4) Chemical properties of monosaccharide.
а) oxidation of monosaccharide (gluconic, glucaric, glucoronic acids)
b) reduction of monosaccharide
c) formation of О- and N-glycosides
a) oxidation of monosaccharide (gluconic, glucaric, glucoronic acids)
Monosaccharides are reducing sugars if their carbonyl groups oxidize to give carboxylic acids.
In the Benedict’s text, D-glucose is oxidized to D-gluconic acid. Glucose is a reducing sugar.
R
eaction
of reduction.
When a cyclic monosaccharide reacts with an alcohol:
A glycoside is produced.
The bond is glycosidic bond
5) Qualitative reactions on monosaccharide
Molisch’s Test:
This is a common test for all carbohydrates larger than tetroses. The test is on the basis that pentoses and hexoses are dehydrated by conc. Sulphuric acid to form furfural or hydroxymethylfurfural, respectively. These products condense with α-naphthol to form purple condensation product.
Furfural α-Naphthol
Benedict’s Test:
As in Fehling’s test, free aldehyde or keto group in the reducing sugars reduce cupric hydroxide in alkaline medium to red colored cuprous oxide. Depending on the concentration of sugars, yellow to green color is developed . All monosaccharides are reducing sugars as they all have a free reactive carbonyl group. Some disaccharides like maltose have exposed carbonyl groups and are also reducing sugars but less reactive than monosaccharides.
Seliwanoff’sTest :
Seliwanoff’s Test – differentiates between ketohexoses and aldohexoses. It also uses concentrated for the dehydrating agent.But the condensation agent is resorcinol. It is a color reaction specific for ketoses. When conce: HCl is added. ketoses undergo dehydration to yield furfural derivatives more rapidly than aldoses. These derivatives form complexes with resorcinol to yield deep red color. The test reagent causes the dehydration of ketohexoses to form 5-hydroxymethylfurfural. 5-hydroxymethylfurfural reacts with resorcinol present in the test reagent to produce a red product within two minutes (reaction not shown). Aldohexoses reacts so more slowly to form the same product.
Bial’sTest :
Bial’s test is used to distinguish between pentoses and hexoses. They react with bial’s reagent and is converted to furfural. Orcinol and furfural condense in the presence of ferric ion to form a colored product. Appearance of green colour or precipitate indicates the presence of pentoses and formation of muddy brown precipitate shows the presence of hexoses.
Tollen’s Test:
Tollen’s Test – also involves a mild oxidizing agent. In the Tollen’s Test, silver (i) ion reduced to metallic silver. The formation of a reducing sugar. The Tollen’s reagent must be freshly prepared to produce the proper results.
6) Derivativesof monosaccharide: deoxysacarose, aminosacarose, neiramine acid, ascorbic acid.
Monosaccharides such as fructose and galactose are widely used individually throughout the body, despite their chemical simplicity, unlike amino acids, fellow biological building blocks. However, like amino acids, monosaccharides have many useful derivatives. There are six ways in which derivatives of monosaccharides can be formed:
Sugar phosphates: Phosphate esters of sugars, found significantly in metabolic pathways. Produced through hydrolysis of an acid and an alcohol, then attatchedbyhydrolysis again to the monosaccharide. Naming conventions attach the number of the carbon serving as the linkage and the number of phosphates. For example, with a glucopyranose monosaccharide in ring form, one can have α-D-Glucopyranose-6-phosphate, α-D-Glucopyranose-1,6-bisphosphate, α-D-Glucopyranose-1,5,6-trisphosphate, etc.
Deoxy sugars: One of the OH groups of the monosaccharide is replaced with hydrogen. An example of this is, coincidentally, DNA.
Amino sugars: One of the OH groups is again replaced, except this time with an amino group, NH2. An example of one of these is α-D-glucosamine, used in cartilage.
Sugar alcohols: Created through the reduction of the carbonyl oxygen to a hydroxyl group. A polyhydroxy alcohol is produced. The naming convention is to distich the standard carbohydrate ‘ose’ ending and replaces it with 'itol'. So, for example, ribose goes to ribitol.
Sugar acids: In these derivatives, either the aldehyde carbonyl is oxidized to COOH, or the highest number carbon is oxidized to COOH.
Ascorbic acid This is a unique one. Ascorbic acid is really just another name for vitamin C. The molecule is a derivative of D-glucoronate.