MONOSACCHARIDES SUGAR: Simple sugars include monosaccharides and disaccharides. The chemical names of the six sugars to be discussed, end in "- ose" which means sugar. DISACCHARIDES POLYSACCHARIDES
MONOSACCHARIDES Are linked by glycosidic bonds (joining of 2 hydroxyl groups of sugars by splitting out water molecule) to create larger structures. The monomers of carbohydrates are called monosaccharides and are also called simple sugars. Monosaccharides include these simple sugars : Glucose, Fructose, Galactose The prefix mono means "one" these sugars consist of a single ring of atoms. Monosaccharides are the building blocks of other carbohydrates, such as disaccharides and starch. Glucose is the most abundant sugar in nature. Fructose is the sweetest natural sugar. it is about 1.3 times as sweet as white sugar. Galactose is almost always linked to glucose to make milk sugar which is a disaccharides.
DISACCHARIDES Contains two or more monosaccharides units linked together. They are also called double sugars which includes : Sucrose, Maltose, Lactose Sucrose (GFS) is commonly two simple sugar joined together (Glucose and fructose). Sucrose is the chemical name which is called table sugar, white sugar, granulated sugar or simply sugar. Maltose (G²M) is simply two glucose units connected together. It is fairly abundant in germination of seeds and is produced in the manufacturing of beer. Lactose (GLUGALALA) is called milk sugar which is present in milk and milk products. As per the sweetness is considered lactose is ranked as the lowest sugar. Most carbohydrates are considered as plant products, lactose is one of the few considered exclusively from the animal products.
POLYSACCHARIDES Are made up of many monosaccharides bonded together by glycosidic bonds. The type and the arrangements of sugar molecules determine the shape and form of the polysaccharides. Some polysaccharides have an orderly linear appearance, whereas others are shaped like branches. The three most common polysaccharides are starch, glycogen, fiber.
Furfural formation Enolization Oxidation Reduction Osazone formation
A reducing sugar is a carbohydrate that is oxidized by a weak oxidizing agent (an oxidizing agent capable of oxidizing aldehydes but not alcohols such as the Tollen s test) in basic aqueous solution. The characteristic property of reducing sugars is that, in aqueous medium, they generate one or more compounds containing an aldehyde group. eg. 1: α-d-glucose, which contains a haematical group and, therefore, reacts with water to give an openchain form containing an aldehyde group.
FURFURAL FORMATION When treated with concentrated sulfuric acid glucose undergoes dehydration and results in the formation of hydroxy methyl furfural.
Monosaccharides are normally stable to dilute acids, but are dehydrated by strong acids. D-ribose (Pentoses) when heated with concentrated HCl yields furfural (cyclic anhydride). D-glucose(Hexoses) under the same conditions yields 5-hydroxymethyl furfural The furfural derivatives can condense with phenolic compounds to give colored products. This forms the basis for Molisch test. This test is a sensitive test but it is nonspecifically given by all carbohydrates. Alpha nephthol is used in this test. A purple colored ring develops if carbohydrate is present. Similar to this Seliwanoff Test is undertaken with Resorcinol, a cherry red color is produced if fructose is present. The other tests are Anthrone test and Bial s test.
Sugars are weak acids and can form salts at high ph. A1,2-enediol salt is formed as the result. This allows the interconversion of D-mannose, D-fructose and D-glucose. The reaction is known as the Lobry de Bruyn-Alberta von Eckenstein reaction. Enediols obtained by the action of base are quite susceptible to oxidation when heated in the presence of an oxidizing agent. Copper sulfate is frequently used as the oxidizing agent and a red precipitate of Cu2O is obtained. Sugars which give this reaction are known as reducing sugars. Some of the frequently used solutions for detecting the presence of reducing sugars in biological fluids are as follows- 1) Fehling s solution: KOH or NaOH andcuso4 2) Benedict s solution: Na2CO3 and CuSO4 3) Clinitest tablets are used to detect urinary glucose in diabetics
oxidation is the loss of electrons. It happens when an atom or compound loses one or more electrons. Some elements lose electrons more easily than others. These elements are said to be easily oxidized. When an atom or compound is oxidized, its properties change. For example, when an iron object undergoes oxidation, it is transformed because it has lost electrons.
REDUCTION (sugar alcohol formation) Sugar alcohol are produced by the reduction of of the carbonyl group (Aldehye/Ketone group) of monosaccharides. The reaction can be represented as follows : a) Reduction of aldoses take place at C-1 to form sugar alcohol. b) Reduction of Ketone sugars takes place at C-2 to form sugar alcohol.
Under specific conditions of temperature and pressure, sugars can be reduced in the presence of hydrogen. The resultant product a polyol or sugar alcohol (alditol) but reduction of ketose sugar produces a new asymmetric carbon atom (See figure), thus two types of sugar alcohols can be produced, which are epimer of each other. Examples 1) Glucose form Sorbitol (glucitol) 2) Mannose forms mannitol 3) Fructose forms a mixture of mannitol and sorbitol 4) Galactose forms Dulcitol 5) Glyceraldehyde gives glycerol 6) Ribose forms Ribitol Structure and Significance of some Sugar alcohols- 1) Sorbitol- In Diabetes Mellitus excess of Glucose is converted to Sorbitol. The osmotic effect of Sorbitol is responsible for many of the complications of diabetes mellitus e.g. Cataract formation in lens. Clinically sorbitol is dehydrated and nitrated to form Isosorbide mono and dinitrate, both of which are used for the treatment in Angina. 2) Mannitol- Mannitol is also osmotically active and is used as an infusion to lower the intracranial tension by producing forced diuresis. 3) Dulcitol- excess of galactose in galactosemia is converted to Dulcitol. The osmotic effect of Dulcitol is similar to Sorbitol and is responsible for premature cataract formation in affected patients of galactosemia.
4) Xylitol- is produced in Uronic acid pathway of Glucose utilization; it is subsequently oxidized to produce D- Xylulose. 5) Glycerol- is produced from Glyceraldehyde. Glycerol is used for the formation of Triglycerides and phospholipids. Clinically glycerol is nitrated to form Nitroglycerine, that is used for the treatment of angina. 6) Myo- Inositol- It is hexahydroxy alcohol, also considered a vitamin It is present in the plasma membrane and acts as a second messenger for the action of hormones. 7) Ribitol- is used in the formation of vitamin B2- (Riboflavin
Osazones are a class of carbohydrate formed when sugars are reacted with excess of phenylhydrazine.the reaction to identify sugars whose stereochemistrydiffered by only one chiral carbon. The reaction involves formation of a pair of phenylhydrazone functionalities, concomitant with the oxidation of the hydroxymethylene group adjacent to the formyl center. The reaction can be used to identify monosaccharides. It involves two reactions. Firstly glucose with phenylhydrazine gives glucosephenylhydrazone by elimination of a water molecule from the functional group. The next step involves reaction of one equivalent of glucosephenylhydrazone with two equivalents of phenylhydrazine (excess). First phenylhydrazine is involved in oxidizing the alpha carbon to a carbonyl group, and the second phenylhydrazine involves in removal of one water molecule with the formyl group of that oxidized carbon and forming the similar carbon nitrogen bond. The alpha carbon is attacked here because its more reactive than the others. Osazones are highly coloured and crystalline compounds and can be easily detected. Glucose gives broomstick or needle shaped crystals with this whereas maltose gives sunflower shaped crystals.
Acetal or ketal derivatives formed when a monosaccharide reacts with an alcohol are called glycosides. They are formed by the reaction of the hydroxyl group of anomeric carbon (hemiacetal or hemiketal)of monosaccharide with hydroxy group of second molecule with the loss of an equivalent of water. The second molecule may be- 1) Another sugar (Glycon)- e.g. formation of disaccharides and polysaccharides. 2) Non Carbohydrate (Aglycon)- such as Methanol, Glycerol, Sterol or Steroids etc. In naming of glycosides, the ose suffix of the sugar name is replaced by oside, and the alcohol group name is placed first. For example, D-glucose reacts with methanol in an acid-catalyzed process: the anomeric carbon atom reacts with the hydroxyl group of methanol to form two products, methyl α -D-glucopyranoside and methyl β -D-glucopyranoside. These two gluco pyranosides differ in the configuration at the anomeric carbon atom. The new bond formed between the anomeric carbon atom of glucose and the hydroxyl oxygen atom of methanol is called a glycosidic bond specifically, an O-glycosidic bond. See figure below.
Showing Methyl Glucopyranoside The anomeric carbon atom of a sugar can be linked to the nitrogen atom of an amine to form an N-glycosidic bond. Nucleosides are adducts between sugars such as ribose and amines such as adenine (the linkage between them is N-Glycosidic linkage). Examples of Glycosides-Glycosides are present in many drugs, spices and in the constituents of animal tissues. Glycosides comprise several important classes of compounds such as hormones, sweeteners, alkaloids, flavonoids, antibiotics, etc. The glycosidic residue can be crucial for their activity or can only improve pharmacokinetic parameters. 1) Cardiac Glycosides Cardiac glycosides all contain steroids or genin component as aglycone in combination with sugar molecules. These include derivatives of digitalis and strophanthus such as oubain. 2) Other glycosides such as streptomycin are used as antibiotics. Phloridzin is another glycoside which is obtained from the root and bark of apple tree. It blocks the transport of sugar across the mucosal cells of small intestine and also renal tubular epithelium. It displaces Na+ from the binding site of carrier protein and prevents the binding of sugar molecule and produces Glycosuria. 3) Glycosides of vitamins, both hydrophilic and lipophilic often occur in nature. Glycosylated vitamins have an advantage over the respective aglycone in their better solubility in water (especially the lipophilic ones), stability against UV-light, heat and oxidation, reduction of the bitter taste and odor(e.g., thiamine), and resistance to an enzymatic action. Some of the vitamin glyco conjugates have altered or improved Pharmacokinetic properties.