Selective Bi‐directional Amide Bond Cleavage ofN‐Methylcysteinyl Peptide

Abstract

AbstractA selective bi‐directional peptide bond cleavage mediated byN‐methylcysteine (MeCys) in Xaa‐MeCys‐Yaa peptides (Xaa and Yaa, non‐cysteine residues) leading to thioesters and thiolactones is described. Rate and product analyses showed that an Nα‐amide bond cleavage occurred at the Xaa‐MeCys bond by an N–S acyl shift to generate an Xaa‐S‐(MeCys‐Yaa) thioester at pH 1–5, whereas under strongly acidic conditions ofH0= –5, the MeCys‐Yaa bond underwent a Cα‐amide bond cleavage via an oxazolone intermediate, which was trapped by thiocresol (TC) as an Xaa‐MeCys‐TC thioester. This thioester was then transformed into an Xaa‐MeCys‐β‐thiolactone at pH 4–5. Replacing MeCys by a Cys residue did not result in significant bi‐directional peptide bond cleavage, which suggests thatN‐methylation in a MeCys residue is important for the N–S acyl shift reaction and formation of oxazolone. The isomerization of amides and thioesters was successfully used to prepare cyclic peptides.