An efficient N-linked glycosylation reaction between glycosylamines and p-nitrophenyl thioester peptides has been developed. The reaction conditions are mild and compatible with the C-terminal free carboxylic acid group and the unprotected N-linked sialyloligosaccharide. By means of this convergent strategy, a versatile N-glycopeptide fragment containing an N-terminal Thz and a C-terminal thioester was readily prepared, which is available for the synthesis of long glycopeptides and glycoproteins using the protocol of native chemical ligation.

Adipic acid diselenoester was developed as an efficient cross-linker for covalent protein conjugation with a variety of small molecular haptens, including mono- and disaccharides, peptide, fluorescence dye, and nicotine. Compared to the counterparts of N-hydroxysuccinimide (NHS) and p-nitrophenyl (PNP) linkers, the diselenoester linker demonstrates improved balance between reactivity and stability and coupling of haptens to proteins under mild conditions with high incorporation efficiency.

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Along with N-hydroxysuccinimidyl, p-nitrophenyl, and phenylseleno esters, tetra- and penta-fluorophenyl esters were comparatively evaluated in term of their reactivity and hydrolytic stability. Their homobifunctional cross-linkers were prepared to conjugate proteins with small molecules, including carbohydrates, fluorescent dyes, and poly(ethylene glycol) monomethyl ether. The conjugations proceeded under mild conditions, affording the corresponding protein conjugates with good efficiency.