Guaiaverin

The following substances and solvents were used in the study: procyanidins B1, B2, C1, A1, A2, A4, (+)-catechin, (–)-epicatechin, quercetin, quercitrin (quercetin-3-O-rhamnoside), isoquercitrin (quercetin-3-O-glucoside), avicularin (quercetin-3-O-arabinofuranoside), guaiaverin (quercetin-3-O-arabinopyranoside), rutin (quercetin-3-O-rutinose), reynoutrin (quercetin-3-O-xyloside), kaempferol, nicotiflorin (kaempferol-3-O-rutinoside), afzelin (kaempferol-3-O-rhamnoside), astragalin (kaempferol-3-O-glucoside), chlorogenic acid (3-O-caffeoylquinic acid), neochlorogenic acid (5-O-caffeoylquinic acid), cryptochlorogenic acid (4-O-caffeoylquinic acid), p-coumaric acid, arbutin, hydroquinone, α-amyrin, β-amyrin, β-sitosterol, lupeol, erythrodiol, maslinic acid, oleanolic acid, acetonitrile, acetone, and methanol, which were purchased from Sigma–Aldrich (Steinheim, Germany); hyperoside (quercetin-3-O-galactoside), procyanidin B3, uvaol, friedelin, 6″-O-acetylisoquercitrin (quercetin-3-O-(6″-acetylglucoside)), betulin, and betulinic and corosolic acids from Extrasynthese (Genay, France); ursolic acid from Carl Roth (Karlsruhe, Germany); and trifluoroacetic acid from Merck (Darmstadt, Germany). All used chemicals were of HPLC grade. Ultrapure water used in this study was prepared with Milli–Q^® 180 (Millipore, Bedford, MA, USA) water purification system.

St. John’s wort dry extract (European Pharmacopoeia, EP, Reference Standard 01131, code: Y0001050, batch: 2.0), avicularin and guaiaverin were obtained from Sigma-Aldrich Inc. while rutin (L10815B002) from Adooq Bioscience. Hypericin primary reference standard (batch HWI 01814-1) was purchased from HWI ANALITIK GmbH.

Commercially available standards of phenolics mostly present in different apple cultivars, including gallic acid (97.5–102.5%), chlorogenic acid (≥95%), (-)-epicatechin (≥90%), catechin (≥98%), caffeic acid (≥98%), rutin hydrate (≥94%), quercetin (≥95%), guaiaverin (≥99.3%), quercitrin (≥97%), hyperoside (≥98.35%), hirsutrin (≥90.1%), reynoutrin (≥97%), phloridzin (≥99%), and phloretin (≥99%), were purchased from Sigma–Aldrich (Steinheim am Albuch, Germany). The LC/MS grade solvents acetonitrile (ACN), methanol (MeOH), and absolute ethanol (EtOH, ≥99.7%) were supplied by Sigma–Aldrich (Prague, Czech Republic). Formic acid (≥98%) was purchased from Honeywell (Seelze, Germany). Ultrapure CO₂ (>99.99%) in cylinders with a dip tube was provided by Messer (Prague, Czech Republic). Ultrapure water was acquired using a Milli-Q purification system (Millipore, Billerica, MA, USA).