Monosaccharides aldopentoses

As glucose acceptor, however, such different substances as inorganic phosphate, keto-monosaccharides and an aldopentose can be used in... 

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. 

By means of the cyanohydrin reaction, higher sugars of the heptose. octosc, and nonosc types have been prepared. A monosaccharide such as an aldohexosc may be converted into the next lower monosaccharide, such as an aldopeniosc. by oxidation to the acid, which corresponds to the aldohexose. then treating the calcium salt solution of this acid with a solution or ferrous acetate plus hydrogen peroxide. Carbon dioxide is evolved and aldopentose formed. 

The simplest carbohydrates are the monosaccharides which under specified conditions are structurally characterised as polyhydroxy aldehydes or polyhydroxy ketones these are termed aldoses and ketoses respectively. Aldoses and ketoses are sub-classified, according to the number of carbons present in each molecule, into aldotetroses, aldopentoses, aldohexoses, etc., or ketotetroses, ketopentoses, etc. 

Aldopentose and aldohexose may exist in furanose and pyranose forms. The former favors the furanose form while the latter prefers the pyranose form. Would the energetic differences between the two forms as exemplified by the following two pairs of monosaccharides be sufficient to rationalize the preferences. 

Glucose is an aldohexose, where aldo- indicates that it is an aldehyde, -hex- designates the number of carbons, and -ose is the suffix used for carbohydrates. Some other common monosaccharides are fructose, a 2-ketohexose that is isomeric with glucose, and ribose, an aldopentose that contains one fewer carbons than glucose. 

Phenyl-methyl-pyrazolone-labeled aldopentose and monosaccharides Nucleotides AMP. ADP, and ATP... 

Certain monosaccharides— notably aldopentoses and ketohexoses—form furanose rings, not pyranose lii, in solution. The same principles apply to drawing these structures as for drawing pyranose rings, except the ring size is one atom smaller. 

In contrast, the separation of the various monosaccharides is much easier, which is surprising because the affinities of these compounds are not very different. However, when the pH value of the mobile phase is lowered to a value corresponding to the pK value of the respective compounds, even small differences in the dissociation behavior contribute to the separation. Thus, the chromatogram in Fig. 3-108 is obtained using a dilute sodium hydroxide solution of c = 0,001 mol/L. Under these conditions, even epimeric compounds such as glucose, mannose, and galactose are baseline resolved. Two important aldopentoses - arabinose and xylose - can also be determined in the same analysis. Remarkable is the elution order, which cannot be correlated with the carbon chain length of the monosaccharides. 

Oxidation of aldoses by chlorites also results in the formation of the corresponding aldonic acids. The aldopentoses react faster than aldo-hexoses, and monosaccharides are oxidized more rapidly than are the disaccharides. The ketoses remain unaffected unless drastic conditions are employed. The principal reaction has been shown to be as follows. 

The forms of glucose in aqueous solution are quite complex and small proportions exist as furanose structures. These arise from the reaction between the OH group at C-4 and the aldehyde group in aldoses. The five-membered ring system is quite stable but pyranose forms are favoured over furanose forms by most monosaccharides notable exceptions are the ketohexose fructose and the aldopentose ribose. Too much strain would be involved in reducing bond angles to produce smaller rings, so trioses and tetroses do not form intermolecular cyclic systems. 

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