SYNTHESIS OF TWO ANALOGS OF A CYCLIC HEXAPEPTIDE WITH DISULFIDE BRIDGE CORRESPONDING TO BOVINE PROTHROMBIN PRECURSOR SEQUENCE 18–23: Extent of Carboxylation by Vitamin K‐Dependent Carboxylase

The synthesis of two analogs of sequence 18–23 of bovine prothrombin precursor is described. Hexapeptides Boc-Cys(Acm)-Leu-Glu(OBzl)-Glu (OBzl)-Pro-Cys(Acm)-OBzl and Ac-Cys(Acm)-Leu-Glu(OBzl)-Glu(OBzl)-Pro-Cys(Acm)-OMe were synthesized in solution by stepwise addition of Boc-amino acids using dicyclohexylcarbodiimide/N-hydroxybenzotriazole as the coupling reagent. The acetamidomethyl groups were cleaved and oxidized, using iodine in methanol, to the protected cyclic disulfide in 62–69% yield. The 0-benzyl groups were removed either by treatment with anhydrous hydrogen fluoride or hydrogen bromide in trifluoroacetic acid to give the cyclic hexapeptide disulfides, R1-Cys-Leu-Glu-Glu-Pro-Cys-OR2 where R1, = H or Ac and R2 = H or CH3. The cyclic hexapeptides were evaluated as substrates for vitamin K-dependent carboxylase. Both peptides are unusually poor substrates for the carboxylase, and each appears to inhibit carboxylation of Phe-Leu-Glu-Glu-Leu, a good substrate for the enzyme.