Structural studies of the pectic polysaccharide from duckweed Lemna minor L.

The pectic polysaccharide of duckweed Lemna minor L. termed lemnan (LM) was shown to contain the ramified, "hairy" region. Using partial acid hydrolysis and Smith degradation followed by NMR spectroscopy of the fragments obtained, some structural features of the hairy region of LM were elucidated. Partial acid hydrolysis of LM afforded the crude polysaccharide fraction LMH that was separated into two polysaccharide fractions: LMH-1 and LMH-2. In addition, the oligosaccharide fraction LMH-3 contained 97% D-apiose was obtained from the supernatant. A further more rigorous acidic hydrolysis of LMH led to the crude polysaccharide fraction LMHR which was separated in to two fractions: LMHR-1 and LMHR-2. Smith degradation of LMH afforded the polysaccharide fragment LMHS differed in low contents of apiose residues. Unfortunately, NMR-spectroscopy failed to provide significant evidence concerning the structure of LMH-1 due to the complexity of the macromolecule. The structure of the 1H/13C-NMR spectroscopy including the correlation 2D NMR spectroscopy. As a result, alpha-1,4-D-galactopyranosyluronan was confirmed to be the main constituent of the LM backbone. In addition, the ramified, "hairy" region of the macromolecule appeared to contain segments consisting of residues of terminal and beta-1,5-linked apiofuranose, terminal and alpha-1,5-linked arabinofuranose, terminal and beta-1,3- and beta-1,4- linked galactopyranose, the terminal and beta-1,4-linked xylopyranose, and beta-1,4-linked 2-mono-O-methyl xylopyranose. Analytical and NMR-spectral data of LMHS confirmed the presence of considerable amounts of the non-oxidized of 1,4-linked D-galactopyranosyl uronic acid residues. Thus, some side chains of the ramified region of lemnan appeared to attach to D-galactopyranosyl uronic acid residues of the backbone.