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Abstract
Random distribution of D-galactose rsidue in side chains of Hibiscus cannabinus mannan has been
confirmed by NMR analysis.
Purified polysaccharide isolated from H. cannabinus seeds on hydrolysis with M trifluoroacetic acid (4h at
1000
c), using paper chromatography and solvent n-butanol; Isopropanol : water (11:6:3) as solvent revealed
the presence of galactose : Mannose in the ratio of 1:3. The structure estabished by usual chemical method.
Investigation has been done by 1H and 13C NMR spectra of galactomannon. Present structure determination
based on complete assignments for protons and carbons as below:
The 1H NMR spectrum showed, in the anomeric region, two absorptions. The signals at 4.9 ppm (J1 ~ 2.9
Hz) and 4.6 ppm (J1,2~1.0 Hz) are assigned to H-1 of α-D-galactopyranosyl and β-D-mannopyranosyl units,
respectively2 which, therefore, should be in the expected 4C1 conformation2
. The integration of the anomeric
proton resonances indicated a 1:3 ratio of galactose to mannose in the polymer.
In the 13C NMR spectrum all the different carbon lies are well separated and are in accord with those
reported for gum3
. Their chemical shifts are in good agreement with those reported in the literature3-5
. The
spectrum clearly differentiated the three structural units of the polymer namely, 6-0-substituted and
unsubstituted internal (1 4) linked β-D-mannopyranosyl units of the backbone and terminal (nonreducing) α-D-galactopyranosyl units. Integration of the anomeric carbon resonances at 101.8 and 103.0
ppm indicated a D-galcto—D-mannan molar ratio of 1.0:3. The peaks at 63.6 and 64.1 ppm were observed
for the methylene carbon atoms3 5. Three resonances in the C-4 (Man) region were observed. Accordingly,