Galactans methylation

According to some, pectin appears to be a mixture of methyl esterified galacturonan, galactan, and araban in which some of the galacturonan molecules... 

The present authors and their collaborators have also investigated arabans derived from apple and citrus pectins and here again it is to be noted that the arabans are associated in nature with galactan and pectic acid. The methylated derivatives of all these arabans have very similar properties and on hydrolysis give approximately the same proportions of 2,3,5-trimethyl-L-arabofuranose, 2,3-dimethyl-L-arabinose and... 

The first polymerizations reported by Kops and Schuerch147 were those of l,4-anhydro-2,3,6-tri-0-methyl-/3-D-galactopyranose and 1,4-anhydro-2,3-di-0-methyl-a -L-arabinopyranose. The latter compound was slightly contaminated with l,4-anhydro-2,3-di-0-methyl-a-D-xy-lopyranose, but the course of the polymerization could nevertheless be monitored reasonably accurately. For the most part, the polymerizations were conducted at 10% concentration (g/mL) in dichloro-methane, or aromatic hydrocarbons, with 1-5 mol% of phosphorus pentafluoride, or boron trifluoride etherate. At low temperature (—78 to —97°), the d.p. of both polymers produced was —90 at increasing temperatures of polymerization, termination processes became more severe, and the d.p. lower. Usually, the reaction times were long (perhaps unnecessarily so), and the conversions were 50 to 90%. The specific rotations of the D-galactans prepared at —28 and —90° differ by only —10° ( — 85 to — 95°), but those of the L-arabinans varied from + 6... 

Those galactans that have been studied (see the preceding) have degrees of polymerization ranging from 33 in white willow161 to 50 in sycamore-cell primary-walls.183 These values were obtained by vapor pressure and osmosis studies,161 and by comparing the ratio of terminal to intrachain galactose units by methylation analysis.163... 

Structural aspects of L-arabino-D-galactan glycoprotein from radish leaves have been studied by using /3-D-galactonase, /J-D-galactosidase, and a-L-arabinofuranosidase, in conjunction with methylation analysis.3786... 

Pectins are present in plant juices. These are now considered to be polymers containing a basic chain, designated pectic acid, of 4-linked D-galactopyranuronic acids, the-carboxyl groups of which exist in part as methyl esters and in part as salts. Variable amounts of an araban and a galactan are associated with this polysaccharide.12 A method of approximate assay for pectic substances13 depends upon saponification... 

Compound 160 was used as donor for the preparation of methyl 4,6-dideoxy-a-L-/yxo-hexopyranosyl-(l -> 3)-2-acetamido-2-deoxy-a-D-glucopyranoside, the 4 -deoxy analogue of the linkage disaccharide to the galactan chain in mycobacteria.235... 

The first structural study of the arabinogalactan or e-galactan from European larch was carried out by Campbell, Hirst, and Jones,126 who showed the polysaccharide to contain residues of D-galactose and L-arabinose in the proportion of 6 1. Fractionation of the methylated polysaccharide gave, as the main component, a methylated galactan containing... 

Further knowledge of the detailed structure of European-larch e-galactan has come from later investigations.126 127 In these studies, however, no evidence could be found for heterogeneity of the type previously encountered in the methylated derivative, and the e-galactan itself was ultracentrifugally homogeneous. 

In agreement with the earlier findings,126 approximately equimolecular proportions of 2,3,4,6-tetra-, 2,3,4-tri-, and 2,4-di-O-methyl-D-galacto-pyranose were isolated as the main products of hydrolysis of methylated e-galactan, but, in addition, small proportions of 2,3,4-tri-O-methyl-L-arabinose, 2,5-di-O-methyl-L-arabinose, and 2,4,6-tri- and 2-mono-0-methyl-D-galactose were found. Evidence in favor of partial structure XLI for the galactan framework was obtained by application to the polysac-... 

Mild, acid hydrolysis of European-larch arabinogalactan results in the formation of 3-0-/3-L-arabinopyranosyl-L-arabinose (XXXIX). 26 Although this disaccharide has been isolated as an acid-reversion product from L-arab-inose,131 it is accompanied under these conditions by two other arabinose-containing disaccharides, neither of which was detected in the larch polysaccharide hydrolyzate. Since hydrolysis of methylated -galactan gave approximately equimolar proportions of 2,3,4-tri-O-methyl-L-arabinose and 2,5-di-O-methyl-n-arabinose, traces of 2,3,5-tri-O-methyl-L-arabinose, and no mono-O-methyl-L-arabinose, it is clear that the L-arabinose units must arise from an arabinogalactan rather than from an arabinan, and that the majority of these must be present in 3-0-/3-L-arabinopyranosyl-L-arabino-furanose side-chains (XLV) linked, in some way as yet unknown, to the framework of D-galactose units (XLI). 

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