¹⁴ N Solid-State NMR Spectroscopy of Amino Acids

¹⁴ N 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 ¹⁴ N quadrupolar parameters were measured for the sp³ nitrogen moieties (quadrupolar coupling constant, C_Q , values ranged from 0.8 to 1.5 MHz). With the aid of plane-wave DFT calculations of the ¹⁴ N electric-field gradient tensor parameters and orientations, the moieties were grouped into three categories according to the values of the quadrupolar asymmetry parameter, η_Q : 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 η_Q values than for those with low η_Q values (this variation arose from different intermolecular hydrogen-bonding arrangements). Strategies for increasing the efficiency of ¹⁴ N SSNMR spectroscopy experiments were discussed, including the use of sample deuteration, high-power ¹ H decoupling, processing strategies, high magnetic fields, and broadband cross-polarization (BRAIN-CP). The temperature-dependent rotations of the NH₃ groups and their influence on ¹⁴ N transverse relaxation rates were examined. Finally, ¹⁴ N 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 ¹⁴ N SSNMR spectra that to date was not matched by other proposed methods.