Purification, structural characterization and antioxidant activity of a new arabinogalactan from Dorema ammoniacum...

Gum ammoniacum is a polymer obtained from Dorema ammoniacum and its medicinal use was already known to the ancient times. In this study, a new D. ammoniacum carbohydrate (DAC-1) with a molecular weight of 27.1 kDa was extracted by hot water and then purified on DEAE-52-cellulose and Sephadex G-100 columns. The structural features of DAC-1 were investigated by partial acid hydrolysis, fourier-transform infrared spectroscopy (FT-IR), methylation, gas chromatography-mass spectrometry (GC-MS), gas chromatography-flame ionization detection (GC-FID), and 1D and 2D nuclear magnetic resonance spectroscopy (1D & 2D NMR). The results indicated that DAC-1 was an arabinogalactan including galactose, arabinose, rhamnose, glucuronic acid and 4-O-methyl-β-d-glucopyranosyl uronic acid (meGlcpA) with a relative percentage of 44.63%, 23.30%, 13.46%. 12.47%, and 6.14%. The structure units of DAC-1 were elucidated as 3,1)-β-_D-Galp-(6 → 1)-β-_D-Galp-(3,6 → containing four branch chains of →1,6)-β-_D-Galp-(3 → 1)-α-_L-Araf-(5 → 1)-β-_D-GlcpA-(4 → 1)-α-_L-Rhap-T (two times), →1,6)-β-_D-Galp-(3→1)-β-_D-Galp-(3 → 1)-β-_D-Galp-(3 → 1)-β-_D-Galp-(3  →  1)-α-_L-Araf-T and →1,6)-β-_D-Galp-(3 → 1)-α-_L-Araf-(5 → 1)-β-_D-meGlcpA-T. X-ray diffraction (XRD) pattern indicated a semi-crystalline structure. Thermal behavior of the polysaccharide was evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) and revealed temperatures higher than 200 °C as dominant region of weight loss. DAC-1 showed acceptable antioxidant activity when analyzed by DPPH, ABTS, FRAP, and OH radical removal methods.