Reducing sugar
In biochemistry , reducing sugars are mono- , di- or oligosaccharides whose molecules have a free aldehyde group in solution . In the case of simple sugars, these are called aldoses . Also ketoses can have a reducing effect when used in α-position a hydroxyl group have and thus an acyloin ( α-hydroxy ketone ) is. Well-known reducing sugars are glucose , fructose , galactose , maltose and lactose . The quantitative determination of reducing sugars is possible using the Luff-Schoorl method .
Monosaccharides
Almost all monosaccharide aldoses are predominantly in solution as a cyclic hemiacetal form that has no free aldehyde group. Since ring formation is an equilibrium reaction , there is also a small percentage of the open-chain aldehyde form that can be oxidized. The open-chain form is used up by the oxidation and is constantly being replenished from the cyclic form. This means that these monosaccharides such as glucose , mannose or galactose react positively to the Fehling's sample and a carboxylic acid is produced ( e.g. gluconic acid from glucose). The same applies to α-hydroxy ketones such as fructose : only the open-chain form can be oxidized and is constantly being supplied from the cyclic form. Here, however, an α- diketone is formed instead of the carboxylic acid .
Di- and oligosaccharides
In the case of disaccharides, this occurs when the two monosaccharides are linked via the anomeric carbon atom (C-1) of the first building block with a non-anomeric alcoholic group, for example on the C-4 atom of the second monosaccharide building block. An acetal is formed at the non-reducing end , while the anomeric center of the second building block remains free and thus serves as the reducing end . Since the cyclic and the open-chain form are also in equilibrium in solution with di- and oligosaccharides, the aldehyde group of the second monosaccharide building block of 1,4-linked forms can react positively in the Fehling or Tollens test. This is oxidized to the carboxy group .
Sucrose
The cane or beet sugar used in the household, sucrose , is not a reducing sugar, unlike the reducing disaccharide maltose, because its molecules in solution do not have a free aldehyde function due to the 1,2-bond between the two anomeric carbon atoms. Therefore this disaccharide reacts negatively in the Fehling and Tollens test.
The repeatedly formulated claim that the Fehling and Tollens test can be used to detect sugar must therefore be restricted to reducing sugars. This is because aldehyde groups are detected.
proof
In addition to the Fehling's sample, reducing sugars can also be detected with the help of the Benedict reagent (through the color of the precipitated product), with the Nylanders reagent , 3,5-dinitrosalicylic acid (through spectroscopic tests) or the decolouration of a potassium permanganate solution.
Individual evidence
- ↑ Organic Chemistry - Biomolecules: Carbohydrates ( Memento of the original from September 11, 2012 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. , University of Zurich
