Solution synthesis of the glyco-hex a peptide sequence of the human oncofetal fibronectin defined by monoclonal antibody FDC-6

The glyco-hexapeptide sequence H-Val-(GalNAc-α)Thr-His-Pro-Gly-Tyr-OH, was synthesized in solution by the segment condensation procedure and the stepwise procedure. A peracetylated, O-galactosaminyl threonine derivative was used for incorporating the glycosylated amino acid residue into the peptide chain. A consistent racemization occurred during the acylation of H-His-Pro-Gly-Tyr(Bzl)-OBzl with Z-Val-GalNAc(Ac)₃-α]Thr-OH by the BOP-HOBt procedure and the D-allothreonine containing glycohexapeptide was isolated in about 20% yield. Stepwise elongation of the C-terminal tetrapeptide with Frnoc-GalNAc(Ac)₃-a]Thr-OH and Z-Val-OH, in the presence of the same coupling reagents, yielded the l -threonine containing diastereoisomer without detectable racemization. A side product, the N^im-ethoxy-carbonylated hexapeptide derivative, formed during the EEDQ-mediated condensation of Fmoc-GalNAc(Ac)₃-α]Thr-OH with the C-terminal tetrapeptide, was isolated and characterized. Preliminary studies showed that the synthetic glycohexapeptide is a good competitive inhibitor of the binding of the FDC-6 monoclonal antibody to the oncofetal fibronectin, supporting the idea that it should represent the minimum essential structure required for the FDC-6 activity.