Synthesis of selenocysteine peptides and their oxidation to diselenide‐bridged compounds

Abstract

Using the Fmoc/tBu protection scheme and the p-methoxybenzyl derivative of selenocysteine, the synthesis of related peptides in the selenol-protected form could be optimized by operating the coupling steps in the absence of auxiliary bases and by reducing the piperidine treatment to the minimum time required for quantitative Fmoc cleavage. Under these conditions, beta-elimination of the p-methoxybenzylselenol as the main side reaction of these syntheses, as well as epimerization of the protected selenocysteine, was largely suppressed. Conversion of the selenol- and thiol-protected bis-selenocysteine and selenocysteine, cysteine peptides into the related cyclic monomeric forms by iodine-mediated oxidation failed since a complex mixture of compounds was produced. Cleavage of the selenoether bond with mercuric acetate was found to proceed smoothly, but displacement of the heavy metal ions by treatment with excesses of thiols or hydrogen sulphide was unsuccessful since a stable Hg2+ diselenide complex was obtained. However, oxidation was achieved in good yields by the dimethylsulphoxide/trifluoroacetic acid procedure and the peptides were then used for determining the redox potential of the diselenide and selenide/sulphide bridge, respectively.

Selenocysteine Derivatives for Chemoselective Ligations

“…The currently most useful derivative to accomplish this goal contains a p-methoxybenzyl (PMB) protecting group on the selenol moiety. [22,23] Preparation of Fmoc-Sec(PMB)-OH has been accomplished by treating a b-halogenated alanine derivative with a diselenide anion, generated by reduction of selenium powder with either lithium triethylborohydride or sodium borohydride. [9,22,24,25] The resulting selenocystine derivative was subsequently reduced and reacted with PMBCl resulting in the protected selenol.…”

“…In addition, once incorporated into peptides, PMB-protected Sec tends to deselenate to some extent during iterative piperidine deprotection steps, yielding piperidine adducts to the initially formed dehydroalanines. [23] Moreover, in our hands, iodine-mediated deprotection of Sec(PMB)-containing peptides requires optimization for each peptide, and the presence of other Cys residues in the sequence can complicate matters. [9] Although these impediments require special precautions, the difficulties are greatly diminished when selenocysteine is only present at the N terminus (see below) and, therefore, is not exposed to multiple rounds of piperidine treatment during Fmoc-based SPPS.…”

Preparation of the β2‐Homoselenocysteine Derivatives Fmoc‐(S)‐β2hSec(PMB)‐OH and Boc‐(S)‐β2hSec(PMB)‐OH for Solution and Solid‐Phase Peptide Synthesis

“…On the other hand, an even better leaving group, PhSe, is used for the generation of dehydroalanine moieties in the use of convergent ligation strategies for the preparation of peptide conjugates [14]. Today, PMB is the most widely used protecting group for Sec, and it can be removed under various conditions [19] [20].…”

“…Therefore, the coupling is usually performed with DIC/HOBt 3 ) activation or with pre-formed pentafluorophenyl (Pfph) esters rather than by methods requiring tertiary base during coupling (e.g., HATU/DIEA). To minimize elimination during Fmoc or final deprotection, the reactions are carried out with much shorter reaction times, lower temperatures, and at lower concentrations of basic or acidic reagents [19].…”

Disulfide-Depleted Selenoconopeptides: Simplified Oxidative Folding of Cysteine-Rich Peptides

“…One recent advancement in the oxidative folding methods is the use of polymer-supported oxidation 17–19. Simplifying oxidative folding by replacing disulfide bridges with isosteric diselenide bridges 20–27 is another attractive strategy, but it has yet to be widely used by peptide chemists. The advantage of the selenopeptide technology is the redox-favored formation of the diselenide bridge compared to presence of disulfide bridges, ensuring the regioselective formation of crosslinks.…”