N -Acyl- N -Alkyl Sulfonamide Probes for Ligand-Directed Covalent Labeling of GPCRs: The Adenosine A 2B Receptor as Case Study.

Small molecular tool compounds play an essential role in the study of G protein-coupled receptors (GPCRs). However, tool compounds most often occupy the orthosteric binding site, hampering the study of GPCRs upon ligand binding. To overcome this problem, ligand-directed labeling techniques have been developed that leave a reporter group covalently bound to the GPCR, while allowing subsequent orthosteric ligands to bind. In this work, we applied such a labeling strategy to the adenosine A 2B receptor (A 2B AR). We have synthetically implemented the recently reported N -acyl- N -alkyl sulfonamide (NASA) warhead into a previously developed ligand and show that the binding of the A 2B AR is not restricted by NASA incorporation. Furthermore, we have investigated ligand-directed labeling of the A 2B AR using SDS-PAGE, flow cytometric, and mass spectrometry techniques. We have found one of the synthesized probes to specifically label the A 2B AR, although detection was hindered by nonspecific protein labeling most likely due to the intrinsic reactivity of the NASA warhead. Altogether, this work aids the future development of ligand-directed probes for the detection of GPCRs.