Detection and homogeneity of cell wall pectic polysaccharides of Lemna minor

Abstract

A method was developed for determining the size homogeneity of purified pectic polysaccharide based on analysis of polysaccharide in individual diethylaminoethyl (DEAE) column fractions by high performance size exclusion chromatography. When the method was used in combination with laser light scattering, absolute M w and M n values of pectic polysaccharides were obtained. The method was used to show that four pectic polysaccharides were present in the 22 °C ammonium oxalate-soluble fraction of cell walls of Lemna minor. The four polysaccharides, designated PS-I-PS-IV, had the following M w ranges: PS-I, 50,200–75,400; PS-II, 18,800–99,700; PS-III, 24,200–150,000; and PS-IV, 6,170–163,000. PS-IIb and PS-IVb, the major portions of PS-II and PS-IV, respectively, were heterogeneous with respect to molecular size but were chemically homogeneous. The peak M w range of PS-IIb was 28,100–99,700, while the range for PS-IVb was 53,300–163,000. PS-IIb was composed of: GalA (54.5 ± 3.2%) (std dev), Api (39.3 ± 1.4%), Xyl (2.5 ± 1.4%), Ara (1.4 ± 0.8%), and Rha (2.3 ± 0.3%), while PS-IVb was composed of: GalA (96.3 ± 0.8%), Api (2.1 ± 0.7%), and Xyl (1.6 ± 0.3%). Small amounts (< 0.5 mole %) of fucose, mannose, and glucose and rhamnose, fucose, arabinose, mannose, and glucose were detected in PS-IIb and PS-IVb, respectively. The degree of methyl esterification of both PS-IIb and PS-IVb was zero. The isolation of a pectic polysaccharide of high galacturonic acid content (96%) and low degree of methyl esterification (∼ 0%) under mild isolation conditions has not been reported previously.