Structural and bioactive characterization of purified polysaccharide from deep-freeze pretreated Sorghum bicolor L. leaf sheath

This study determined the effect of deep-freeze pretreatment of sorghum leaf sheath on the extraction yield of ultrasound-extracted purified polysaccharides (DFUP-1), and confirmed the physicochemical, structural and bioactive characteristics of DFUP-1. Deep-freeze pretreatment induced breaks in the cell walls and resulted in a higher yield (14.35%) of crude polysaccharide extract than without pretreatment (9.23%). The crude extract was purified using DEAE-52 cellulose ion exchange and Sephadex G-100 gel-filtration chromatography to obtain DFUP-1. The 1H and 13C NMR spectra of DFUP-1 showed the presence of β-glycosidic linkages and a substitution at C6 of the sugars which may be O-methyl substitution or due to β (1−6) linkages. Peaks for α-anomeric linkages were not detected. The FTIR spectrum of DFUP-1 confirmed its carbohydrate functional groups. Periodate oxidation and formic acid analysis of DFUP-1 showed 1-, 1–6, 1–2, 1–3 and 1–4 linkages. GC-MS analysis of monosaccharide residues of DFUP-1 showed presence of glucose, galactose, arabinose, mannose and xylose. Congo red assay showed that DFUP-1 was not of triple-helix configuration. The ABTS+ and DPPH radical scavenging assay showed an IC50 of 0.10 mg/mL and 1.19 mg/mL for DFUP-1 respectively. At 0.2 mg/mL DFUP-1 showed anti-inflammatory (anti-egg denaturation) activity of 41.52%. The DFUP-1 sample showed lower inhibitory effect against proliferation of leukemia cells at an IC50 of 37.43 µg/mL compared to curcumin with IC50 of 0.98 µg/mL. Taken together, deep-freeze pretreatment coupled with ultrasound extraction increases yield of sorghum leaf sheath polysaccharides with potential nutraceutical and pharmaceutical applications. [Display omitted] •Deep-freezed pretreatment influenced the mass transfer of intracellular content.•Polysaccharide showed β-glycosidic linkages and O-methyl substitution at C6.•L-arabinose and D-galactose were the predominantly abundant monosaccharides.•Extracts showed remarkable antioxidant, anti-inflammatory and cytotoxic properties.