Solid-state 14N NMR of amino acids and polypeptides

Amino acids play a vital role as the building blocks of proteins and their arrangement in the polypeptide chain controls protein structure and folding. These systems have been extensively investigated by NMR using 13C and 15N as the probe nuclei. Unfortunately, due to the low natural abundances of the probe nuclei (especially of 15N), costly isotopic labelling is often necessary to obtain high quality spectra. Nitrogen has two NMR active nuclei (14N and 15N), the former is 99.63% naturally abundant and perhaps may have use as an alternative probe nucleus, despite difficulties associated with 14N NMR. Nitrogen-14 is an integer-spin nucleus (I = 1) with a non-zero electric quadrupole moment (eQ = 20.44 mb) and a low gyromagnetic ratio (γ = 1.93 × 107 rad T−1 s−1) [1]. Due to the quadrupolar nature of this nucleus, it couples to the surrounding electric field gradient (EFG), which is dependent upon the local ground state electronic environment. Solid-state 14N NMR spectra of micro-crystalline samples often yield powder patterns with widths exceeding 1 MHz [1]. Spectral broadening low Larmor frequencies make acquisitions of 14N powder patterns quite difficult. Utilizing the 21.1 T NMR spectrometer and the WURST-QCPMG pulse sequence [2,3], we have been able to acquire high quality powder patterns of the majority of known amino acids with generall