EUROISMAR 2019

Accurately determined conformations of biological agents are valuable assets in drug discovery. By enabling rational modifications, knowledge of the conformational envelope of biological ligands may enable the design of increased binding affinity, reduced off-target effects, or the discovery of an entirely new chemical series through “scaffold hopping”. The use of NMR to study the range of conformations adopted by molecules in solution (their “dynamic 3D-structure”) is becoming more widespread, however the success of this approach can be undermined by a sparsity of ¹H nuclei, poor signal dispersion, or exchange processes.

The fenamates are a group of small molecules (<300 Da) that have biological activity as non-steroidal anti-inflammatory drugs, and have been used to understand crystal nucleation, growth and polymorphism. Their structure is such that that no conformationally-dependent ¹H-¹H scalar couplings and very few distance-defining NOE measurements can be made. Signal dispersion is also poor, deepening the challenge that these molecules present. Although there was no single set of measurements that permitted the determination of their solution conformational behaviour, the dynamic 3D-structures of two very similar members of the group $–$ mefenamic acid and flufenamic acid $–$ were accurately determined by the synergistic combination of orthogonal NMR parameters.

The results reveal clearly measurable differences in the conformational envelopes of the two molecules, which are in agreement with their conformational behaviour as studied by small molecule and protein crystallography. Not only does this further demonstrate our ability to precisely determine molecular shape for use in rational design, these results provide direct experimental evidence for a link between behaviour in different phases, which has implications on the interpretation and use of data collected by different experimental techniques, and may further our understanding of self-assembly and nucleation processes.