Quercetin

Leucine enkephalin was purchased from Waters Corporation (Milford, MA, USA). Chromatography-grade methanol, mass-spectrometry-grade acetonitrile, and formic acid were purchased from Thermo Fisher Scientific (Waltham, MA, USA). Folin & Ciocalteu phenol reagent was bought from Sigma-Aldrich, Co., Ltd. (St. Louis, MO, USA). Ferrous sulfate, ferric chloride, and analytical-grade ethyl alcohol were procured from Tianjin Xinbo Chemical Co., Ltd. (Tianjin, China). Analytical-grade anhydrous sodium carbonate (Na₂CO₃), aluminum chloride (AlCl₃), potassium acetate, and gallic acid monohydrate (purity 99.0%) were obtained from Tianjin Yongsheng Fine Chemical Co., Ltd. (Tianjin, China). The phenolic standards such as rutin, quercetin, ferulic acid, caffeic acid, isorhamnetin, kaempferol 3-O-beta-rutinoside, p-coumaric acid, chlorogenic acid, and isoquercetin were purchased from Anpel Laboratory Technologies Inc. (Shanghai, China). Hydrochloric acid was purchased from Xi’an Chemical Reagent Factory (Xi’an, China). The 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), 1,1-diphenyl-2-picrylhydrazyl (DPPH), 2,4,6-tris (2-pyridyl)-s-triazine (TPTZ), and sodium acetate were provided by Beijing Solarbio Science & Technology Co., Ltd. (Beijing, China). Ultrapure water was purchased from Watsons Group Ltd. (Hong Kong, China).

Pure standards of myricetin-3-O-α-L-rhamnoside (myricitrin, purity ≥ 98%), quercetin 3-O-α-L-rhamnopyranoside (quercitrin, purity ≥ 98%), cyanidin-3-glucoside (C3G, purity ≥ 99%), myricetin (purity ≥ 98%), quercetin (purity ≥ 99%), quercetin 3-O-β-D-galactopyranoside (hyperoside, purity ≥ 98%), and kaempferol (purity ≥ 99%) were obtained from Anpel Scientific Instrument Co., Ltd. (Shanghai, China). Methanol, ammonium formate, and formic acid used for liquid chromatography were of chromatographic grade and purchased from Aladdin Co., Ltd. (Shanghai, China). Ultrapure water was purified from a Water Purification System (Bedford, MA, USA, Millipore Milli-Q Advantage A10). Macroporous resin D101 (particle size 0.3~1.2 mm) used for separation was obtained from China National Pharmaceutical Group Chemical Reagent Co., Ltd. (Shanghai, China). All other chemicals and reagents used were of analytical grade unless otherwise noted.

Jujube fruit (Ziziphus jujuba Mill. cv. Junzao) was harvested in December 2020 in Hotan, Xinjiang province (latitude, 79°18′22″; longitude, 37°13′9″). The local climate is temperate continental desert with a high temperature and dry climate, and the annual average rainfall is <50 mm. Approximately 10 kg of dry mature jujube fruit (wet base moisture content, 21%) was collected in foam boxes covered with ice. The samples were sent to the laboratory at Xinjiang Shihezi University within one day. The screened samples were cleaned with double-distilled water (ddH₂O), and stored in a dry and odorless cold storage at 4 °C until drying and analysis were complete.
Caffeic acid, ferulic acid, phloroglucinol, cinnamic acid, chlorogenic acid, p-hydroxybenzoic acid, p-coumaric acid, vanillic acid, gallic acid, catechin, protocatechuic acid, epicatechin, rutin, quercetin, and procyanidins B1 and B2 were obtained from Anpel Co. (Shanghai, China). A total of 37 fatty acid mixed standards, 17 amino acid mixed standards, asparagine, glutamine, citrulline, valine, tryptophan, and 21 hydroxyproline and sarcosine standards were obtained from Sigma Aldrich (St. Louis., Mo, USA).