Sugar-Assisted Ligation in Glycoprotein Synthesis

Abstract

Sugar-assisted ligation (SAL) presents an attractive strategy for the synthesis of glycopeptides, including the synthesis of cysteine-free β-O-linked and N-linked glycopeptides. Here we extended the utility of SAL for the synthesis of α-O-linked glycopeptides and glycoproteins. In order to explore SAL in the context of glycoprotein synthesis, we developed a new chemical synthetic route for the α-O-linked glycoprotein diptericin ε. In the first stage of our synthesis, diptericin segment Cys(Acm)³⁷-Gly⁵² and segment Val⁵³-Phe⁸² were assembled by SAL through a Gly-Val ligation junction. Subsequently, after Acm deprotection, diptericin segment Cys³⁷-Phe⁸² was ligated to segment Asp¹-Asn³⁶ by means of native chemical ligation (NCL) to give the full sequence of diptericin ε. In the final synthetic step, hydrogenolysis was applied to remove the thiol handle from the sugar moiety with the concomitant conversion of mutated Cys³⁷ into the native alanine residue. In addition, we extended the applicability of SAL to the synthesis of glycopeptides containing cysteine residues by carrying out selective desulfurization of the sulfhydryl-modified sugar moiety in the presence of acetamidomethyl (Acm) protected cysteine residues. The results presented here demonstrated for the first time that SAL could be a general and useful tool in the chemical synthesis of glycoproteins.