¹⁴N Solid-State NMR Spectroscopy of Amino Acids

14N ultra-wideline solid-state NMR (SSNMR) spectra were obtained for 16 naturally occurring amino acids and four related derivatives by using the WURST–CPMG (wideband, uniform rate, and smooth truncation Carr–Purcell–Meiboom–Gill) pulse sequence and frequency-stepped techniques. The 14N quadrupolar parameters were measured for the sp3 nitrogen moieties (quadrupolar coupling constant, CQ, values ranged from 0.8 to 1.5 MHz). With the aid of plane-wave DFT calculations of the 14N electric-field gradient tensor parameters and orientations, the moieties were grouped into three categories according to the values of the quadrupolar asymmetry parameter, ηQ: low (≤0.3), intermediate (0.31–0.7), and high (≥0.71). For RNH3+ moieties, greater variation in N−H bond lengths was observed for systems with intermediate ηQ values than for those with low ηQ values (this variation arose from different intermolecular hydrogen-bonding arrangements). Strategies for increasing the efficiency of 14N SSNMR spectroscopy experiments were discussed, including the use of sample deuteration, high-power 1H decoupling, processing strategies, high magnetic fields, and broadband cross-polarization (BRAIN-CP). The temperature-dependent rotations of the NH3 groups and their influence on 14N transverse relaxation rates were examined. Finally, 14N SSNMR spectroscopy was used to differentiate two polymorphs of l-histidine through their quadrupolar parameters and transverse relaxation time constants. The strategies outlined herein permitted the rapid acquisition of directly detected 14N SSNMR spectra that to date was not matched by other proposed methods.