Monosaccharides and Disaccharides:- Monosaccharides Are Reducing Agents

Monosaccharides can be oxidized by relatively mild oxidizing agents such as ferric (Fe³⁺) or cupric (Cu²⁺) ion (Fig. 7–10a). The carbonyl carbon is oxidized to a carboxyl group. Glucose and other sugars capable of reducing ferric or cupric ion are called reducing sugars. This property is the basis of Fehling’s reaction, a qualitative test for the presence of reducing sugar. By measuring the amount of oxidizing agent reduced by a solution of a sugar, it is also possible to estimate the concentration of that sugar. For many years this test was used to detect and measure elevated glucose levels in blood and urine in the diagnosis of diabetes mellitus. Now, more sensitive methods for measuring blood glucose employ an enzyme, glucose oxidase (Fig. 7–10b).

FIGURE 7–10 Sugars as reducing agents. (a)Oxidation of the anomeric carbon of glucose and other sugars is the basis for Fehling’s reaction. The cuprous ion (Cu⁺) produced under alkaline conditions forms a red cuprous oxide precipitate. In the hemi acetal (ring) form, C-1 of glucose cannot be oxidized by Cu²⁺. However, the open-chain form is in equilibrium with the ring form, and eventually the oxidation reaction goes to completion. The reaction with Cu²⁺ is not as simple as the equation here implies; in addition to D-gluconate, a number of shorter-chain acids are produced by the fragmentation of glucose. (b)Blood glucose concentration is commonly determined by measuring the amount of H₂O₂ produced in the reaction catalyzed by glucose oxidase. In the reaction mixture, a second enzyme, peroxidase, catalyzes reaction of the H₂O₂ with a colorless compound to produce a colored compound, the amount of which is then measured spectrophotometrically.

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Monosaccharides and Disaccharides:- The Common Monosaccharides Have Cyclic Structures

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