Preparation and structural characterization of poly-mannose synthesized by phosphoric acid catalyzation under microwave irradiation

• Poly-mannose was rapidly synthesized under microwave irradiation for the first time.
• The structural features were studied upon methylation analysis.
• The structural features were studied upon NMR spectroscopy analysis.
• The main chain was (1 → 3)-linked β-d-Manp, (1 → 3)-linked and (1 → 6)-linked α-d-Manp.
• Poly-mannose was branched at the O-2, O-3, O-4, O-6 position.

Poly-mannose with molecular weight of 2.457 kDa was synthesized using d-mannose as substrate and phosphoric acid as catalyst under the condition of microwave irradiation for the first time. The optimum reaction conditions were microwave output power of 900 W, temperature 115 °C, proton concentration 2.5 mol/L, and microwave irradiation time 5 min. The actual maximum yield was 91.46%. After purified by Sepherdex G-25 column chromatography, the structural features of poly-mannose were investigated by high-performance anion-exchange chromatography (HPAEC), high-performance gel-permeation chromatography (HPGPC), infrared (IR) spectroscopy, methylation analysis and NMR spectroscopy analysis (1H, 13C, COSY, TOCSY, HMQC, and HMBC). HPAEC analysis showed that the composition of synthetic polysaccharides was d-mannose, its purity was demonstrated by HPGPC as a single symmetrical sharp peak, and additionally IR spectra demonstrated the polymerization of d-mannose. Methylation analysis and NMR spectroscopy revealed that the backbone of poly-mannose consisting of (1 → 3)-linked β-d-Manp, (1 → 3)-linked α-d-Manp, and (1 → 6)-linked α-d-Manp residues, and the main chain were branched at the O-2, O-3, O-4, O-6 position.