All of the conventional S-protecting groups of cysteine which were tested could be selectively converted to the 3-nitro-2-pyridinesulfenyl (Npys) group after treatment with an appropriate Npys halide. Unidirectional formation of an unsymmetrical disulfide bond is possible when Cys (Npys) is mixed with a free thiol of another Cys residue. Some of these features were exploited during the solid phase synthesis of lysine 8-vasopressin.
