Guaran hydrolysis

In 1951, Whistler and Durso reported68 the isolation of epimelibiose by partial, acid hydrolysis of the polysaccharide guaran, followed by charcoal-column fractionation of the products. ThA material was obtained in anhydrous crystalline form from a mixture of methyl and butyl alcohols it had m.p. 201°, [a]26D + 120.9° — +124.6° in 36 hr. (c 2, in water). The yield was 2.2% of the weight of guaran used. Epimelibiose has also been produced72" in crystalline form following the ammonia-catalyzed alkaline isomerization of melibiose. 

Partial hydrolysis is also useful in the study of some problems connected with heteroglycans. Whistler and Durso hydrolyzed guaran to 79 % completion and isolated 4-0-/3-D-mannopyranosyl-D-mannose, 6-0-a-n-galacto-pyranosyl-D-mannose, and 6-0-a-D-galactopyranosyl-4-0- 3-D-mannopyran-osyl-D-mannose from the hydrolysate. This enabled an unambiguous... 

Much of the information on acid hydrolysis of polysaccharides is in the form of unpublished, empirical observations, although inferences (such as, that during the acid hydrolysis of guaran, the D-galactopyranose residues are readily removed, whereas D-mannose is set free somewhat more slowly ") may be found in the literature. 

Methylation of the polysaccharide with subsequent hydrolysis and separation of products leads to the isolation (1) of 2,3,4,6-tetra-O-methyl-D-galactose, 2,3,6-tri-0-methyl-l>-mannose, and 2,3-di-0-methyl-l>-mannose. These results, confirmed by others (11) f show that D-galactopyranose units occur as nonreducing terminal units. Since the branches must be short, the most likely structure of guaran, based on evidence to this point, is that shown. 

Preparation. The sugar may be prepared by charcoal-column fractionation of the partial acid hydrolysis products of guaran (4 ) and by the alkaline isomerization of melibiose ( S). 

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