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Contrary to the conventional wisdom of the peptide synthesis field, N,S-protected derivatives of cysteine can undergo substantial levels of racemization with widely-used reagents and protocols for stepwise incorporation. A systematic study of this problem has been carried out as a function of coupling conditions and β-thiol protecting groups, i.e., S-acetamidomethyl (Acm), S-triphenylmethyl (trityl or Trt), S-2,4,6-trimethoxybenzyl (Tmob), and S-9H-xanthen-9-yl (Xan), taking advantage of a convenient and quantitative model system assay involving HPLC resolution of H-Gly-l-Cys-Phe-NH₂ from H-Gly-d-Cys-Phe-NH₂. For example, standard protocols for couplings mediated by phosphonium and aminium salts, e.g., (benzotriazolyloxy)tris(dimethylamino)phosphonium hexafluorophosphate (BOP), N-[(1H-benzotriazol-1-yl)(dimethylamino)methylene]-N-methylmethanaminium hexafluorophosphate N-oxide (HBTU), N …