Metabolites of Siberian Raspberries: LC-MS Profile, Seasonal Variation, Antioxidant Activity and, Thermal Stability of <i>Rubus matsumuranus</i> Phenolome

<i>Rubus matsumuranus</i> H. Lev. & Vaniot, a famous Siberian shrub of the Rosaceae family, is used in the folk medicine of nomads (Buryats, Yakuts, Soyots, and Mongols) as a remedy for the treatment of diseases of the respiratory and hepatobiliary systems. The lack of scientific information on <i>R. matsumuranus</i> leaves contributed to the investigation of the metabolomic profile and biological activity of this plant. In this study, metabolites of <i>R. matsumuranus</i> leaves in three stages (active growth, flowering, and fruiting) were characterised using high-performance liquid chromatography with photodiode array and electrospray ionisation triple quadrupole mass spectrometric detection (HPLC-PDA-ESI-tQ-MS). In total, 63 compounds were identified, including gallic acid derivatives, hydroxycinnamates, catechins, procyanidins, flavonols, and ellagitannins. Lambertianin C (57.11 mg/g of dry weight, DW), miquelianin (39.63 mg/g DW), and kaempferol-3-<i>O</i>-glucuronide (31.18 mg/g DW) were the major compounds in <i>R. matsumuranus</i> leaves. As a result of the HPLC-PDA-based assay to determine the antioxidant activity, it was revealed that lambertianin A, sanguiin H6, lambertianin C, and sanguiin H11 were effective scavengers of free radicals (2,2-diphenyl-1-picrylhydrazyl, DPPH^•) and possessed Fe²⁺-chelating activity. After an investigation of the phenolic content in infusions and decoctions obtained by extraction with water at different temperatures, it was revealed that a hot infusion (80 °C) is a phenolic-rich preparation of <i>R. matsumuranus</i> leaves. Our research suggests that <i>R. matsumuranus</i> leaves are a rich source of phenolic compounds with high antioxidant properties and that this could be a prospective plant for new functional products.