(14) N solid-state NMR spectroscopy of amino acids

¹⁴N ultra-wideline solid-state NMR (SSNMR) spectra were obtained for sixteen naturally occurring amino acids and four related derivatives using the WURST-CPMG pulse sequence and frequency-stepped techniques. The ¹⁴N quadrupolar parameters were measured for the sp³ nitrogen moieties (CQ ranges from 0.8 to 1.5 MHz). With the aid of plane-wave DFT calculations of the ¹⁴N EFG tensor parameters and orientations, the moieties are 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 is observed for systems with intermediate ηQ values than for those with low ηQ values (this variation arises from different intermolecular hydrogen-bonding arrangements). Strategies for increasing the efficiency of ¹⁴N SSNMR experiments are 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 are examined. Finally, ¹⁴N SSNMR is used to differentiate two polymorphs of histidine, via their quadrupolar parameters and transverse relaxation time constants. The strategies outlined herein permit for the rapid acquisition of directly-detected ¹⁴N SSNMR spectra that is not matched by other proposed methods to date.