Preparations of Boc-Cys(S-Pyr)-OH and Z-Cys(S-Pyr)-OH and their applications in orthogonal coupling of unprotected peptide segments

Boc-Cys(S-Pyr)-OH and Z-Cys(S-Pyr)-OH were prepared by addition of their cysteine derivatives to 3 equiv of 2,2′-dipyridyldisulfide in one portion. 2-Mercaptopyridine was removed by addition of 0.1 M CU(NO₃)₂ to the solution. Both derivatives are white solids and can be used to facilitate the formations of heterodisulfide bonds. Two methods of synthesizing peptides with N-terminal Cys(S-Pyr) were also provided. Two peptide thiocarboxylic acids H-Tyr-Ser-Ala-Glu-Leu-Val-SH and H-Tyr-Ser-Ala-Glu-Leu-Gly-SH were prepared on the thioester benzhydryl resin with the cleavage condition of 1.0 m TFMS A/TFA instead of HF. From the orthogonal couplings of these peptides with H-Cys(S-Pyr)-Tyr-Ser-Glu-Leu-Ala-NH₂, both intramolecular acyl transfers finished at pH 7 at about 15 to 20 min. The intermediate acyl disulfide peptide was collected by high-performance liquid chromatography and identified by liquid chromatography-mass spectrometry.