In situ NMR investigation of the native chemical ligation (NCL) of N-terminal cysteines to alginate.

Polysaccharide-peptide conjugates constitute an important class of biomaterials for tissue engineering scaffolds and glycoconjugate vaccines. Yet, benign and chemoselective synthetic strategies that operate under aqueous, physiologically relevant conditions remain scarce. Herein, we present a proof-of-concept approach for the traceless conjugation of N-terminal l-cysteines to alginate. A glycine-derived thioester functionality was synthesized and covalently introduced onto the uronic acid residues of alginate (DS = 0.47), as confirmed by 1H and 13C NMR and ATR-IR spectroscopy. The resulting alginate-thioester derivative served as a hydrolytically stable and chemoselective reactive handle for the native chemical ligation (NCL) of l-cysteine hydrochloride (l-cys·HCl) and the dipeptide l-cysteinylglycine (l-cys-gly). The ligation reactions were monitored in situ by kinetic 1H NMR spectroscopy. Control experiments of the alginate-thioester derivative with alternative biological nucleophiles glycine and l-lysine showed no reactivity (1H NMR), thus confirming the selectivity toward the thiol functionality of N-terminal l-cysteines under the provided conditions (0.1 M PO43- buffer, pH 7, 25 °C). This work establishes a convenient aqueous method to prepare peptide-polysaccharide conjugates in which the peptide is linked to the polysaccharide exclusively through stable amide bonds, without the incorporation of exogenous linker molecules or the use of conventional coupling agents or protecting groups.