Efficient Substitution Reaction from Cysteine to the Serine Residue ofGlycosylated Polypeptide: Repetitive Peptide Segment Ligation Strategyand the Synthesis of Glycosylated Tetracontapeptide Having Acid LabileSialyl-T_NAntigens

摘要

This paper reports the synthesis of a 40-residue glycopeptide having two antigenic sialyl-TN (NeuAc-a-(2,6)-GalNAc-Thr) residues in the MUC1 sequence. This target glycopeptide is a tandem-repeat form of 20-residue glycopeptides.For the synthesis of this large molecule, native chemical ligation (NCL) at the serine site was used (CysNCLSer).Theconcept of CysNCLSerrelies on the following: (1) conventional NCL between peptide-a-thioester and the cysteineresidue of another peptide segment; (2) methylation of the thiol that was used for NCL; (3) acidic CNBr conversionof the cysteine residue to the serine residue forming an O-ester linkage; and (4) an O- to N-acyl shift to couple thetwo glycopeptides through a native amide bond. To synthesize glycopeptide having an acid-labile sugar moiety, a20-residue glycopeptide-a-thioester and 20-residue glycopeptide having a cysteine residue at the N-terminal weresynthesized by solid phase glycopeptide synthesis, and then coupled by CysNCLSer.As the result of extensiveinvestigation, CNBr activation with an additional acid (trifluoroacetic acid) was found to be essential to obtain goodreactivity and yield, and this condition afforded a tandem repeat form of 40-residue sialylglycopeptide having twosialyl-TN residues. In addition to this, it was demonstrated that the cysteine thiol protected by the acetoamidomethyl(Acm) group did not react with the CNBr reagent, and therefore CysNCLSercan be used for repetitive native chemicalligation in the presence of a protecting N-terminal cysteine residue with an Acm group.