Enabling the Identification of Dopamine Receptor Ligands from Complex Matrices Using a Cyclic Tetrapeptide

G protein-coupled receptors (GPCRs) are targets for 36% approved drugs. However, identifying GPCR ligands remains cumbersome owing to their inherent instability and inevitably associated salts and detergents. Here, we resolve these bottlenecks by developing a stereoselective cyclic tetrapeptide (CP1) for identifying dopamine (DA) receptor ligands from complex matrices. This innovative strategy enables online coupling with liquid chromatography to reduce matrix complexity and accurate identification of putative ligands based on the high mass spectral resolution combined with the predictable pronounced mass-to-charge ratio shifts. CP1 is revealed to recognize the shared scaffolds or catecholic groups of the ligands, which is further proven tunable via conformational or submolecular modifications. Thereby, the first identification of β-phenylethylamine (β-PEA, an agonist of DA receptors) in underground water and the identification of 1-phenylguanidine (PG) and 1,3-diphenylguanidine (DPG) as antagonists of DA receptor D1 (DRD1) are achieved, raising concerns about their potential unintended effects on human dopamine receptors. This study demonstrates oligopeptides as a transformative class of gas-phase molecular tools for GPCR ligand discovery, effectively overcoming bottlenecks of conventional receptor-based methodologies.