Redox‐active bis‐cysteinyl peptides. I. Synthesis of cyclic cystinyl peptides by conventional methods in solution and on solid supports

Abstract

Cyclic mono-cystinyl active-site fragments of thioredoxin and thioredoxin reductase were synthesized as N-acetyl and C-amide octapeptides by conventional methods of peptide synthesis in solution and on solid supports. Using a side-chain protection based on acid-labile tert-butanol-derived groups and on the S-tert-butylthio unsymmetric disulfide for the thiol functions, in combination with N alpha-Z- or N alpha-Nps derivatives in the chain elongation steps, the synthesis in solution was carried out in straightforward manner yielding the fully protected octapeptides as well characterized compounds. Upon deprotection with trifluoroacetic acid and reduction of the unsymmetrical disulfides with tri-butylphosphine, the resulting bis-cysteinyl-octapeptides were oxidized in dimethylformamide with azodicarboxylic acid di-tert-butyl ester to produce the desired cyclic compounds in good overall yields. For the synthesis on solid supports a similar acid-labile side-chain protection was applied in combination with the N alpha 9-flourenylmethyoxycarbonyl derivatives in the chain elongation steps. Thereby acylations were performed with the related amino acid N-carboxyanhydrides (UNCAs) or by the O-(1H-benzotriazol-1-yl)-N,N,N',N'-tetramethyluronium-tetraf luoroborate/1- hydroxybenzotriazole (TBTU/HOBt) procedure. The solid phase synthesis of the two octapeptides led to unexpected difficulties in terms of recovery of peptidic material from the resins in the final acidolytic cleavage step as well as of racemization at the level of the cysteine residues by the TBTU/HOBt coupling method. Racemization was efficiently suppressed by employing the related pentafluorophenyl ester and this method led to crude octapeptide products of a degree of purity comparable to those obtained by the synthesis in solution. However, the recovery of the peptides from the resin, i.e., irreversible reattachment of cleaved peptidic material via alkylation of various side-chain functions, could not be avoided even using the most efficient scavengers or their cocktails.

Photomodulation of conformational states. Synthesis of cyclic peptides with backbone-azobenzene moieties

“…In previous syntheses of bis-cysteinyl-octapeptides related to the active sites of thiol protein oxidoreductases, racemization of the cysteine residues, whatever the thiol protecting group, was found to occur at extents of up to 30% using the standard HBTU/HOBt/DIEA (1:1:2) coupling procedure [30,32]. This high degree of racemization of cysteine residues was fully confirmed recently by a detailed study by Han et al [33].…”

“…This high degree of racemization of cysteine residues was fully confirmed recently by a detailed study by Han et al [33]. It can, however, be largely suppressed by the use of pentafluorophenyl esters [30,33]. For the synthesis of the side-chain protected nonapeptide 8 on a solid support the chlorotrityl-resin [34] was chosen and the Fmoc/tBu strategy [35].…”

“…For assembly of the linear nonapeptide 7 according to Scheme 3, the hexapeptide derivative 6 was synthesized in solution essentially following previous syntheses in this field [30]. Fragment condensation was performed, upon silylation of the hexapeptide 6 with N,O-bis(trimethylsilyl)acetamide, via the pentafluorophenyl ester of the synthon 4 without auxiliary base.…”

Amino Acid-Protecting Groups

“…322 Create PDF files without this message by purchasing novaPDF printer (http://www.novapdf.com) -Reattachment to the resin: resin bound carbocations generated in the acidolytic cleavage from resins can react with both protected and unprotected Cys, thus causing reattachment of the peptide to the resin. 323 -Transfer of Acm (Acetamidomethyl) group to Ser, Thr, Gln and Tyr during Acm removal. 324 -Racemization: Cys is highly prone to racemize during the anchoring to the solid support or during the couplings.…”

Design and synthesis of heterotrimeric collagen peptides with a built‐in cystine‐knot Models for collagen catabolism by matrix‐metalloproteases

“…Briefly, the three chains eO[25,Cys(Acm)], c~2126,Cys(StBu),Cys(Acm)] and cd'[26,Cys(Acm)], suitably protected at the cysteine thiol functions, were synthesized on Wang resin using Fmoc/tBu protection [21] and HBTU/HOBt in the coupling steps [22] except for cysteine residues which were coupled as pentafluorophenyl esters to avoid racemization [23]. Cleavage from the resin was achieved with 95% aqueous TFA containing 1% triethylsilane.…”