Pectic substances from red beet ( Beta vulgaris conditiva). Part I. Structural analysis of rhamnogalacturonan I using enzymic degradation and methylation analysis

Abstract

Cell wall material from ripe red beet ( Beta vulgaris L. var. conditiva) was isolated as alcohol insoluble residue (AIR). The chelator-soluble pectin obtained by cyclohexane- trans-1,2-diaminotetraacetate (CDTA) extraction of the AIR was fractionated by anion exchange chromatography (AEC). The main fraction was further fractionated by gel filtration chromatography (GFC). Fractions from both chromatographic systems were stepwise degraded by endo-polygalacturonase, endo-β-(1→4)- d-galactanase, endo-α-(1→5)- l-arabinanase and α- l-arabinofuranosidase. Degradation products were fractionated by GFC or by AEC. Polymeric fractions were investigated by methylation analysis after carbodiimide-activated reduction with NaBD 4. Selected fractions were additionally methylated with trideuteromethyliodide to enable the detection of O-methyl substituted sugars. The results indicate that the CDTA-soluble pectins of red beet cell walls are composed of three different sub-units: a homogalacturonan, which accounts for about 75%, a highly ramified rhamnogalacturonan I (RG-I) and a typical rhamnogalacturonan II (RG-II). RG-I consists of a highly ramified backbone composed of nearly equal amounts of rhamnose and galacturonic acid. Side chains, mainly arabinans, galactans and type-II arabinogalactans are attached to the RG-I backbone. Some arabinans are connected via short galactan chains directly or indirectly to this backbone. Type-II arabinogalactans are formed by “inner” chains consisting of (1→3)-linked galactans and short “outer” chains composed of an average number of one to three (1→6)-linked galactose residues. Terminal arabinofuranoses are linked via the O-3-position to galactose residues. Nearly all non-reducing ends consist of glucuronic acid. Approximately 65% of the glucuronic acid residues are substituted by a methyl ether group and approximately 10%, most probably, by a terminally linked rhamnose.