Pentoses quantitative determination

In as far as other analytical methods are concerned, many specific reactions have been elaborated for the quantitative determination of 2-deoxy aldoses. 2-Deoxy-D-ribose (2-deoxy-D-erythro-pentose), a compound which was recognized early as playing an important role in biological systems, has been of particular interest. Overend and Stacey (43) have given a critical review of the methods available until 1952 for the estimation of 2-deoxy pentoses. A recent summary of specific methods for the identification and quantitative estimation of the different classes of deoxy sugars has been prepared by Dische (13). 

Among the other sugars detected in the soil, only the 6-deoxyhexoses have been quantitatively determined. In Delaware soils, rhamnose and fucose, determined by quantitative paper-chromatography, amounted to 20% of the sugars. Under the conditions of furfural formation from pentoses, the 6-deoxyhexoses yield 5-methyl-2-furaldehyde this has been determined by the differential solubilities of the phloroglucides in alcohol. The proportion of 6-deoxyhexoses in some cases exceeded that of pentoses. 

The cystine-H2S04 reaction is a very good colorimetric method for determining pentoses quantitatively (D3). Roe s (R5) method is probably the best method for quantitative determination of aldopentoses. This method uses 2% p-bromoaniline in acetic acid saturated with... 

Glyceraldehyde (2,3-dihydroxypropanal), acetol, and dihydroxyace-tone form 1-5% of biacetyl and a number of other products, including pyrocatechol and 33, after exposure to aqueous alkali at 300°. Such trioses as glyceraldehyde and dihydroxyacetone have been shown to form various hexoses by aldol reaction. Aldolization, followed by retro-aldoliza-tion, is undoubtedly a major consideration when three-, four-, and five-carbon sugars are subjected to elevated temperatures. Differences in thermolysis products, partially quantitative, are noticeable at 100°, but, at temperatures near 300°, it is quite difficult, if not impossible, to determine if the starting material was a triose, a tetrose, or a pentose. 

Isbell and co-workers (51) have tried to minimize the oxygen reaction and to maximize the peroxide reaction. When a large excess of peroxide and a low temperature were maintained, they found that the monosaccharides are converted almost quantitatively to formic and two-carbon acids. Table II presents results for the peroxide oxidation of 14 sugars. The total acid produced from aldo-hexoses under favorable conditions is about six moles, consisting almost entirely of formic acid. Aldopentoses react more rapidly than aldo-hexoses and yield about five moles of formic acid per mole of pentose. Fructose and sorbose yield approximately five moles of total acid of which four moles are formic acid. Glycolic acid was identified qualitatively but not determined quantitatively. L-Rham-nose and L-fucose yield about five moles of acid, including four moles of formic acid. Acetic acid was identified only qualitatively. 

The researches have dealt with each phase of the pentosan determination. Many modifications have been introduced in the distillation procedure, in attempts to obtain ma.ximal yields of furfural. For e.xample, some workers prefer to distil with 23 % hydrobromic acid, rather than with the conventional 12% hydrochloric acid. Others distil in the presence of added sodium chloride, to avoid changes in acid concentration. Steam distillation has been used by a number of workers, who claim theoretical yields of furfural from pentoses, but Launer and Wilson found no advantage either in salts or in steam in the analysis of pulps and papers. Interfering substances are of two types materials other than pentosans which form furfural in the pentosan analysis, and substances which yield products which may be determined as furfural. TJronic acids and polyuronides yield furfural, although not quantitatively, and, in the case of materials containing appreciable quantities of these substances, it is usual to make a correction. The value of the correction to be applied has been determined experimentally by several workers, with somewhat differing results. 

Pentose sugars and pentosans may be quantitatively estimated by conversion into furfural by distillation with hydrochloric acid. The amount of furfural is determined gravimetrically after precipitation with phloroglu-cinol, barbituric acid, or thiobarbituric acid, or volumetrically by titration with bromine or phenylhydrazine. Approximately theoretical yields of furfural are obtained if the furfural is removed rapidly from the reaction mixture by steam distillation ... 

Monosaccharides can be identified by Curie-point pyrolysis (358°) and g.c. analysis (packed column) of the volatile products since fragments characteristic of the sugar class (e hexose, pentose, deoxy-sugar, uronic acid, alditol and amino-sugar) were obtained. Sugars and polyols in biological fluids have been determined quantitatively by g.c. of their pertrimethylsilyl ether derivatives. ... 

General color reactions of this type include the Molisch test with a-naphtfiol and concentrated sulfuric acid and the anthrone reaction with anthrone and sulfuric acid. The latter is used for quantitive colorimetric determinations. Reaction conditions can be chosen so that only ketoses react (HCl - - resorcinol, Seliwanoff s test) or only pentoses (HCl -b orcinol. Rial s test). These reactions help to distinguish the various sugars. 

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