3.1. Materials and Reagents

DNR samples were purchased from a local Traditional Chinese Medicine market (Anguo, Hebei, China). The samples were identified by Associate Professor Long Guo, and the voucher specimens have been deposited in Hebei University of Chinese Medicine, Shijiazhuang, China. The DNR samples were dried in the shade and powered by a disintegrator, then sieved through 60 mesh and stored in the desiccator before use.

Choline chloride (ChCl), betaine (Bet), L-proline (Pro), ethylene glycol (Eg), glycerol (Gly), xylitol (Xyl), D-sorbitol (Sor), lactic acid (Lac), malonic acid (Mal), DL-malic acid (Maa), Oxalic acid (Ox), Citrate acid (Ca), l-Methylurea (Mu), Acetamide (Am), and Urea (Ur) were purchased from Aladdin Biochemical Technology (Shanghai, China). The reference compounds of protodioscin, protogracillin, pseudoprotodioscin, and pseudoprotogracillin (purities > 95%) were obtained from Chengdu Biopurify Phytochemicals Company (Chengdu, China). The chemical structures of the four steroidal saponins are shown in Figure 6. Acetonitrile and methanol (HPLC grade) were purchased from Fisher Scientific (Geel, Belgium). Deionized water was prepared by a Synergy water purification system (Millipore, Billerica, USA). Other reagents and chemicals were of analytical grade.

Chemical structures of protodioscin, protogracillin, pseudoprotodioscin, and pseudoprotogracillin.