Araban from peanut

Pectin preparations usually show the presence of an araban and a galactan along i ith the pectic acid. By using methods that would not hydrolyze furanosidc linkings it has been possible to isolate an araban from the pectin of peanuts, apples, citnis fruits and from the seed of Lupinus albus. A galactan was also isolated from Lupinus albus. 

We may return now to the polysaccharides present in the peanut for a brief consideration of the relationship of the other components present in the pectic materials to the araban constituent. All the evidence indicates that the pectic acid portion of the peanut is identical with normal pectic acid and, as was indicated in the previous section, this material, which is very stable to acid hydrolysis and possesses a high positive rotation contains a main chain which is built up of D-galac-turonic acid residues of the pyranose type. If, therefore, the araban associated with the pectic acid had been derived directly from the pectic acid by decarboxylation without intermediate hydrolysis of the poly-galacturonide, the sugar residues in the araban should also be in the pyranose form. The experimental evidence shows clearly, however, that the arabinose residues in araban are furanose in type and it follows that any hypothesis concerning the direct conversion of pectic acid into the araban by decarboxylation is untenable. 

Since L-arabinose is a constituent unit of many natural products a number of methylated derivatives of this sugar have been isolated by the hydrolysis of methylated polysaccharides. Thus methylated peanut (Arachis hypogea) araban and sugar beet araban give trimethyl-L-arabofuranose, 2,3-dimethyl-L-arabinose and 2-methyl-L-arabinose in equal proportions.84 From methylated gum arabic trimethyl-L-arabo-furanose and 2,5-dimethyl-L-arabinose have been isolated.85 These same compounds have also been isolated from methylated cherry gum86 and methylated damson gum,87 whereas methylated mesquite gum88 affords 3,5-dimethyl- and 2,3,5-trimethyl-L-arabinose on hydrolysis. 

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