The ligands in that study are amide-capped pseudopeptides, which contain a varied central amino acid (X) flanked by a modified phosphotyrosine (pY) and an asparagine (N) (Figure ). For each flexible ligand (fpYXN,Fig. ), a constrained analog (cpYXN, Fig. ) was synthesized and tested under identical experimental conditions. The cpYXN ligands are conformationally constrained by cyclization of the phosphotyrosine to form a cyclopropane ring.y