Monosaccharides mixtures

V. Pitthard, P. Finch, GC MS analysis of monosaccharide mixtures as their diethyldithioacetal derivatives Application to plant gums used in art works, Chromatographia, 53, S317 S321 (2001). 

The analysis of monosaccharide mixtures as the permethylated derivatives was proposed early in the application of gas-liquid chromatography to carbohydrates, but the method has now been superseded by more convenient procedures.230,231 There are, however, situations in which this method is useful, such as during a structural study of a polysaccharide by the methylation technique. The mixture of partially methylated monosaccharides obtained by methanolysis may then be fully methylated, and the proportions of the various monosaccharides determined. This approach has been used, for example, in studies on a galactomannan392 and on tamarind-kernel polysaccharide.393 Such an analysis also constitutes a useful check to ensure that no significant change in the composition of the polysaccharide occurred during methylation. 

Insoluble concanavalin A has already been used in the purification of glycoproteins,and, in view of its commercial availability, it will no doubt prove useful in structural, studies. Furthermore, the in-solubilized lectin may be used in column form for the separation of monosaccharide mixtures and of polysaccharide mixtures " it has been demonstrated that mixtures of carbohydrates not separable by complex-formation with concanavalin A in solution may be separated by using the immobilized form,... 

The hydrolysate (monosaccharide mixture) is analyzed by chromatographic methods. The ion exchange chromatography of the borate complexes of neutral monosaccharides is the most common method, which is used as the basis of fully automatic carbohydrate analysis. 

Fructose—Dextrose Separation. Emctose—dextrose separation is an example of the appHcation of adsorption to nonhydrocarbon systems. An aqueous solution of the isomeric monosaccharide sugars, C H 2Dg, fmctose and dextrose (glucose), accompanied by minor quantities of polysaccharides, is produced commercially under the designation of "high" fmctose com symp by the enzymatic conversion of cornstarch. Because fmctose has about double the sweetness index of dextrose, the separation of fmctose from this mixture and the recycling of dextrose for further enzymatic conversion to fmctose is of commercial interest (see Sugar Sweeteners). 

In a similar manner, ketones can react with alcohols to form hemiketals. The analogous intramolecular reaction of a ketose sugar such as fructose yields a cyclic hemiketal (Figure 7.6). The five-membered ring thus formed is reminiscent of furan and is referred to as a furanose. The cyclic pyranose and fura-nose forms are the preferred structures for monosaccharides in aqueous solution. At equilibrium, the linear aldehyde or ketone structure is only a minor component of the mixture (generally much less than 1%). 

Deoxy sugars, as we saw in Section 25.7, have an oxygen atom "missing." That is, an —OH group is replaced by an -H. The most common deoxy sugar is 2-deoxyribose, a monosaccharide found in DNA (deoxyribonucleic acid). Note that 2-deoxyribose exists in water solution as a complex equilibrium mixture of both furanose and pyranose forms. 

The reversible reactions are initiated by an equilibrium between neutral and ionized forms of the monosaccharides (see Fig. 6). The oxyanion at the anomeric carbon weakens the ring C-O bond and allows mutarotation and isomerization via an acyclic enediol intermediate. This reaction is responsible for the sometimes reported occurrence of D-mannose in alkaline mixtures of sucrose and invert sugar, the three reducing sugars are in equilibrium via the enediol intermediate. The mechanism of isomerization, known as the Lobry de Bruyn-... 

Maximum labelling of heparin with F-D was achieved at 5 hours at 25 °C, pH 8.4. In the case of heparin, the efficiency of labelling was not dependent on molecular weight, but solely a function of the ratio of the concentrations of labelling reagent to monosaccharide subunit in the reaction mixture. Similar results were encountered in the labelling of dextrans of different molecular weight (9). 

New technology has been applied to allow more whey to be used. The technology has been applied to convert the lactose into a mixture of dextrose and galactose. These two monosaccharides are both reducing sugars and the mixture is much more soluble than lactose. 

The reaction of sucrose with a combination of 2,2-dimethoxy-propane-N,N-dimethylformamide-p-toluenesulfonic acid has been exploited to give various, interesting, cyclic acetal derivatives.32,83-85 This combination of reagents for acetonation is known to give strained, and otherwise inaccessible, acetals of monosaccharides.86-89 Treatment of sucrose with 2,2-dimethoxypropane in N,N-dimethyl-formamide in the presence of a catalytic proportion of p-toluenesul-fonic acid for 80 min at room temperature afforded a mixture containing 4,6-O-isopropylidenesucrose, 2,1 4,6-di-O-isopropyli-... 

---