Solid-Phase Synthesis of C‑Terminus Cysteine Peptide Acids

Cysteine (Cys) is a key amino acid in many therapeutic peptides. For research and industrial purposes, solid-phase peptide synthesis is the method of choice for the preparation of most peptides. The solid-phase synthesis of C-terminal Cys peptide acids is problematic because it is accompanied by a side reaction, namely, the abstraction of α-H from the Cys residue, which leads to the formation of three side products: the epimer and two N-piperidinyl-Ala epimer peptides. Here, we used a chlorotrityl chloride resin to conduct a rational and in-depth study of this side reaction. The following variables were examined: removal of the fluorenylmethoxycarbonyl (Fmoc) group by different bases, the presence or absence of an acid rectifier for buffering the base, and thiol side-chain protection. In conclusion, the use of Fmoc-Cys protected with tetrahydropyran (Thp) and 4-methoxytrityl (Mmt) along with 30% 4-methylpiperidine in 0.5 M OxymaPure-DMF for Fmoc removal assures minimization of the side reaction, as demonstrated in a model peptide and confirmed for the elongation of somatostatin