Scheme 1. Synthesis of phosphinate ABPs. (a) Synthesis of Cbz-protected inhibitors as previously described. (b) Synthesis of novel phosphinate ABPs incorporating azido hexanoic acid or a tripeptide as linker. (c) Structures of the tripeptides employed as linker in P4 probes. Reagents and conditions: (i) CbzNH₂, acetonitrile, reflux, aqueous workup; (ii) phenol, DIC, DMAP, toluene, 80 °C, 4 h; (iii) 33% HBr/AcOH, rt, 1 h; (iv) N₃-linker-OH, HATU, DIPEA, DMF, rt, overnight.