Novel sequential solid-phase synthesis of N-linked glycopeptides from natural sources

Abstract

In the present report a practical and versatile procedure for the solid-phase synthesis of N-linked glycopeptides from natural sources has been demonstrated. The approach is based on the mild hydrazinolysis procedure to release N-linked oligosaccharides in their intact unreduced form from the natural glycoproteins, e.g. fetuin and ribonuclease B and subsequent formation of the corresponding glycosylamines. Treatment of the reducing sugars 1–7 with a saturated solution of ammonium hydrogen carbonate in either water or dimethyl sulfoxide (DMSO) gives in almost quantitative yields the glycosylamines 8–14. Coupling of the unprotected glycosylamines 8–14 to the side-chain-activated aspartic acid derivative Fmoc-Asp(ODhbt)-OBut 16 affords the N-glycosylated asparagine derivatives 17–23. Subsequent acetylation of the carbohydrate hydroxy groups and cleavage of the tert-Bu ester by trifluoroacetic acid (TFA) treatment yields the glycosylated N-linked building blocks 31–37. The building blocks 31–37 are then incorporated into the multiple-column peptide-synthesis protocol of the glycopeptide T-cell epitope analogues 40–46 of the mouse haemoglobin-derived decapeptide Hb (67–76), VITAFNEGLK. The decapeptide sequence VITAFNEGLK binds well to the MHC Class II Ek molecule and is non-immunogenic in CBA/J mice. Syntheses of several natural and unnatural glycosylations, e.g. N-acetylglucosamine, N,N′-diacetylchitobiose, glucose, maltotriose, maltoheptose and di- and tri-antennary complex oligosaccharides on the decapeptide Hb (67–76) affording the N-linked glycopeptides 40–46 are described. The N-linked glycopeptides 40–46 have been fully characterised by 1D- and 2D-1H and 13C NMR spectroscopy and by ES-MS.