C-terminal thiocarboxylate

In this method, the cyclic peptide is generated through cysteine-perthioester cyclization to afford a Xaa-Cys bond whose thiol is then used for the second ligation step to the core through a thiol addition reaction (Scheme 25).[S71 For the perthioester cyclization, the linear precursor 91 consists of an activated N-terminal Cys(Npys) and a C-terminal thiocarboxylic acid which favors cyclization due to intramolecular mixed disulfide formation between them. 

We will encounter yet another post-translationaUy modified amino acid later in this chapter, namely the C-terminal thiocarboxylate used as sulfur donor in the biosynthesis of the universally distributed MoCo cofactor. 

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