Synthesis and conjugation of oligosaccharide fragments related to the immunologically reactive part of the circulating anodic antigen of the parasite Schistosoma mansoni

Abstract

The immunoreactive part of the circulating anodic antigen (CAA) from the parasite Schistosoma mansoni is a threonine-linked polysaccharide consisting of ->6)-[beta-D-GlcpA-(1->3)]-beta-D-GalpNAc-(1-> repeating disaccharides. In the framework of an immunochemical project, as a follow-up of earlier synthesized di- to tetrasaccharide CAA fragments, the synthesis of a spacer-containing pentasaccharide fragment, 3-(2-aminoethylthio)propyl (2-acetamido-2-deoxy-beta-D-galactopyranosyl)-(1->6)-[(beta-D-glucopyranosyluronic acid)-(1->3)]-(2-acetamido-2-deoxy-beta-D-galactopyranosyl)-(1->6)-[(beta-D-glucopyranosyluronic acid)-(1->3)]-2-acetamido-2-deoxy-beta-D-galactopyranoside, is described. Moreover, 1-O-[3-(2-aminoethylthio)propyl]-N-acetyl-beta-D-galactosamine was synthesized. Oxidation steps in the synthesis of tri- to pentasaccharide CAA fragments were performed using pyridinium dichromate and acetic anhydride. TEMPO-catalyzed oxidations were explored in the synthesis of 1-O-[6-aminohexyl]-beta-D-glucuronic acid and 3-aminopropyl (2-acetamido-2-deoxy-beta-D-galactopyranosyl)-(1->6)-[(beta-D-glucopyranosyluronic acid)-(1->3)]-2-acetamido-2-deoxy-beta-D-galactopyranoside, affording short reaction times and high yields. All synthesized compounds, including the earlier described 3-(2-aminoethylthio)propyl-spacered di-, tri-, and tetrasaccharide CAA fragments, were conjugated to BSA using squaric diester chemistry with coupling efficiencies in the range of 30-90%. The efficiency decreased when larger oligosaccharides were coupled to BSA. Finally, conformational analyses of the tri- and tetrasaccharide fragments were performed using Molecular Mechanics (MM) and Molecular Dynamics (MD) calculations.