Molecular structure of pectic substances with particular reference to sugar-beet araban
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1961Author
Allan, John Lewis
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Abstract
An araban-rich polysaccharide was extracted from dry, sugar-free
beet pulp with hot saturated aqueous calcium hydroxide. Hydrolysis
followed by chroma to graphic analysis of the araban gave arabinose and
galactose in approximately equal proportions, together with smaller
quantities of galacturonic acid and rhamnose.
The araban was acetylated and then fractionated by a chromatographic
method involving adsorption on a charcoal column, followed by gradient
elution with acetone/chlorofom, increasing the proportion of the
latter, and finally using pyridine as the eluting agent.
Catalytic oxidation of the araban was then carried out in aqueous
solution by means of gaseous oxygen in presence of finely divided
platinum (Adams’ catalyst), the temperature and pH of the solution
being controlled. One of the samples of oxidised araban was subjected
to controlled hydrolysis, the course of the reaction being followed
polarimetrically. The increase in uronic anhydride content of the
araban on oxidation was lower than expected* The catalytic oxidation
procedure was then applied to the methyl glycoside a-methyl-g-mannoside
as a check. The oxidation was in this case successful, and the
corresponding hexuronic acid methyl glycoside, a-methyl-D-mannuronoside
was isolated and characterised.
Preliminary periodate oxidative studies were then followed by an
application of the Smith degradation, involving oxidation of the
araban with periodate, reduction of the oxypolysaccharide with
borohydride, and finally mild hydrolysis of the "polyalcohol" so
produced.
Oxidations of araban with permanganate were also carried out.
Both small and large scale extractions were carried out.
Bellowing a preliminary extraction of the milled hop leaves with organic
solvents, the pectin was extracted from the residual "hop straw" in two
fractions, using first boiling water and then dilute aqueous ammonium
oxalate. Both fractions gave on hydrolysis galacturonic acid,
galactose, and arabinose as the main components, and also smaller
quantities of rhamnose and an unidentified sugar which may have been
galactobiose.
The extracted material was grossly contaminated and several methods
of purification were applied. The most successful of these was
treatment with cold trichloroacetic acid.