1. Fmoc-based synthesis of peptide alpha-thioesters using an aryl hydrazine support
Julio A Camarero, Benjamin J Hackel, James J de Yoreo, Alexander R Mitchell J Org Chem. 2004 Jun 11;69(12):4145-51. doi: 10.1021/jo040140h.
C-Terminal peptide thioesters are key intermediates in the synthesis/semisynthesis of proteins and of cyclic peptides by native chemical ligation. They are prepared by solid-phase peptide synthesis (SPPS) or biosynthetically by protein splicing techniques. Until recently, the chemical synthesis of C-terminal alpha-thioester peptides by SPPS was largely restricted to the use of Boc/Benzyl chemistry due to the poor stability of the thioester bond to the basic conditions required for the deprotection of the N(alpha)-Fmoc group. In the present work, we describe a new method for the SPPS of C-terminal thioesters using Fmoc/t-Bu chemistry. This method is based on the use of an aryl hydrazine linker, which is totally stable to conditions required for Fmoc-SPPS. When the peptide synthesis has been completed, activation of the linker is achieved by mild oxidation. This step converts the acyl hydrazine group into a highly reactive acyl diazene intermediate which reacts with an alpha-amino acid alkyl thioester (H-AA-SR) to yield the corresponding peptide alpha-thioester in good yield. This method has been successfully used to prepare a variety of peptide thioesters, cyclic peptides, and a fully functional Src homology 3 (SH3) protein domain.
2. A reversible protection strategy to improve Fmoc-SPPS of peptide thioesters by the N-Acylurea approach
Santosh K Mahto, Cecil J Howard, John C Shimko, Jennifer J Ottesen Chembiochem. 2011 Nov 4;12(16):2488-94. doi: 10.1002/cbic.201100472. Epub 2011 Sep 12.
C-terminal peptide thioesters are an essential component of the native chemical ligation approach for the preparation of fully or semisynthetic proteins. However, the efficient generation of C-terminal thioesters by Fmoc solid-phase peptide synthesis remains a challenge. The recent N-acylurea approach to thioester synthesis relies on the deactivation of one amine of 3,4-diaminobenzoic acid (Dbz) during Fmoc SPPS. Here, we demonstrate that this approach results in the formation of side products through the over-acylation of Dbz, particularly when applied to Gly-rich sequences. We find that orthogonal allyloxycarbonyl (Alloc) protection of a single Dbz amine eliminates these side products. We introduce a protected Fmoc-Dbz(Alloc) base resin that may be directly used for synthesis with most C-terminal amino acids. Following synthesis, quantitative removal of the Alloc group allows conversion to the active N-acyl-benzimidazolinone (Nbz) species, which can be purified and converted in situ to thioester under ligation conditions. This method is compatible with the automated preparation of peptide-Nbz conjugates. We demonstrate that Dbz protection improves the synthetic purity of Gly-rich peptide sequences derived from histone H4, as well as a 44-residue peptide from histone H3.
3. Native Chemical Ligation via N-Acylurea Thioester Surrogates Obtained by Fmoc Solid-Phase Peptide Synthesis
Judith Palà-Pujadas, Juan B Blanco-Canosa Methods Mol Biol. 2020;2133:141-161. doi: 10.1007/978-1-0716-0434-2_7.
Native chemical ligation (NCL) enables the direct chemical synthesis and semisynthesis of proteins of different sizes and compositions, streamlining the access to proteins containing posttranslational modifications (PTMs). NCL assembles peptide fragments through the chemoselective reaction of a C-terminal α-thioester peptide, prepared either by chemical synthesis or via intein-splicing technology, and a recombinant or synthetic peptide containing an N-terminal Cys. Whereas the generation of C-terminal α-thioester proteins can be achieved via the recombinant fusion of the sequence of interest to an intein domain, chemical methods can also be used for synthetically accessible proteins. The use of Fmoc solid-phase peptide synthesis (Fmoc-SPPS) to obtain α-thioester peptides requires the development of novel strategies to overcome the lability of the thioester bond toward piperidine Fmoc-removal conditions. These new synthetic methods enable the easy introduction of PTMs in the thioester fragment. In this chapter, we describe an approach for the synthesis and use of C-terminal α-N-acylbenzimidazolinone (Nbz) and α-N-acyl-N'-methylbenzimidazolinone (MeNbz) peptides in NCL. Following stepwise peptide elongation, acylation with p-nitrophenylchloroformate and cyclization affords the Nbz/MeNbz peptides. The optimization of the coupling conditions allows the chemoselective incorporation of the C-terminal amino acid (aa) on the 3,4-diaminobenzoyl (Dbz) and prevents undesired diacylations of the resulting o-aminoanilide. Following synthesis, these Nbz/MeNbz peptides undergo NCL straightforwardly at neutral pH catalyzed by the presence of arylthiols. Herein, we apply the Nbz technology solid phase synthesis, NCL-mediated cyclization and folding of the heterodimeric RTD-1 defensin, an antimicrobial peptide isolated from the rhesus macaque leukocytes.
