Controlled peptide‐protein conjugation by means of 3‐nitro‐2‐pyridinesulfenyl protection‐activation

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Abstract
The disulfide bond in S‐3‐nitro‐2‐pyridinesulfenyl (S‐Npys) compounds is stable towards the acid treatment used in solid‐phase peptide synthesis, yet the lability of S‐Npys‐peptides towards nucleophiles enables the conjugation to proteins to proceed under mild conditions. Thus Boc‐Cys(Npys)‐OH was coupled as N‐terminal residue to a resin‐linked peptide chain. After deprotection and cleavage from the resin the Npys‐cysteinylpeptide was attached to a properly functionalized protein by reaction with a mercapto group. The amount of peptide conjugated to the protein was determined by measuring the amount of 3‐nitro‐2‐thiopyridone liberated. The cysteinylpeptide which was detached from the protein by reduction of the disulfide bond was shown to be identical with the product obtained by reduction of the Npys‐cysteinylpeptide.