Arabinan-rich rhamnogalacturonan-I from flaxseed kernel cell wall

• Arabinan was first time found in flaxseed, and its fine structure was studied.
• The linkage types of arabinans from different plant origins were summarized.
• Increasing utilization of flaxseed dietary fibre is further potential outcome.

The structure of ammonium sulphate supernatant fraction from 1 M KOH extracted flaxseed kernel polysaccharides (KPI-ASF) was elucidated by methylation/GC-MS, NMR spectroscopy, and MALDI-TOF-MS analysis techniques. The dominate sugar residues of KPI-ASF molecule comprised of (1,5)-linked-α-l-Arabinofuranose, (1,3,5)-linked-α-l-Arabinofuranose, (1,2,3,5)-linked-α-l-Arabinofuranose, terminal α-l-Arabinofuranose, and rhamnogalacturonan-I (RG-I) repeating disaccharide units. Weight average molecular weight (Mw) of KPI-ASF was 596 kDa with around 60 mol% of arabinans. The total phenol content of KPI-ASF was calculated as 0.96 ± 0.01 mg GAE/g DW by Folin-Ciocalteu method. A possible structure of KPI-ASF was proposed as RG-I bridge-linked arabinans as below: R1 (8–12 mol%) could be arabinoglactan, and/or arabinoxylan; R2 (1–12 mol%) could be terminal arabinose and/or ester-linked phenolic compounds; and R3 (12–18 mol%) could be xylogalacturonan. As branched arabinans are widely distributed in the cell wall of oilseeds and legumes, the detailed structures of KPI-ASF might help explain the roles of cell wall polysaccharides of flaxseed kernel on the protection of polyunsaturated fatty acids. Moreover, the utilization of flaxseed dietary fibres in related food products are further potential outcomes from this enhanced understanding of molecular structure.

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