Cysteine-Free Native Peptide Ligation for the Assembly of Glycoproteins

Homogeneous, structurally defined glycoproteins can be assembled by coupling synthetic glycopeptides to synthetic or bacterially-derived protein fragments using the native chemical ligation (NCL) reaction. A limitation of NCL is the requirement for an Nterminal cysteine residue in one of the peptide fragments. One method for cysteine-free peptide ligation utilizes thiol acyl transfer auxiliaries which effect ligation and can then be removed under mild conditions. We developed new, rapid routes to 1-phenyl-2-mercaptoethyl and 2-mercaptobenzyl auxiliaries. The key steps involved: 1) introduction of a suitably protected thiol to auxiliary precursors; 2) direct reductive amination of the auxiliary aldehyde or ketone to afford the auxiliary-amine, which can be conjugated to a peptide via the “sub-monomer” approach, or the auxiliary-amino acid “cassette” for use in conventional solid phase peptide synthesis. Overall yields are 53-83 %. A glycopeptide was then assembled via auxiliary-mediated ligation at a Gly-Gly junction, which was complete within 48 hours. Thioester- and auxiliary-peptides were assembled to investigate the scope and limitations of auxiliary-mediated ligation for non Gly-Gly junctions. The 1-phenyl-2-mercaptoethyl auxiliary effected ligation at Ala-Gly, Lys-Gly and Gly-Ala junctions in 24-70 % yield, whereas the 2-mercaptobenzyl auxiliary effected ligation at a Gly-Ala junction in 42 % yield. Excess thiol was found to inhibit ligation, indicating a change in rate-determining step relative to cysteine ligation