A novel post-ligation thioesterification device enables peptide ligation in the N to C direction: synthetic study of human glycodelin.

Human glycodelin consists of 162 amino acid residues and two N-linked glycans at Asn(28) and Asn(63) . In this study, we synthesized it by a fully convergent strategy using native chemical ligation (NCL) in N to C direction. The four peptide segments corresponding to 1-31, 32-65, 66-105 and 106-162 sequences were synthesized by 9-fluorenylmethoxycarbonyl based solid-phase peptide synthesis. At the C-terminus of the second segment, N-ethyl-S-acetamidomethyl-cysteine was attached as a post-ligation thioesterification device. The N-terminal two segments were condensed by the homocysteine-mediated NCL at Leu-Met site, and the product was methylated to convert homocysteine to methionine. After deprotection of acetamidomethyl group on the N-ethylcysteine residue, the peptide was thioesterified by N-alkylcysteine-assisted method. The product was then ligated with the C-terminal half, which was obtained by the NCL of third and fourth segments, to give the full-length glycodelin.