Reducing and nonreducing disaccharides

Reducing and nonreducing disaccharides exhibit different retention behaviors, but can be analyzed in a single run with a gradient elution technique. To reduce the analysis time, small amounts of acetic acid are added to the sodium hydroxide eluent. Figure 3.235 shows a separation of different disaccharides on a CarboPac PAl anion exchanger. 

There are many publications and comprehensive handbooks on the thin-layer chromatography (TLC) of carbohydrates (e.g., Refs. 1 and 2). The reason is their great importance in life science and the great diversity of cases monosaccharide, disaccharide, trisaccharide, oligosaccharide, polysaccharide, aldose, ke-tose, triose, tetrose, pentose, hexose, as well as reducing and nonreducing sugars. In addition, when extracted from natural products or produced by fermentation, carbohydrates are accompanied by many impurities. That is why separation methods are used predominantly for their analysis. 

The disaccharide trehalose, shown next, is the major reason why some plants and animals can withstand prolonged periods of dessication (severe dehydration). It is widely used in cosmetics and commercial foods, because of its high water retention properties. Is it a reducing or nonreducing disaccharide ... 

A disaccharide is formed when two monosaccharides become joined by a glycosidic bond. The bond may be an a- or (3-bond depending on the configuration of the anomeric carbon atom involved in the bond. Usually the anomeric carbon atom of only one of the two monosaccharides is involved in the bond so that the disaccharide still has one free aldehyde or ketone group and is still reducing. However, in sucrose both anomeric carbon atoms are bonded together so that sucrose is a nonreducing disaccharide. 

The nature of the sugars in a nonenzymic browning reaction determines their reactivity. Reactivity is related to their conformational stability or to the amount of open-chain structure present in solution. Pentoses are more reactive than hexoses, and hexoses more than reducing disaccharides. Nonreducing disaccharides only react after hydrolysis has taken place. The order of reactivity of some of the aldohexoses is mannose is more reactive than galactose, which is more reactive than glucose. 

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