D. L. Stevenson

Characteristics and distributions of ester groups in propylene glycol alginates

Abstract The nature of the ester groups in propylene glycol alginate (PGA) has been investigated by 13 C-NMR spectroscopy and chemical saponification studies. Two types of ester, namely primary 2-hydroxyprop-1-yl and secondary 1-hydroxyprop-2-yl, were detected by 13 C-NMR spectroscopy by virtue of the methyl carbon resonances. From model compound studies the two forms were shown to be due to the type of ester present and not to the type of sugar residue (β- d -mannuronic or α- l -guluronic acid) to which the ester may have been bound. Alkaline saponification studies using a variety of analytical monitoring systems, and model compounds, has shown that the two types of ester exist in an equilibrium with the more thermodynamically favourable primary ester predominating in the ratio 4:1. The nature of the sugar residues, namely β- d -mannuronic and α- l -guluronic acid, to which the esters were bound, did not have any bearing on the rate of ester hydrolysis. The rate of hydrolysis of ester groups in PGA was, however, dependent upon the nature of the ester groups present, namely primary or secondary. On the basis of the evidence presented, the thermodynamically more stable primary ester predominates in PGA, as would be expected from the preparation conditions.

The chromatographic behaviour of a series of fructo-oligosaccharides derived from levan produced by the fermentation of sucrose by Zymomonas mobilis

Abstract Levan, a (2 → 6)-β- d -fructofuranosyl linear polysaccharide has been produced by fermentation of sucrose by Zymomonas mobilis. Complete hydrolysis of levan and HPLC analysis have shown that the molecular weight of the polymer is in the order of 106 daltons. Mild acid hydrolysis of levan produced a series of (2 → 6)-β- d -fructofuranosyl oligosaccharides of up to DP 14, which were resolved by GPC on Bio-Gel P2. The elution positions of these oligosaccharides were different from those of (1 → 4)-α- d - and (1 → 6)-β- d -glucopyranosyl oligosaccharides but similar to those of (1 → 4)-α- d -glucopyranosyl oligosaccharides. These differences have been explained in terms of anomeric configuration for each series of oligosaccharides.

Analysis of the oligosaccharides from the roots of Arnica montana L., Artemisia absinthium L., and Artemisia dracuncula L.

Abstract The oligosaccharides extracted from the roots of Arnica montana L., Artemisia absinthium L. and Artemisia dracunculus L. have been analysed by thin-layer and gel-permeation chromatography to assess their applicability as ‘guide’ substances for pharmacological activity. Differences observed in the oligosaccharide component composition of such extracts might be more related to the vegetative stage of the plants at time of harvest than to the species themselves. In addition to a series of non-reducing oligofructosides, a series of reducing inulin-type oligosaccharides was found at the initial stages of growth, whereas in later stages of growth only non-reducing oligofructosides were present. These differences have been related to different stages of biosynthetic activity within the plants.

Studies on the chemical stability of propylene glycol alginate esters

Abstract The chemical stability of propylene glycol alginates (PGAs) has been examined. Under acidic conditions the ester groups in PGA are stable to hydrolysis but hydrolytic degradation of the glycosidic linkages in the polysaccharide backbone occurs. Under alkaline conditions the ester groups are hydrolysed with the primary 2-hydroxyprop-1-yl ester groups being more susceptible than secondary 1-hydroxyprop-2-yl ester groups, with little degradation of the polysaccharide backbone. Sodium carbonate-bicarbonate buffer was a much more effective hydrolysing reagent than sodium hydroxide at the same concentration and pH, and the rate of hydrolysis was greatly accelerated by increasing the hydrolysis temperature. Acetate, citrate and phosphate ions accelerated the rate of hydrolysis of the ester groups in PGA when added to the sodium hydroxide hydrolysing reagent. Hydrolysis of the ester groups in PGA with sodium hydroxide was unaffected by the addition of imidazole. However hydrolysis of the ester groups in PGA with sodium hydroxide in the presence of 1-aminobutane led to the formation of an alginate amide in which only the primary 2-hydroxylprop-1-yl ester groups were present, suggesting that a nucleophilic substitution of primary ester groups by amine groups is involved in the reaction.