Synthesis of an O-acyl isopeptide by using native chemical ligation in an aqueous solvent system

O-Acyl isopeptides, in which the N-acyl linkage on the hydroxyamino acid residue (e.g. Ser and Thr) is replaced by an O-acyl linkage, generally suppress unfavorable aggregation properties derived from the corresponding parent peptides. Here, we report the synthesis of an O-acyl isopeptide of 34-mer pyroGlu-ADan (2), a component of amyloid deposits in hereditary familial Danish dementia, by using native chemical ligation. Native chemical ligation of pyroGlu¹-ADan(1-21)-SCH₂CH₂SO₃⁻Na⁺ (3) and Cys²²-O-acyl isopeptide (4), in which the amino group of the Ser²⁹ residue at the isopeptide moiety was protected by an allyloxycarbonyl group, proceeded well in an aqueous solvent to yield a ligated O-acyl isopeptide (5). Subsequent disulfide bond formation and deprotection of the allyloxycarbonyl group followed by HPLC purification gave 2 with a reasonable overall yield. 2 was converted to the parent peptide 1 via an O-to-N acyl migration reaction. The sequential method, namely (i) native chemical ligation of the O-acyl isopeptide, (ii) HPLC purification as the O-acyl isopeptide form, and (iii) O-to-N acyl migration into the desired polypeptide, would be helpful to solve problems with HPLC purification of hydrophobic polypeptides in the process of chemical protein synthesis. Copyright © 2014 European Peptide Society and John Wiley & Sons, Ltd.

We here reported the synthesis of an O-acyl isopeptide by using native chemical ligation in an aqueous solvent, in combination with orthogonal allyloxycarbonyl protecting group at the amino group of the isopeptide moiety. The O-acyl isopeptide was quantitatively converted to the parent native peptide via an O-to-N acyl migration reaction. The present method would be helpful to solve the problem of HPLC purification of hydrophobic polypeptides in the process of chemical protein synthesis by using ligation chemistry.