A commercial apple pectin was sequentially digested with the cloned enzymes endopolygalacturonase, galactanase, arabinofuranosidase, xylogalacturonase, and rhamnogalacturonan hydrolase. The rhamnogalacturonan hydrolase-generated oligosaccharides were separated by ultrafiltration, anion exchange, and size-exclusion chromatography. Fractions from the ion exchange chromatography were pooled, lyophilized, and screened by MALDI-TOF MS. An oligosaccharide (RGP14P3) was identified and its structure, α-D-GalpA--α-L-Rhap--α-D-GalpA--α-L-Rhap--α-D-GalpA, determined by 1D and 2D NMR spectrometry. This oligosaccharide probably represents a direct connection between homogalacturonan and rhamnogalacturonan in pectin. Alternatively, it could indicate that the nonreducing end of rhamnogalacturonan starts with a galacturonic acid residue. 1. Introduction Pectin is probably the most complex macromolecule in nature, being composed of as many as 17 different monosaccharides [1, 2]. These monosaccharides are organized in a number of distinct structural regions. The backbones of these regions fall into two groups: those containing 1,4-linked α-D-GalA residues (galacturonans) and those made up of a repeating disaccharide unit --L-Rha---D-GalA- (RG-I). Three classes of galacturonans are recognized: the unsubstituted homogalacturonan (HG), short stretches of galacturonan with closely spaced complex and simple side chains, rhamnogalacturonan II (RG-II), and xylogalacturonan (XGA) consisting of an HG backbone with multiple side branches of xylose linked to O-3 of the GalA residues. Until recently structural models have depicted pectin as HG (smooth regions) and RG (hairy regions) forming one continuous backbone [3, 4]. Recently, a model of pectin was suggested in which HG and XGA are depicted as side chains of RG [2, 5] and Yapo [4] suggested a model which is a hybrid of the two. Round et al. [6] found by atomic force microscopy coupled with mild acid hydrolysis that much of the HG extracted from green tomato fruit with sodium carbonate was devoid of any link to RG-I. The RG-I was all associated with high molecular weight complexes possibly covalently linked to additional HG. In this paper, we describe the purification and structural characterization of an oligosaccharide (RGP14P6), which supports the linear model for the way that HG and RG are connected together in apple pectin. Its structure was determined by MALDI-TOF MS and TOCSY, NOESY, HMQC, and HMBC NMR spectroscopy. As a connection between HG and RG-I, the oligosaccharide supports the model of HG and RG-I forming one
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