A series of novel α-ketoamides incorporating stereoisomeric residues with different electronic properties at the P1'-position were synthesized to study the electronic requirements for inhibitor binding to the S1'-subsite of calpain I. The results of the study suggested the presence of an acidic amino acid residue at the S1'-subsite of calpain I. For example, ester 1a (Cbz-l-Leu-l-Phe-CO-d-Phe-OMe) was over 450-fold more potent than ...