Molecular structure of pectic substances with particular reference to sugar-beet araban

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.