The transcyclization reaction transforms the thiosuccinimide linker into a thiazine linker (Figure , structure IV). This reaction also generates a new pseudopeptide bond in the thiazine linker structure (highlighted in red in Figure ). For the transcyclization reaction the cysteine residue involved in the thiosuccinimide linker should be located at the N-terminal end of the peptide; and its amino terminal group should be free, and nonprotonated .