N) distances obtained from the DFT‐optimized structures . Each of these individual 2D spectra was constructed by forming the product Re[ S ( ω 1 )] ⋅ Re[ S ( ω 2 )], where the column vector S ( ω 1 )is the spectrum for an ideal 14 N second‐order quadrupolar lineshape simulated with Simpson assuming NMR parameters calculated with DFT methods, and the row vector S ( ω 2 ) is a simple Lorentzian lineshape simulated with Simpson, associated with a 1 H spin with an isotropic shift given by DFT methods.…”

“…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 …”

Polymorphic Forms of Valinomycin Investigated by NMR Crystallography

“…For all the aforementioned structures, their intermolecular interactions; the key internal coordinates including parameters of hydrogen bonds; and selected SSNMR data are discussed. These results should be useful also in analyses of other pharmaceutically active cyclic peptides like cyclosporins [19,20].…”

Natural abundance oxygen-17 solid-state NMR of metal organic frameworks enhanced by dynamic nuclear polarization

“…[7] The NMR parameters thus obtained can then be compared with those calculated by means of DFT methods [8,9] and serve to validate different structural hypotheses and assist structural refinement. [10][11][12][13][14][15][16] The combination of MAS NMR spectroscopy and DFT calculations has become known as "NMR crystallography" and has found successful applications in a multitude of studies. [17] For the structural elucidation of a MOF, an accurate description of the constitutive inorganic building units (IBUs) and their sites is a crucial step.…”