To rationalize the design of our fluorescent CCR2 ligand 14 in a retrospective manner, we synthesized and tested 27 as the CCR2 binding unit of 14 as well as the 14-related ligandlinker-conjugate 44 (Figure ). Using our displacement assay, we detected nanomolar CCR2 affinities for 27 (Ki = 6.22 ± 1.33 nM) and 44 (Ki = 98.2 ± 6.2 nM), thereby corroborating the structure-based design of 14.