The use of 2,2′‐dithiobis(5‐nitropyridine) (DTNP) for deprotection and diselenide formation in protected selenocysteine‐containing peptides

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In contrast to the large number of sidechain protecting groups available for cysteine derivatives in solid phase peptide synthesis, there is a striking paucity of analogous selenocysteine Se-protecting groups in the literature. However, the growing interest in selenocysteine-containing peptides and proteins requires a corresponding increase in availability of synthetic routes into these target molecules. It therefore becomes important to design new sidechain protection strategies for selenocysteine as well as multiple and novel deprotection chemistry for their removal. In this paper, we outline the synthesis of two new Fmoc selenocysteine derivatives [Fmoc-Sec(Meb) and Fmoc-Sec(Bzl)] to accompany the commercially available Fmoc-Sec(Mob) derivative and incorporate them into two model peptides. Sec-deprotection assays were carried out on these peptides using 2,2'-dithiobis(5-nitropyridine) (DTNP) conditions previously described by our group. The deprotective methodology was further evaluated as to its suitability towards mediating concurrent diselenide formation in oxytocin-templated target peptides. Sec(Mob) and Sec(Meb) were found to be extremely labile to the DTNP conditions whether in the presence or absence of thioanisole, whereas Sec(Bzl) was robust to DTNP in the absence of thioanisole but quite labile in its presence. In multiple Sec-containing model peptides, it was shown that bis-Sec(Mob)-containing systems spontaneously cyclize to the diselenide using 1 eq DTNP, whereas bis-Sec(Meb) and Sec(Bzl) models required additional manipulation to induce cyclization.