Fmoc-Sec(Xan)-OH: synthesis and utility of Fmoc selenocysteine SPPS derivatives with acid-labile sidechain protection

Abstract

We report here the synthesis of the first selenocysteine SPPS derivatives which bear TFA-labile sidechain protecting groups. New compounds Fmoc-Sec(Xan)-OH and Fmoc-Sec(Trt)-OH are presented as useful and practical alternatives to the traditional Fmoc-Sec-OH derivatives currently available to the peptide chemist. From a bis Fmoc-protected selenocystine precursor, multiple avenues of diselenide reduction were attempted to determine the most effective method for subsequent attachment of the protecting group electrophiles. Our previously reported one-pot reduction methodology was ultimately chosen as the optimal approach toward the synthesis of these novel building blocks, and both were easily obtained in high yield and purity. Fmoc-Sec(Xan)-OH was discovered to be bench-stable for extended timeframes while the corresponding Fmoc-Sec(Trt)-OH derivative appeared to detritylate slowly when not stored at -20 °C. Both Sec derivatives were incorporated into single- and multiple-Sec-containing test peptides in order to ascertain the peptides' deprotection behavior and final form upon TFA cleavage. Single-Sec-containing test peptides were always isolated as their corresponding diselenide dimers, while dual-Sec-containing peptide sequences were afforded exclusively as their intramolecular diselenides.

Keywords: selenocysteine; sidechain protection; solid phase peptide synthesis; xanthenyl protection.

Figure 1

Traditional sidechain protection architecture for Sec SPPS derivatives.

Figure 2

bis-Fmoc selenocystine synthetic intermediate 1 and Fmoc-Sec(Xan)-OH 2 and Fmoc-Sec(Trt)-OH 3 derivatives presented in this study.

Figure 3

Traditional literature synthetic approaches toward Sec SPPS derivatives.

Figure 4

Chromatograms showing MS Peak analysis of crude Mmp3-I peptide isolates.

Figure 5

HPLC chromatograms derived from crude Lys16 Grx 10 17 peptide isolates.

Figure 6

Possible mechanism to explain the existence of adducts 5 and 6 in MMP3-I test peptide product profile.