p-Nitrobenzyl protection for cysteine and selenocysteine: a more stable alternative to the acetamidomethyl group.

This study evaluated the acidic lability of the acetamidomethyl (Acm), trimethylacetamidomethyl (Tacm), and the p-nitrobenzyl (pNB) as protecting groups for cysteine and selenocysteine (Sec) during the tert-butyloxycarbonyl (Boc)-chemistry solid-phase peptide synthesis of oxytocin (OT). Two novel Sec building blocks (Nalpha-tert-butyloxycarbonyl-Se(acetamidomethyl)-L-selenocysteine (Boc-L-Sec(Acm)-OH) and Nalpha-tert-butyloxycarbonyl-S(4-nitrobenzyl)-L-selenocysteine (Boc-L-Sec(pNB)-OH)) were developed for this study. Six partially protected thio- and seleno-OT analogues were synthesized, purified, and exposed to neat trifluoroacetic acid (TFA) at temperatures of 25, 40, 50, and 60 degrees C for 1 h, and HF treatment at 0 degrees C for 1 h. Significant losses were observed for the Acm and Tacm group in TFA at temperatures greater than 25 degrees C and during HF treatment at 0 degrees C, whereas the pNB group remained intact. Removal of the pNB was achieved via reduction to the p-aminobenzyl group either with zinc in acetic acid in solution or via tin chloride in hydrochloric acid on solid support, followed by oxidative cleavage with iodine yielding the corresponding disulfide or diselenide bond. No major side reactions were observed. This study confirms the occasionally described Acm instability and underpins the development of the pNB group as an alternative for cysteine and Sec protection.