Ready to Use Cysteine Thiol Protecting Groups in SPPS, A Practical Overview

Protecting group chemistry plays a pivotal role during peptide synthesis. For the synthesis of cysteinyl peptides, this is more significant as cysteine (Cys) is involved in the formation of two key peptide bonds─the amide and the disulfide. Cys also poses itself as one of the most sensitive amino acid residues during peptide synthesis, especially for the occurrence of side reactions such as racemization, elimination, alkylation, etc. The magnitude of these side reactions is dependent on several factors including the choice of appropriate protecting groups for Cys thiol. The regioselective synthesis of multidisulfide peptides is a sequential process and it requires different sets of Cys thiol protections with unique removal conditions for each set of protecting groups used. Here, the selection of Cys side-chain protections is significant in obtaining the final peptide with the desired conformation and a good yield. With the proper knowledge of the chemistry of Cys thiol protecting groups trouble-free synthesis of multicysteinyl peptides can be achieved with minimization/elimination of a plethora of side reactions related to Cys thiol. In this current review, ready to use/commercialized Cys thiol protecting groups have been discussed with a focus on their use for the synthesis of multidisulfide peptides.