Chemical Properties of Carbohydrates with special reference to Glucose
1. Formation of Glycosides
What are glycosides? Glycosides are molecules in which a sugar (carbohydrate) is bound to a non-sugar moiety (non-carbohydrate). In glycoside molecule, the sugar part is known as glycone and the non-sugar part is known as aglycone par. In the formation of glycosides, hydroxyl group (OH) of anomeric carbon of sugar part reacts with hydroxyl group of non sugars part through glycosidic linkage e.g. Amygdalin. This linkage is formed at anomeric carbon that can be of α or β configuration so the glycosides may α or β glycosides.
The aglycone may be attached through –OH or –NH2 group forming O– or N– glycosides respectively. O-glycosides are more common in nature. Oligosaccharides and polysaccharides contain O-glycosidic bonds. N-glycosidic bonds occur in nucleotides and in glycoproteins.
2. Formation of Osazone
What is Osazone? Osazone is yellowish, crystalline compound, produced as a result of heating sugars solutions with phenylhydrazine. Osazones are formed by those sugars which contain a free aldehyde or ketone group. For example one molecule of glucose reacts with three molecules of phenyl hydrazine to form glucosazone. On the other hand sucrose doesn’t possess free aldehyde or ketone group and thus cannot form Osazone until it is first hydrolyzed to monosaccharides.
 Glucosazone (x 250) |  Maltosazone (x 250) |
 Galactosazone (x 160) |  Lactosazone (x 250) |
Figure showing different types of Osazone crystals
3. Formation of sugar alcohols
The aldehyde or ketone group of both aldoses and ketoses can be reduced to form the corresponding polyhydroxy alcohols. Glucose reduces to form sorbitol and fructose reduces to form sorbitol and mannitol.
| Monosaccharides | Corresponding Alcohol | |
| Glucose | Sorbitol | |
| Mannose | Mannitol | |
| Galactose | Dulcitol | |
| Fructose | Sorbitol & Mannitol | |
| Ribose | Ribitol | |
| Glyceraldehyde | Glycerol | |
| Dihydroxyacetone | Glycerol |
Mannitol is frequently use in the patients of cerebral edema because it act as osmotic diuretic and decrease the water content of the body and thus decrease the brain swelling. Sorbitol is getting deposited in the lens of the eye especially in the patients of diabetes mellitus and contributes to the early cataract formation.
4. Formation of sugar acid
Carbohydrates form the sugar acid on their oxidation. When glucose (aldoses) is oxidized under proper conditions, then it yields three types of sugar acids; namely gluconic acid, glucuronic acid, and glucaric acid.
i. Gluconic acid is formed under mild conditions and due to oxidation at C-1. Gluconic acid is used in the formation of salts of different drugs e.g. antimalarial drugs.
Glucuronic acid is formed by oxidation at C-6. Glucuronic acid is formed in the body. It is of great physiological importance because it is use in the body as detoxifying agent and inactivates the many substances like camphor, benzoic acid, steroid hormones and bilirubin etc.
iii. Glucaric acid is formed via oxidation of glucose at C-1 & C-6.
5. Reducing Properties of sugar in alkaline solutions.
Almost, all carbohydrates containing a free aldehyde or ketone group except sucrose. That is oxidized especially in alkaline pH. This means that they are good reducing agents in an alkaline medium. They readily reduce oxidizing ions such as Ag+, Hg2+, Bi3+, Cu2+, and ferricyanide3+. \
This reaction is the basis for the Benedict’s test and Fehling’s test.
6. Action of acids on carbohydrates
Monosaccharides (such as glucose) are resistant to the action of hot diluted mineral acids. Strong acids dehydrate all carbohydrates leading to the formation of furfural (with pentoses) or 5-hydroxymethyl furfural (with hexoses). These products condense with phenols to yield characteristic colored products.
This reaction is the basis of the color test, known as Molisch’s test for sugars.
7. Action of bases on carbohydrates
Dilute basic solutions at low temperature can bring about re-arrangement of groups at the anomeric carbon atoms and its adjacent carbon atom. For example glucose can be changed to fructose and mannose. Higher concentration of bases can cause the further changes i.e. more carbon atoms show the rearrangement of the groups. Fragmentation and polymerization can also result.
8. Esters formation
Hydroxyl group of sugar can be esterifies with phosphates, acetates, propionates and stearates etc. Sugar phosphates are of great biological significance. Nucleoproteins of cells also contain the sugar phosphate in combination with various nitrogen bases.
9. Amino sugars formation
A hydroxyl group of the monosaccharides can be replaced by an amino group (– NH2) forming an amino sugar. For example D-glucosamine, D-galactosamine, D-fructosamine. In all these – NH2 group is attached at C-2. These are present in nature. As they are derived by hexoses so they are also called hexosamine.
Glucosamine is constituent of hyaluronic acid. Galactosamine is present in chondroitin. Mannosamine is an important constituent of mucoproteins. Aminosugars also occur in many antibiotics e.g. erythromycin. In most cases amino sugar is N-acetylated.
10. Fermentation
Fermentation is the process of converting a larger complex molecule into simple molecules by means of enzymes. Some of the hexoses sugars are converted to ethanol and CO2 by a group of enzymes called as zymases.
C6H12O6 → 2(C2H5OH) + 2CO2
Glucose Ethanol Carbondioxide
Glucose can be changed to fructose and mannose. Glucose, fructose and mannose can be readily fermented by common baker yeast. Galactose is fermented to negligible amount.
Importance of Carbohydrates
1. Carbohydrates are widely distributed in plants and animals and have important structural and metabolic roles.
2. In plants, glucose is synthesized from carbon dioxide and water by photosynthesis and stored as starch or used to synthesize cellulose.
3. Animals can synthesize carbohydrate from lipid, glycerol and amino acids, but in most animal carbohydrate is derived from plants sources.
4. Glucose is the most important carbohydrate.
5. Most dietary carbohydrate is absorbed into the bloodstream as glucose, and other sugars are converted into glucose in the liver.
6. Glucose is the major metabolic fuel of mammals (except ruminants) and a universal fuel of the fetus.
7. It is the precursor for synthesis of all the other carbohydrates in the body, including glycogen for storage; ribose and deoxyribose in nucleic acids; and galactose in lactose of milk. Glucose is also present in glycolipids and in glycoproteins.
8. Various diseases associated with carbohydrate metabolism include diabetes mellitus, galactosemia, glycogen storage diseases, and lactose intolerance.
9. Oligosaccharides are present in combination with proteins at all cell membranes on the extracellular face.
10. These are also present in secreted proteins such as antibodies and blood clotting factors.
11. Complexes of carbohydrates with proteins (glycoproteins) have been shown to act as receptors on cell membranes which are thus involved in molecular recognition.
12. Carbohydrate derivatives such as heparin sulphate (a glycoprotein) are involved in the adhesion of one neuron to the other during the development of nervous system. For example the aggregation of the retinal neurons.
13. The glucosaminoglycans are the integral constituents of the gel like extracellular matrix.
Ribose is an integral part of high energy phosphate compounds i.e., ATP, GTP (guanosine triphosphate), UTP (uridine triphosphate) and CTP (cytidine triphosphate) and secondary messengers such as cAMP (cyclic adenosine monophosphate) and cGMP ((cyclic guanosine monophosphate).
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