Coupling strategies in solid-phase synthesis of glycopeptides.

N-beta-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-asparaginyl peptides [Asn(GlcNAc)] corresponding to T-helper cell determinants were synthesized on solid-phase. Various amino-terminal- and carbohydrate-protecting groups were used on the glycosylated asparagine residue which was coupled to the peptide chain. We found that coupling rates decreased with increased size of the protected carbohydrate part of the acylating agent. Double couplings with an O-unprotected saccharide, as in Fmoc-Asn(GlcNAc)-OH resulted in acceptable coupling rates even with a synthetically difficult sequence corresponding to the T-cell epitopic peptide from the C-terminus of pigeon cytochrome c. The observed coupling rates on this peptide as well as on a T-cell epitopic pentapeptide, derived from the rabies virus N-protein, were comparable to those of conventional solid-phase peptide syntheses. The Fmoc-Asn(GlcNAc)-OH used can be prepared easily from commercially available components. The described glycopeptides will be used to probe effects of N-glycosylation on the immune recognition of viral glycoproteins.