Abstract
The acquisition of 14N 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 14N 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 14N 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.
Highlights
The acquisition of 14N NMR spectra in solid samples is challenging due to quadrupolar couplings with magnitudes up to several MHz
We present the structural study of a biologicallyrelevant polypeptide by means of 14N NMR spectroscopy at a very fast magic-angle spinning (MAS) rate νR = 100 kHz35–39
The structure of cyclosporin reported in the literature[42], initially obtained by X-ray diffraction techniques, was optimized with planewave-pseudopotential density functional theory (DFT) methods by means of the VASP code[43,44,45]
Summary
Solid-state NMR is an invaluable tool to obtain structural details of arbitrary materials with atomic resolution Inhomogeneous interactions such as anisotropic chemical shifts, dipolar and quadrupolar couplings reveal a wealth of information on the local environment on an atomic scale[1]. The possibility of computing both isotropic and anisotropic interactions expected for a molecular model can be used to assess the agreement between experimental evidence and different structural hypotheses These computational tools are useful for the prediction of 14N NMR spectra since the wide frequency dispersion of the resonances and their inhomogeneous lineshapes (so-called “powder patterns”) are determined by both anisotropic chemical shifts and second-order quadrupolar interactions
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