Single-Crystal NMR for 17O in Alanine Enantiomers

TL;DR

The paper addresses how to resolve chiral environments in amino acids at the level of nucleus-specific electronic tensors by using single-crystal ssNMR of [17]O in alanine enantiomers, integrated with X-ray diffraction and DFT (GIPAW) calculations. By mounting and rotating single crystals along three orthogonal axes, the authors assign eight magnetically nonequivalent [17]O resonances and extract quadrupolar and CSA tensor parameters, validating them against DFT predictions. The study reveals antisymmetric components of chemical-shielding tensors and demonstrates mirror-symmetric ACS behavior across enantiomers, linking experimental observations to chirality. This integrated approach advances the understanding of chiral environments in amino acids and points to future avenues for observing ACS effects via satellite transitions and higher-field measurements with potential implications for prebiotic chemistry.

Abstract

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. 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.

Figures (7)

• Figure 1: (a): Photograph of front view of the probe. The arrow shows rotation of the goniometer inside the RF coil. Inset shows L-alanine crystal glued to the tenon. The tenon may be mounted on three different orientations in the goniometer so that three orthogonal rotations of the sample can be achieved. (b): Photograph of side view of the probe showing goniometer mechanism and probe coils. (c): Rotation of tenon around an axis that is perpendicular to magnetic field. The three mutually perpendicular rotations are achieved by mounting the tenon plate into dovetails for $\text{-}x^T$, $y^T$, and $\text{-}z^T$ rotations. (This Figure is modified from Ref. vosegaard_single-crystal_2021 under CC BY 4.0 license)
• Figure 2: Orthorhombic crystal structure of alanine with crystallographically nonequivalent O nuclei labeled as O1 and O2 and their shielding tensors represented in orange and green colors. All the eight O nuclei are magnetically nonequivalent. The arrows represent directions and scaled magnitudes of the computed chemical shielding tensors in the crystal frame. O, C, and N nuclei are shown in red, gray, and blue, respectively. (H nuclei omitted for clarity.)
• Figure 3: Single-crystal [17]O NMR spectra at 14.1 T for L-alanine. The spectra were recorded at $15^\circ$ increments for rotation about (a) mounting -$x^T$, (b) mounting $y^T$, and (c) mounting -$z^T$
• Figure 4: Single-crystal [17]O NMR spectra at 14.1 T for D-alanine. The spectra were recorded at $10^\circ$ increments for rotation about (a) mounting -$x^T$, (b) mounting $y^T$, and (c) mounting -$z^T$
• Figure 5: Rotation plots for [17]O central transition in L-alanine showing experimental resonances with two crystallographically nonequivalent [17]O sites each having four magnetically nonequivalent [17]O nuclei (marked as $\bigcirc, \square, \bigtriangleup, \cross$) under rotation of the three orientations of the crystal sample. The curves are constructed from optimized coefficients (see Table \ref{['tab:Lala fit coeff']}).