2 - Amide Formation, Deprotection, and Disulfide Formation in Peptide Synthesis

Abstract

The amide-forming reaction is one of the most crucial steps in peptide synthesis. An appropriate activation is required to perform the coupling under mild conditions because harsh chemical conditions can harm complex peptides. Two types of reactions are available theoretically, namely, amino activation and carboxyl activation. The former methods have rarely been applied to practical peptide synthesis because of the drastic reaction conditions and racemization. Carboxyl activation has been used in almost all recent peptide couplings. The other crucial steps in peptides synthesis are protection and deprotection. Deprotecting steps are of two types: deprotection of α-amino groups to give peptides for further extension and final deprotection to give free peptides. Most frequently, trifluoromethanesulfonic acid has been used for the final deprotection in combination with selective deprotection of temporary α-amino protecting groups. Another step that critically influences the synthesis of a cystine-containing peptide is disulfide bond formation. Particularly in the syntheses of peptides and proteins containing two or more disulfides, the final disulfide forming reactions often occur in poor yields due to the formation of side products such as polymers or isomers with disulfide locations different from those of the desired product. These side reactions are unavoidable whenever the peptide chains are synthesized either by chemical means or by genetic engineering. This chapter also elucidates the aspects of the folding and disulfide formation of proteins.