Single-Crystal NMR for ¹⁷O in Alanine Enantiomers

Single-crystal solid-state nuclear magnetic resonance (ssNMR) spectroscopy, which enables detailed analysis of the electronic structures of crystalline molecules, offers a unique opportunity to investigate molecular chiralityan essential feature with broad implications for understanding the origin and function of life. In this study, we employ single-crystal ssNMR spectroscopy, in combination with X-ray diffraction and density functional theory (DFT) calculations, to examine the electronic structure of ¹⁷O nuclei in crystalline forms of alanine enantiomers. Eight magnetically nonequivalent ¹⁷O resonances within the unit cell were observed and successfully assigned, and their corresponding NMR tensor parameters were determined. These resonances are comprised of pairs of chemically distinct oxygens in each of four symmetrically related sites. The experimental findings were compared with previous NMR studies as well as with DFT calculations performed in this work. The DFT results not only supported the assignment of crystallographically distinct ¹⁷O sites but also revealed previously unobserved antisymmetric components of the chemical shift tensors. This study presents the first comprehensive characterization of ¹⁷O NMR tensors in alanine enantiomers and underscores the power of integrating single-crystal ssNMR with X-ray diffraction and DFT calculations to advance our understanding of molecular chirality in amino acids.