Glycopeptide Synthesis
Although various homogeneous N-glycans were prepared in large scale, its application in SPPS was still troublesome due to two reasons: the acid-labile sialyl linkage and lack of proper coupling conditions of glycosyl asparagine with peptide on a solid support. In order to incorporate semi-synthesized disialyloligosaccharide into SPPS efficiently, our laboratory reported a method for SPPS-compatible disialylglycopeptide synthesis by utilizing protection of sialic acid and optimized coupling conditions.[22] Based on these optimized coupling conditions, glycopeptide containing two sialyloligosaccharides was synthesized by SPPS and NCL. (Fig3-1: A protocol to incorporate disialo complex type N-glycan into Fmoc-SPPS for the synthesis of glycopeptide).
Although the protocol for solid-phase synthesis of glycopeptide was established, the chemical synthesis of glycopeptide as thioester form was still challenging. The general method toward peptide thioester relied on Boc-SPPS which required strong acidic condition (HF) for global deprotection, which might lead to the cleavage of acid-labile sialyl linkage. In 2008, the Kajihara group reported two methods for synthesizing glycopeptide thioester: a minimal protection strategy based on Boc-SPPS directly, and a coupling strategy using peptide derived from Fmoc-SPPS.[23] (Fig2-2: Synthesis of glycopeptide thioester based on Boc-SPPS or Fmoc-SPPS)
To develop a more efficient method for the synthesis of glycopeptide thioester, Maki and coworkers reported the liquid phase glycopeptide synthesis with a triantennary glycan.21 (Fig3-3: liquid phase synthesis of glycopeptide). The authors further succeeded in synthesizing erythropoietin with a complex type triantennary sialyloligosaccharide.
