Synthesis and biological activity of new conformationally restricted analogues of pepstatin*

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Abstract
A new statine derivative, 3‐hydroxy‐4‐amino‐5‐mercaptopentanoic acid; cysteinylstatine (CySta), was synthesized and used to prepare a series of conformationally restricted analogues of pepstatin (Iva‐Val‐Val‐Sta‐Ala‐Sta) in which the conformational constraint was introduced via a bis‐sulfide connecting the appropriately substituted residues in the P1 and the P3 inhibitor side chains. The precursor peptide, Iva‐Cys‐Val‐CySta‐Ala‐Iaa, was synthesized and alkylated with a series of dibromoalkanes and alkenes to produce the cyclic structures. This strategy permitted the carbon atom spacing between the P1 and the P3 inhibitor side chains to be systematically varied so as to produce inhibitors with 15‐, 16‐, and 17‐membered ring systems. Additional non‐cyclic analogues were synthesized as controls by alkylating the bisthiol intermediates with methyl iodide. The inhibitory potency of the analogues were determined against porcine pepsin and penicillopepsin by using standard enzyme kinetic assays. The cyclic inhibitor were found to be potent inhibitors of both aspartic proteases; inhibitor that contained a trans‐2‐butene link between the two sulfur atoms was found to be the most potent inhibitor with a Ki less than 1 nm against pepsin and 3.94 nm against penicillopepsin. This series of compounds illustrates a new type of conformational restriction that can be used to probe for the bioactive conformation of peptides.