Reducing sugar

The reducing sugar is known in biochemistry mono-, di -or oligosaccharides, the molecules in solution have a free aldehyde group. In the case of simple sugars called these aldoses. Also ketoses are reducing, if they have a hydroxyl group in α - position and therefore are a acyloin ( α -hydroxy ketone ). Known reducing sugars include glucose, fructose, galactose, maltose and lactose. The quantitative determination of reducing sugars is achieved by means of the method of Luff -Schoorl.

Monosaccharides

Almost all monosaccharide aldoses are in solution predominantly as cyclic hemiacetal form before, which has no free aldehyde group. Since the annular formation is an equilibrium reaction, a small percentage of the open chain aldehyde form is found which can be oxidized. The open-chain form is consumed by the oxidation and steadily replaced by the cyclic form. In order for these monosaccharides such as glucose, mannose or galactose respond positively to the Fehling's test and there is a carboxylic acid (such as gluconic acid from glucose ). The same applies mutatis mutandis to α -hydroxy ketones such as fructose: only the open-chain form is oxidized and is constantly replenished from the cyclic form. But here arises instead of the carboxylic acid is an α -diketone.

Di -and oligosaccharides

The disaccharides this case occurs when the combination of the two monosaccharides on the anomeric carbon atom ( C-1) is done the first block with a non- alcoholic anomeric group about the C -4 atom of the second monosaccharide building block. Is formed at the non- reducing end of an acetal, during the anomeric center of the second block remains completely free and it serves as a reducing end. Since the cyclic and open-chain form of the present even when di-and oligosaccharides in solution is in equilibrium, the aldehyde group of the monosaccharide second block of 1,4-linked form at the Fehling or Tollensprobe can respond positively. In this case, this is oxidized to the carboxyl group.

Sucrose

The used in the household cane or beet sugar, sucrose, however, is not a reducing sugar because its molecules by the combination of the two 1,2- anomeric C- atoms have no free aldehyde in solution. Therefore, this disaccharide reacted negatively to the Fehling's and Tollensprobe.

The ever formulated assertion, with Fehling and Tollensprobe as one sugar to must therefore be restricted to the reducing sugars. In fact we can prove aldehyde groups.

Proof

In addition to the Fehling's test can also reducing sugars using the Benedict 's reagent ( by the color of the failing product), with the Nylanders reagent, 3,5-dinitrosalicylic acid ( by spectroscopic tests) or the discoloration of a potassium permanganate solution are proved.