Use of the Npys thiol protection in solid phase peptide synthesis Application to direct peptide‐protein conjugation through cysteine residues

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Abstract
The protection of the thiol function of cysteine with the 3‐nitro‐2‐pyridylsulfenyl (Npys) group has been successfully applied in the solid phase synthesis of nine peptides. A reexamination of the chemical stability of the protecting group has shown that, while Npys is essentially suitable for standard Boc/benzyl synthesis conditions, it is inadequate for the Fmoc strategy. Its proven stability to “high” HF acidolysis can not be extended to “low‐high”conditions without significant thiol deprotection. On the other hand, the Npys group is quite compatible with standard photolytical cleavage conditions. The stability of Npys to HF and its thiol‐activating character allow its application in peptide‐carrier protein conjugation reactions by specific coupling through cysteine residues in the peptide.