Comprehensive analysis of the chemical composition of seeds from five Russian cultivars of flax (Linum Usitatissimum): Search for correlations with cyclic peptide content

Flax ( Linum usitatissimum ) is a valuable source of bioactive cyclic peptides (CPs), including cyclolinopeptides (also known as linusorbs or orbitides). This study presents the first comprehensive characterization of CPs in cold-pressed oils from five Russian flax cultivars—Biryuza, Fliz, RFN, Y-117, and Nilin—and investigates their statistical correlations with the biochemical composition of flaxseeds. Seeds were analyzed for macronutrients (protein: 21.5–25.4%, fat: 43.3–51.0%, carbohydrates: 13.2–22.2%), moisture (6.0–7.3%), ash (3.3–3.9%), and water activity (0.44–0.55). Oils exhibited high levels of polyunsaturated fatty acids (64.86–71.90%), with α-linolenic acid ranging from 36.70% to 53.61%. Tocopherol content varied from 208.0 to 261.0 μg/g, with γ-tocopherol as the predominant form. The total polyphenol content ranged from 34.7 μg/ml to 52.1 μg/ml, and the DPPH radical scavenging activity ranged from 64.6% to 73.2%. CPs were extracted from oil using 70% aqueous ethanol and identified via HPLC-DAD-MS; all cultivars contained the same 11 CPs, including six non-oxidized (CLA, CLB, CLL, CLM, CLO, CLP) and five methionine-oxidized forms (CLC, CLE, CLD, CLG, CLN). Correlation analysis revealed that oxidized peptides CLC and CLE showed strong positive associations with seed α-linolenic acid (C18:3), α-tocopherol, protein content, and color index a*. In contrast, non-oxidized peptides CLA, CLO, CLB, and CLP correlated positively with seed copper (14.5–23.2 μg/g) and iron (56.2–75.9 μg/g) content and plant height (65–85 cm). Peptides CLL and CLM were closely linked to total polyphenolic content, DPPH activity, and oil oxidative stability (induction period: 4.8–6.5 h at 90 °C). These findings demonstrate that individual CPs are differentially associated with specific seed hemical and morphological traits, suggesting distinct biological roles and potential for targeted breeding of flax cultivars with enhanced functional properties.