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

<i>Rhodiola</i> species are an integral part of traditional Chinese medicine, with <i>Rhodiola crenulata</i> (<i>R. crenulata</i>) being the only medicinal species officially listed in the 2020 edition of the Chinese Pharmacopeia. Polysaccharides are recognized as key bioactive components of <i>Rhodiola</i>, yet efficient extraction methods have not been systematically reported. In this study, polysaccharides from <i>R. crenulata</i> 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 <i>Rhodiola</i> species (<i>R. crenulata</i>, <i>R. wallichiana var.</i>, <i>R. kirilowii</i>, and <i>R. himalensis</i>). Total sugar content and monosaccharide composition were quantified. DES-1 as the extraction solvent yielded the highest total sugar content across four <i>Rhodiola</i>, with <i>R. himalensis</i> 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 <i>Rhodiola</i> polysaccharides using DES-1.