Rhamnopyranose structure

Five different crystalline products were separated and characterized from the reaction mixture obtained from the condensation of L-rhamnose with paraformaldehyde. None of the products isolated corresponded to the acetal isolated earlier by Lobry de Bruyn and Alberda van Ekenstein. The expected 1,2 3,5-di-0-methylene-a-L-rhamnofuranose was identified on graded hydrolysis, this gave the 3,5-0-methylene acetal. However, the possibility of the 3,4-0-methylene-L-rhamnopyranose structure was not eliminated. The main product was 3,4-0-(oxidodimethylene)-L-rhamnose a smaller yield of 2,3-0-(oxidodimethylene)-a-L-rhamnose was obtained. 

Approximately 400 different glycosyltransferases are necessary in order to ensure the synthesis of those bacterial polysaccharides whose structures have thus far been elucidated. This estimate is based on the results of an analysis of the structures, made in order to ascertain how many different disaccharide fragments are present. An example of such an analysis is shown in Table V for the disaccharide sequences L-rhamnopyranosyl-D-galacto-pyranose, D-mannopyranosyl-L-rhamnopyranose, and D-galactopyranosyl-D-mannopyranose that are characteristic for the O-specific polysaccharides of Salmonella serogroups A, B, D, and E, the objects of many biosynthetic studies. Full details of similar analyses for other disaccharide sequences will be published elsewhere, as the resulting Tables are too voluminous for inclusion in this Chapter, but the most interesting results are summarized in Tables VI and VII. 

Simple one-pot synthesis of different 1-0-glycosylboranophosphate diesters (65), the stable analogues of 1-0-glycosylphosphates based on the anomeric H-phosphonate of a-L-rhamnopyranose as common precursor, has been reported. Various phosphodisaccharides, structural analogues of phosphate (66) including deoxy, fluoro and aminodeoxy disaccharide phosphates e.g. (67) have been synthesized. These phosphodisaccharides were tested as acceptor sub-strate/putative inhibitors for the Leishmania-a-D-mannosylphosphate transferase. 

The noncellulosic fraction of SB hull polysaccharides consists mainly of xylose and mannose. The acidic polysaccharides that can be extracted from SB hulls in sequential extractions steps (acidic polysaccharides 1-V) are all very similar in structure with interior chains made up of D-galacturonic acid and L-rhamnopyranose residues. These interior acidic chains are associated with side chains consisting of neutral sugars, including L-arabinose and chains of 1 4 linked P-D-galactopyranose residues. In addition, 2-O-P-D-gal-D-xyl and 2-O-a-L-fuc-D-xyl residues also may be present (Aspinall et al., 1966). Although the acidic polysaccharide fractions I—V are similar structurally, upon hydrolysis they may yield different monosaccharide concentrations. For example, acidic polysaccharide III contains 76% uronic acids, whereas fractions IV and V contained approximately 45% uronic acids. Hydrolysis of the acidic polysac-... 

The problem of similar retention-times occurs not only with isomeric compounds but also in mixtures containing deoxy sugars. Thus, 2,3-di-O-methyl-L-rhamnose and 2,3,4,6-tetra-O-methyl-n-glucose are inseparable as their methyl glycosides, but, after trimethylsilylation of the mixture, resolution is readily achieved, the rhamnoside being eluted first. Larsson and Samuelson used the same principle in a study of the structure of an 0-(a-D-galactopyranosyluronic add)-L-rhamnopyranose 1,2 -r,2-dianhydride. 

Fig. 3.17. Structure of a rhamnogalacturonan I (RG-I). a-o-GalAp a-D-galactopyranoside acid (galacturonic acid) a-L-Rhap a-L-rhamnopyranose (rhamnose)...

Fig. 3.21. Structure of rhamnogalacturonan II (RG-II) (Doco and Brillouet, 1993). a-o-GalAp galacturonic acid a-D-Galp galactopyranoside /3-L-Rhap /3-L-rhamnopyranose (rhamnose) a-L-Ara/ a-L-arabinofuranose (arabinose) a-L-Fucp = a-L-fucopyranose (fucose) 2-0-Me-a-D-xylose 2-0-methyl-a-D-xylose /3-D-GlcAp /3-D-glucuropyranosic acid (glucuronic acid) /3-D-Api/ /3-D-apiofuranose (apiose) KDOp 3-dioxy-D-manno-2-octululopyranosic acid /3-D-DHA/ /3-deoxy-D-lyxo-2-heptulafuronosic acid /3-L-AceA/ /3-3-C-carboxy-5-deoxy-L-xylofuranose (aceric acid) Me methyl...

The basic structural unit of the main okra chain is acidic disaccharide 4)-a-D-GalpA-(1 2)-a-L-Rhap-(l. Side chains contain the disaccharide D-galactobiose, P-D-Galp-(1 4)-P-D-Galp-(1, that is linked by (1 4) bound to about one half of the a-L-rhamnopyranose residues. 

Type 2 polysaccharide has an unusual structure with a repeating tetrasaccharide unit containing a sequence of three a- and -linked L-rhamnopyranose residues linked to position 3, and a single a-D-glucopyranose linked at position 4. The polysaccharide has a branched disaccharide unit of a-D-glucopyranose uronic acid-(l 6)-a-D-glucopyranose linked 1 2 to the second L-rhanmose residue... 

A very extensive set of methyl a- and P-D-glucopyranosides substituted at 0-2 and 0-3 with a range of other sugars has been produced for computer-based structural analytical work, a-Linked D-maimopyranose, L-rhamnopyranose, L-fiicopyranose, and p-linked D-galactose and L-fucose were the substituting sugars. 

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