Structural details of pectic galactan from the secondary cell walls of flax (Linum usitatissimum L.) phloem fibres

Details of the backbone and side chain structure of pectic β-(1 → 4)-galactan from the secondary cell walls of flax phloem fibres were characterised by NMR and mass spectrometry of the fragments obtained after partial hydrolysis with specific endogalactanase and rhamnogalacturonan hydrolase. The proportions of branched and linear rhamnose in the backbone of the polymer equalled 72% and 28%, respectively. Rhamnose branched with a single galactose residue comprised 47% of the total rhamnose; thus, in the bulk of the polymer backbone, rhamnose had 0–1 galactose residues. Within the backbone, residues of rhamnose branched with long galactose chains alternated with linear rhamnose and rhamnose with a single galactose. Oligomeric galactose chains averaged 14 monomers in length. Alternative glycosidic bonds of galactosyl residues were present. The established structural details of cell wall galactan are compared to those of nascent galactan before incorporation into the fibre cell wall, and galactan modifications in muro are discussed.