Determination of Ligand-Binding Affinity (K_d) Using Transverse Relaxation Rate (R₂) in the Ligand-Observed ¹H NMR...

High hit rates from initial ligand-observed NMR screening can make it challenging to prioritize which hits to follow up, especially in cases where there are no available crystal structures of these hits bound to the target proteins or other strategies to provide affinity ranking. Here, we report a reproducible, accurate, and versatile quantitative ligand-observed NMR assay, which can determine K_d values of fragments in the affinity range of low μM to low mM using transverse relaxation rate R₂ as the observable parameter. In this study, we examined the theory and proposed a mathematical formulation to obtain K_d values using non-linear regression analysis. We designed an assay format with automated sample preparation and simplified data analysis. Using tool compounds, we explored the assay reproducibility, accuracy, and detection limits. Finally, we used this assay to triage fragment hits, yielded from fragment screening against the CRBN/DDB1 complex.