Comparison and optimization of synthetic methods for preparing cholecystokinin peptides

Cholecystokinin‐octapeptide sulphate ester was synthesized by four different strategies in the liquid phase. Both sulphation of the non‐sulphated octapeptide and the introduction of tyrosine‐ O ‐sulphate into the peptide molecule were attempted. For sulphation, three different reagents were applied: the classical pyridine‐SO 3 H complex, DCC‐H 2 SO 4 and a new sulphating agent, pyridinium‐acetylsulphate. Although all these methods proved suitable for preparing cholecystokinin‐octapeptide of full biological activity, the mixed anhydride of sulphuric acid‐acetic acid seemed to be the best reagent for sulphation, causing minimal side‐reactions. The sensitive amino acids (methionine, tryptophan) could be protected from oxidation and tert. ‐butylation during the deprotection step by using a new scavenger mixture containing water, thioglycolic acid and dithiothreitol. Both classical liquid chromatography on silica gel and reverse‐phase high‐performance liquid chromatography were suitable for isolation of pure cholecystokinin‐octapeptide.