Optimized extraction and characterization of ramie leaf polysaccharides using deep eutectic solvent and microwave: Antioxidant, metal chelation, and UV protection properties

Ramie leaf polysaccharides (RLP) were extracted using deep eutectic solvent (DES) and microwave. The extraction conditions, i.e., buffer-to-substrate (B:S) ratio (10:1-30:1 w/v), microwave power (90-270 W) and extraction duration (2-4 min) were optimized using response surface methodology. Based on the optimized model, 21.1 mL/g B:S ratio, 263 W microwave power and 2.8 min extraction time had successfully produced RLP with 16.67 ± 1.10 % (w/w) yield and 80.84 ± 1.16 % 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity. The 23.4 ± 0.389 kDa RLP was a neutral polysaccharide with low carbohydrate, protein, and phenolic contents. The low galacturonic acid content (0.89 ± 0.05 mg/g) suggested RLP contained partially pectic-polysaccharide. The major monosaccharides of RLP were rhamnose, glucose, galactose and xylose. RLP was a relatively non-linear, highly branched polysaccharide with short branches based on the monosaccharide ratio. Bioactivity screening had identified the reduction (0.66 ± 0.02 mmol Fe2+/g) and copper chelation (48.5 ± 0.4 %) activities of RLP. The polysaccharide could also absorb ultraviolet (UV) in which it gave major protection against UVB with 8.7 ± 0.3 sun protection factor. These biological activities were related to specific functional groups, monosaccharide units, molecular weight and/or the neutral property of RLP. The current findings provided new insights into the antioxidant, copper chelation and sun protection benefit of RLP.