Fractionation and structural characterization of six purified rhamnogalacturonans type I from flaxseed mucilage

Abstract

An improved method for the qualitative fractionation of water-soluble mucilage (WSM) polysaccharides isolated from flaxseed (Linum usitatissimum) comprising ion-exchange chromatography coupled with an evaporative light scattering detector was developed. One neutral fraction (NWSM) and six acidic fractions (AWSM-1 to AWSM-6) were purified. The molecular weights of NWSM, AWSM-1 - AWSM-6, determined using SEC/MALLS, were 1300 kDa, 756.4 kDa, 718.8 kDa, 505.6 kDa, 457.5 kDa, 354.8 kDa and 593.2 kDa, respectively. Monosaccharide composition and linkage analysis confirmed that NWSM was an arabinoxylan with a (1 → 4)-xylan backbone to which terminal arabinose residues and putative short-chain monosaccharides were attached at positions 2 and/or 3. On the other hand, it was shown that the six acidic fractions had similar rhamnogalacturonan I (RG-I) blocks that can be monosubstituted at O-3 position of rhamnose, mostly by terminal galactose or terminal fucose and occasionally by short side-chains of neutral monosaccharides. The calculated rhamnose/galacturonic acid ratio was from 1.22 to 0.85, which remained virtually unchanged and close to 1. The degree of branching of the isolated RG-I fractions was estimated at 0.33 to 0.65. The structural variability of the acidic fractions appeared not to be related to the RG-I backbone but possibly to the type and degree of substitution. These results might serve as a basis for developing chromatography conditions to contribute towards a better separation of mucilage polysaccharides.