The topic of our March round-table workshop (Thursday March 30, 2023 @ 12:00 PM ET USA) was a discussion of Anisotropic Interactions in NMR (RQCs, RDCs, RCSAS) and their application to the analysis of small molecules. Roberto Gil of Carnegie Mellon University lead the panel that includes Han Sun -Leibniz Research Institute for Molecular Pharmacology, Berlin, Germany, Philippe Lesot – University of Paris-Saclay, Orsay, France, and Robert T. Williamson Univ of North Carolina Wilmington, USA.
The configurational (3D structure) and conformational analysis of small molecule by NMR spectroscopy has been historically performed using NMR parameters such as chemical shift, J-coupling constants and the nuclear Overhauser effect (NOE). However, there are cases in which the use of these three NMR parameters is not enough to lead to a unique solution to the structural problem. This limitation can be lifted by the application of anisotropic NMR parameters such as Residual Dipolar Couplings (RDCs), Residual Chemical Shift Anisotropies (RCSAs) and Residual Quadrupolar Couplings (RQCs). RDCs, RCSAs and RQCs are not directly observed in a solution that solely uses NMR solvents, but they are observable in the presence of alignment media such as physically strained (compressed or stretched) polymers gels or lyotropic liquid crystalline solutions (LLC phases) made with homopolypeptides, polyacetylenes, graphene oxide derivatives and self-assembling peptides. NMR in anisotropic media is a game changer in 3D structure elucidation of small molecules since anisotropic NMR parameters can correlate the relative configuration of remotely located stereocenters regardless of the distance between them. Even if the structural problem can be unambiguously solved using isotropic NMR parameters, the complementary use of anisotropic NMR parameters represents an orthogonal way of 3D structure confirmation. We will a panel discussion of practical applications of NMR in anisotropic media to the structural analysis of small molecule. The members of the panel will show applications to their everyday structural problems.