Anisotropic NMR Techniques for Molecular Structure Elucidation

Anisotropic NMR spectroscopy exploits controlled partial alignment of molecules in solution to detect orientation‐dependent interactions that are averaged to zero in isotropic media. By introducing alignment media such as liquid crystals, stretched gels or polymer films, it becomes possible to measure residual dipolar couplings (RDCs), residual chemical shift anisotropies (RCSAs) and residual quadrupolar couplings (RQCs). These parameters furnish long‐range orientational constraints and tensorial information about bond vectors and shielding or electric field gradient tensors. When combined with conventional scalar coupling and NOE data, anisotropic observables greatly enhance the determination of relative and absolute configuration, conformational ensembles and dynamic behaviour. The interpretation of anisotropic NMR data often relies on alignment tensors and computational fitting, molecular dynamics with tensorial restraints or three‐dimensional alignment simulations such as P3D. These approaches have been applied to small organic molecules, complex natural products and bioactive compounds, enabling unambiguous stereochemical assignments and conformational analyses even for minute samples. The global significance of anisotropic NMR spans drug discovery, natural‐products chemistry and materials science, with ongoing advances in alignment medium design, sensitivity enhancement and integration with quantum‐chemical prediction methods.

Research from Nature Portfolio

Recent studies have demonstrated the power of proton residual chemical shift anisotropies measured in constrained aligning gels or liquid crystals to determine the three‐dimensional structures of organic molecules available in microgram quantities. By exploiting the abundant 1H spin, accurate RCSA measurements in deuterated alignment media have enabled the relative configuration of challenging briarane diterpenes to be established from as little as 35 µg of material. This approach circumvents the limitations of low‐abundance 13C experiments and opens new possibilities for structural elucidation of scarce natural products and synthetic compounds.

Anisotropic NMR Techniques for Molecular Structure Elucidation publication trend

The graph below shows the total number of articles in anisotropic nmr techniques for molecular structure elucidation across all publications each year (not limited to Nature Index journals).

Technical terms

Anisotropic NMR spectroscopy: NMR techniques employing partial molecular alignment to reveal orientation‐dependent interactions.

Alignment medium: A partially ordered environment causing restricted molecular tumbling necessary for anisotropic NMR measurements.

Residual dipolar coupling (RDC): Weak dipolar interactions observed in anisotropic media that provide long‐range orientational constraints between nuclear spins.

Residual chemical shift anisotropy (RCSA): Variation in chemical shift due to orientational dependence of the shielding tensor in partially aligned molecules.

Residual quadrupolar coupling (RQC): Interaction between nuclear quadrupole moments and electric field gradients under partial alignment, offering structural insights for quadrupolar nuclei.

Alignment tensor: A mathematical description of how a molecule is oriented on average within an alignment medium, used to interpret anisotropic NMR parameters.

References

1. Relative configuration of micrograms of natural compounds using proton residual chemical shift anisotropy. Nature Communications (2020).
2. Residual‐Chemical‐Shift‐Anisotropy‐Based Enantiodifferentiation in Lyotropic Liquid Crystalline Phases Based on Helically Chiral Polyacetylenes. Angewandte Chemie International Edition (2023).
3. Resonance and structural assignment in (car)borane clusters using 11 B residual quadrupolar couplings. Chemical Communications (2023).
4. Configuration determination by residual dipolar couplings: accessing the full conformational space by molecular dynamics with tensorial constraints. Chemical Science (2019).
5. Extending the applicability of P3D for structure determination of small molecules. Magnetic Resonance (2021).
6. Conformational analysis of small organic molecules using NOE and RDC data: A discussion of strychnine and α-methylene-γ-butyrolactone. Journal of Magnetic Resonance (2015).