Polysaccharide extraction from Rhodiola species using solvents of varying pH and polarity: a comparative study and mechanistic analysis

Rhodiola species are an integral part of traditional Chinese medicine, with Rhodiola crenulata (R. crenulata) being the only medicinal species officially listed in the 2020 edition of the Chinese Pharmacopeia. Polysaccharides are recognized as key bioactive components of Rhodiola, yet efficient extraction methods have not been systematically reported. In this study, polysaccharides from R. crenulata were extracted using acidic, neutral, and alkaline deep eutectic solvents (DESs), as well as acidic water, deionized water, and alkaline water. Four solvents with distinct pH values, including 0.01 M acidic water (ACW-1), choline chloride:lactic acid deep eutectic solvent (DES-1), neutral water (NW), and 0.01 M alkaline water (ALW-1), were selected for comparative analysis across four Rhodiola species (R. crenulata, R. wallichiana var., R. kirilowii, and R. himalensis). Total sugar content and monosaccharide composition were quantified. DES-1 as the extraction solvent yielded the highest total sugar content across four Rhodiola, with R. himalensis yielding the highest content (67.03 ± 0.81%). Monosaccharide composition indicated that polysaccharides extracted with DES-1 were mainly composed of glucose (Glc) and arabinose (Ara), whereas those from ACW-1, NW, and ALW-1 were rich in galacturonic acid (GalA) and arabinose (Ara). Additionally, density functional theory (DFT) calculations revealed that DES-1 forms a hydrogen-bond network with polysaccharide hydroxyl groups, enhancing solubility and extraction selectivity. In conclusion, this study provides insights into solvent-mediated polysaccharide extraction and elucidates the extraction mechanism of Rhodiola polysaccharides using DES-1.