Pectic substances isolated from apple cellulosic residue: structural characterisation of a new type of rhamnogalacturonan I

The cellulosic residue (CR) of ripe apples (variety ‘Glockenapfel’) was analysed. Uronic acid in the CR indicated the presence of pectic substances. The objectives of the work were to isolate and characterise the CR associated pectic substances and to find possible cross-links between the two cell wall polymers. to characterisation enzymatic degradation of the cellulose was necessary. The degradation products were removed by ultrafiltration and the enzyme resistant material submitted to linkage analysis to obtain information about the structure of the CR-pectin. A model of the CR-pectin consisting of five units was developed: a highly ramified rhamnogalacturonan I backbone containing an equal number of galacturonic acid (1,4-GalAp) and rhamnose (sum of 1,2-Rhap and 1,2,4-Rhap) residues was found. About 80% of the rhamnose residues was origin of side chains. Attached to the backbone was a xylogalacturonan side chain, containing 1,4-GalAp, 1,3,4-Galp, 1,4-Xylp and T-Xylp. Highly ramified arabinans, with a 1,3-linked backbone, made up more than half of the neutral sugar residues. Two galactose containing units were found: a 1,4-linked galactan with few 1,3- and 1,6-linked short side chains (located near to the backbone) and a short strictly linear 1,4-galactooligomer. The postulated model was confirmed by further degradation of the CR with pectin specific enzymes. An interaction between cellulose and CR-pectin is suggested to occur through the galactan side chains.