4-Methoxybenzyloxymethyl Group, a Racemization-Resistant Protecting Group for Cysteine in Fmoc Solid Phase Peptide Synthesis

The 4-methoxybenzyloxymethyl (MBom) group was introduced for sulfhydryl protection of Cys in combination with Fmoc chemistry. The MBom group proved to substantially suppress racemization of Cys during its incorporation mediated by phosphonium or uronium reagents. Furthermore, this group was found to significantly reduce racemization of the C-terminal Cys linked to a hydroxyl resin during repetitive base treatment, in comparison with the usually used trityl (Trt) and acetamidomethyl (Acm) groups

Postsynthetic Modification of Unprotected Peptides via <i>S</i>‑Tritylation Reaction

Tritylation using trityl alcohol (Trt–OH) in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) is a convenient and efficient procedure that can offer <i>S</i>-protection of the Cys located in fully unprotected peptides. The procedure simply requires Trt–OH and HFIP to selectively promote <i>S</i>-tritylation in the presence of peptide nucleophilic functionalities

Synthesis of Cysteine-Rich Peptides by Native Chemical Ligation without Use of Exogenous Thiols

Native chemical ligation (NCL) performed without resorting to the use of thiol additives was demonstrated to be an efficient and effective procedure for synthesizing Cys-rich peptides. This method using tris­(2-carboxyethyl)­phosphine (TCEP) as a reducing agent facilitates the ligation reaction even at the Thr-Cys or Ile-Cys site and enables one-pot synthesis of Cys-rich peptides throughout NCL and oxidative folding

Synthetic Procedure for <i>N</i>‑Fmoc Amino Acyl‑<i>N</i>‑Sulfanylethylaniline Linker as Crypto-Peptide Thioester Precursor with Application to Native Chemical Ligation

<i>N</i>-Sulfanylethylanilide (SEAlide) peptides <b>1</b>, obtainable using Fmoc-based solid-phase peptide synthesis (Fmoc SPPS), function as crypto-thioesters in native chemical ligation (NCL), yielding a wide variety of peptides/proteins. Their acylating potential with N-terminal cysteinyl peptides <b>2</b> can be tuned by the presence or absence of phosphate salts, leading to one-pot/multifragment ligation, operating under kinetically controlled conditions. SEAlide peptides have already been shown to be promising for use in protein synthesis; however, a widely applicable method for the synthesis of <i>N</i>-Fmoc amino acyl-<i>N</i>-sulfanylethylaniline linkers <b>4</b>, required for the preparation of SEAlide peptides, is unavailable. The present study addresses the development of efficient condensation protocols of 20 naturally occurring amino acid derivatives to the <i>N</i>-sulfanylethylaniline linker <b>5</b>. <i>N</i>-Fmoc amino acyl aniline linkers <b>4</b> of practical use in NCL chemistry, except in the case of the proline- or aspartic acid-containing linker, were successfully synthesized by coupling of POCl₃- or SOCl₂-activated Fmoc amino acid derivatives with sodium anilide species <b>6</b>, without accompanying racemization and loss of side-chain protection. Furthermore, SEAlide peptides <b>7</b> possessing various C-terminal amino acids (Gly, His, Phe, Ala, Asn, Ser, Glu, and Val) were shown to be of practical use in NCL chemistry