Reactions of Monosaccharides

Reactions of Monosaccharides 1. Mutarotation :When an aldohexose is first dissolved in water and the polarised light is passed passed through the solution solution the initial optical rotation shown by the sugar gradually gradually changes until a constant fixed rotation rotation characteristic characteristic of the sugar sugar is reached. This phenomenon phenomenon of change of rotation ic called as mutarotation. When D- Glucose is crystallised from water or dilute alcohol at room room temperature , α-D-Glucose separates & when its fresh solution is made , its specific rotation of  polarised light will be +112degree initially ,but after 12-18 hrs it changes to a fixed rotation of of +52.5 degree. degree. When D-Glucose is crystallised from water at 98 degree Celsius β-D-Glucose separates & when its solution is made is made, its initial rotation will be +19 degree which within few hours changes to a fixed rotation of +52.5 degree. This change in the optical rotation rotation is due due conversion conversion of  α-form into a equilibrium mixture of  α & β – β – form. This

means that glucose glucose existing in in isomeric form in solution changes into the same equilibrium mixture regardless of  which form is dissolved. ie,in glucose solution at equilibrium rd

rd

2/3 of the sugar exist as the β-form & 1/3 as α-form.

2. Reaction with acids :Polysaccharides Polysacch arides and the compound carbohydrates when boiled with dilute mineral acid such as HCL & H 2SO4 gets hydrolysed into their constituent monosaccharides. Monosaccharides Monosaccharides when treated treated with conc. H2SO4 undergoes dehydration dehydration with removal of 3 H2O molecules & form furfural furfural derivatives. derivatives. ie, pentoses pentoses with 12%HCL with release of 3 H2O molecules form furfural and the hexose with conc. H2SO4 or HCL with removal of 3 H2O molecules form hydroxymethyl furfural. The furfural derivatives with certain phenolic compounds condense to form coloured products (compounds with characteristic colours). Thus it forms the basis for certain test used for detection of sugars. Example :1. Molisch’s test : with α -naphthol in alchoholic solution sugars gives purple or red violet ring. This is a sensitive test not a specific test. All sugars gives the + test. So it is used as a general test for carbohydrates. carbohydrates. 2. Seliwanoff’s test : with resorcinol resorcinol gives a cherry red colour. This is characteristic of D-fructose.

3 . Reaction with alkalies :With alkalies monosaccharides reacts in various ways. a) In mild alkaline medium the sugar changes to cyclic α & β forms with an equilibrium between the two

isomeric forms. On standing rearrangement occurs & an equilibrated mixture of glucose , fructose & mannose. This is because in weak alkaline solution they are all interconvertible interconvertible . It is because all all three sugars give the same enediol which tautomerises to all three sugars. That is in mild alkaline solution the +H atom attached to the second carbon atom shifts to the first carbon atom & result in the formation of double bond b/w 1 &2 carbon atom (ene group) and to these double bonded carbon atoms two hydroxyl groups are attached (diol). This compound is called enediol. The rearrangement rearrangement of +H +H atom of enediol form give the isomers of the original original sugar . The interconvers interconversion ion of  sugars through a common enediol form is called Lobry de Bruyen-Van Ekenstein transformation . The enediols are highly reactive so they are easily oxidised by O2 & other oxidising agents to form sugar acids by breaking at the double bond . That is they readily reduce oxidising ions such as Cu++, Hg+,Bi+++,Ag+ & Fe(CN)6---. This reducing action sugar in alkaline medium is used for both qualitative qualitat ive & quantitative determination determination of sugars. sugars. Reagents Reagents

containing Cu++ ion is used. Benedict’s reaction used to detect the presence of sugar in urine is based on this action. ie, when sugar solution is boiled wit h Benedict’s reagent containing sodium carbonate, sodium sodium citrate citrate & copper sulphate, sugar gets converted in to enediols, the Cu++ ions take electrons from enediol &oxidise into sugar acid acid & in turn gets reduced to Cu+ Cu+ ions, this in turn combine with the OH- ions to form yellow cuprous hydroxide hydroxide which on heating gets converted to red cuprous oxide. The appearance of yellow to red ppt indicates reduction & the quantity of sugar present can be roughly estimated from colour & amount of ppt . Any sugar with free Aldehyde or Keto group reduce the Benedict’s reagent. As Glucose is a reducing sugar it

gives + test. b) In conc. Alkali the sugars caramelises & gives various decomposition decomposition products ,yellow &brown pigments , salts may be formed, many double bond b/w c-atoms are formed & C to C bonds may rupture. 4.OSAZONE FORMATION :When a mixture of phenylhydrazinehydrochloride & sodium acetate is added to a sugar solution and heated in a boiling waterbath waterbath for 30mt to 45mt 45mt and then cooled cooled slowly by itself, crystals are formed. They are called c alled the osazone. It is a useful means of preparing crystalline derivatives of 

sugars. All reducing sugar form osazones with excess phenylhydrazine phenylhydrazine at boiling temperature. Osazones are 

Insoluble



Have characteristic melting points



Have characteristic crystal form or structure



Have characteristic characteristic precipitation precipitation time.

Thus osazones are valuable in identification of sugars. 3 steps are involved in the osazone formation a. Aldohexose or ketose reacts with phenylhydrazine phenylhydrazine to form phenylhydrozone phenylhydrozone with release of a water molecule. b. Phenylhydrozone Phenylhydrozone then react with a molecule of  phenylhydrazine phenylhydrazine to form intermediate Keto  – derivative with release release of ammonia ammonia molecule molecule & aniline. c. This intermediate Keto – derivative then reacts with another molecule of phenylhydrazine and form osazone with release of H 2O molecule. The reaction involves the carbonyl carbon (aldehyde (aldehyde or ketone group) & the adjacent carbon. Each sugar will have characteristic characteristic crystal form of  osazones. 

Glucosazone are fine , yellow needle shaped crystals arranged like a broom, aggregates or

sheaves or crosses . It is typically described as bundle of hay. MP is 204 to 205degree Celsius. The glucose glucose , fructose & mannose mannose form the same same osazone crystals. 

Lactosazone crystals are irregular clusters of  needle and look like a powder puff.ie, puff.ie , hedgehog or pincushion with pins or flower of touch  –menot- plant.



Maltosazone are star shaped , sunflower shaped or petal shaped crystals. Osazones are used used to differentiate differentiate sugars sugars in biological fluid like urine.

5. Oxidation of sugars:When oxidised under different conditions , the Aldoses form a) Monobasic Aldonic acids or b) Dibasic saccharic acids or c) Monobasic Uronic acids containing aldehyde groups When an aldose is oxidised with the hypobromous acid (HOBr,Br2/H2O) the aldehyde group gets converted into a carboxyl carboxyl group and the aldonic aldonic acid is formed . Thus glucose is oxidised to gluconic acid, mannose to mannonic acid & galactose to galactonic acid. When aldoses is heated with conc.HNO3 under proper conditions conditions the first & last carbon atoms

simultaneously simultaneously oxidise to form dicarboxylic acids known as saccharic or aldaric acids .ie, both aldehyde & primary alcohol group is converted into carboxylic groups (-COOH) . Glucose is oxidised to glucosaccharic acid, mannose to mannaric acid & galactose to mucic acid. The mucic mucic acid forms insoluble insoluble crystals & is the basis for a test for for identification identification of of galactose. When aldehyde group of aldose is protected and the molecule is oxidised the last carbon becomes COOH group ie, the t he primary alcohol group is converted to _COOH group, without oxidation of aldehyde group and uronic acid is produced . Due to the presence of  free _CHO group group they exert reducing reducing action. action. The glucose is oxidised to glucuronic acid , mannose to mannuronic acid & galactose to galacturonic acid. The glucuronic acid is formed in in the liver by uronic acid pathway, an alternative pathway for glucose oxidation. The glucuronic acid is used by the body for conjugation with insoluble molecules to make them soluble for detoxification purpose ie,the glucuronic acid conjugates conjugates with toxic substances, substances, drugs, hormones, even bilirubin bilirubin & converts them to a soluble nontoxic substance, a glucuronide, which is excreted in urine & also for synthesis of heteropolysaccharides. heteropolysaccharides.

6. Reduction of sugars:When treated with reducing agents such as sodium amalgam , hydrogen reduce the sugars to alcohol. Aldose forms forms corresponding corresponding alcohol alcohol & ketose forms forms two alcohols because of appearance of a new asymmetric carbon atom in the process.eg:

Glucose is reduced to sorbitol

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Mannose to mannitol

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Galactose to dulcitol

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Ribose to ribitol

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Fructose to sorbitol & mannitol Sorbitol, Sorbitol, mannitol & dulcitol are used to identify

bacterial bacteria l colonies. Mannitol is also used to reduce intracranial tension tension by forced forced diuresis . The sorbitol & dulcitol when accumulate in abnormal amount produces changes in the tissues by osmotic effect. Eg:- cataract of  lens.

7. Formation of esters or acetylation:Due to the presence presence of alcoholic alcoholic group group -OH groups groups , the sugars react with anhydrides & chlorides of many organic & inorganic inorganic acids like like acetic acid , phosphoric phosphoric acids, acids, sulphuric acid & benzoic acids to form esters of  corresponding corresponding acids.ie,esterified acids.ie,esterified to form acetates, phosphates,sulphates,b phosphates,sulphates,benzoates enzoates etc. Sugar phosphates phosphates are

of great biological Importance. Metabolism of sugars inside the body starts with Phosphorylation . Glucose-6-phosphate & Glucose -1-phosphate are important intermediaries of  glucose metabolism.

8. Forms iodo –compounds:An aldose when heated with conc.HI it loses all of its oxygen and is converted into an iodo-compound. Conc. HI Glucose

→

Iodohexane (C6H13I)

As the resulting compound is a straight chain compound relating to normal hexane , it means that there is’nt any branched chains in structure of glucose.

9. Glycoside formation:Glycosides are compounds containing a carbohydrate & a non carbohydrate group. It is formed when the hemi- acetal group ( hydroxyl group of anomeric carbon ) of a monasaccharide condenses with an alcohol or phenol group . The non carbohydrate group is called aglycone. The aglycone may be methyl alcohol, glycerol, phenol, adenine, sterols, hydroquinones, anthraquinones. The glycosides are named according to the carbohydrate present in them. If contains glucose called glucoside ; if  galactose called galactoside and so on. Glycoside do not reduce Benedict’s reagent because the sugar group is

masked. But if hydrolysed hydrolysed by boiling boiling with dilute dilute acid the sugar group become free & can then reduce copper. Αlpha – glycosides are hydrolysed by maltase from yeast and β – glycosides by Emulsin from almonds. Thus α & β forms can

be distinguished by enzyme hydrolysis. Examples of glycosides of biomedical importance :Glycosides are present in many drugs, spices and in the constituents of animal tissues. They are widely distributed in the plant kingdom. a) Phlorhizin (glucose + phloretin) → obtained from rose bark & root & bark of apple tree. It blocks the transport of sugar across the mucosal cells of small intestine & also renal tubular epithelium; it displaces Na+ from the binding site of carrier protein and prevents the binding of sugar molecule and produces glycosuria. It produces renal damage in animal. b) Digitonin (4 galactose + xylose + digitogenin, a steroid) → obtained from leaves of foxglove. It is a cardiac

stimulant & used in cardiac insufficiency. c) Plant indicant (glucose + indoxyl ) → obtained from leaves of indigofera is used as stain. d) Ouabain → obtained from strophanthus species is a sodium pump inhibitor. It inhibits active transport of  Na+ in cardiac muscle.

Other monosaccharides of biological importance:1) Deoxy sugars:They represents the sugars in which the oxygen of the hydroxyl group are removed. Thus  –CHOH or –CH2OH becomes –CH2 or –CH3.

Important are 

2-deoxy –D-ribose → found in the DNA



6-deoxy-L-galactose 6-deoxy-L-galactose or L-fucose found as constituent of glycoproteins, glycoproteins, blood group antigens & bacterial polysaccharides.

The deoxy sugars will not reduce & will not form osazone. Fuelgen staining staining is specific for 2-deoxy sugars in tissues. So for the DNA. It is based on the reaction of 2deoxy sugar with Schiff reagent (Dye Fuchsine is decolorized by sulphurous acid. Basic fuchsine- a triphenylmethane triphenylmetha ne dye whose dominant component is pararosanilin ). In case of  DNA cells are hydrolysed hydrolysed with with hydrochloric hydrochloric acid to produce produce apurinic acid & the stained with Schiff reagent to produce magenta stained nuclei. 2) Pentoses :They are sugars containing 5 carbon atoms. Important are a) D-ribose : the constituent of RNA, coenzymes FAD(flavin-adenine FAD(flavin-adenine dinucleotide), dinucleotide),

NAD(nicotinamide-adenine NAD(nicotinamide-adenine dinucleotide), dinucleotide), CoA(coenzyme), CoA(coenzyme), ATP(adenosine ATP(adenosine tri phosphate). b) D -2- deoxyribose deoxyribose : constituent of DNA. c) D-ribulose : its phosphates esters is an intermediate in HMP shunt pathway. d) Arabinose is present in cherries & in gycoproteins. e) D-xylulose : its phosphates esters is an intermediate in HMP shunt pathway. L- xylulose is a metabolite of D- glucuronic acid ( uronic acid) pathway. In a hereditary disorder in metabolism it is excreted in urine called pentosuria. ( lyxose –aldopentose,epimeric with arabinose & xylose; lyxulose- keto derivative of lyxose; lyxoflavin-similar lyxoflavin-similar to riboflavin except D-lyxitol present in place of D-ribitol.) f) Xylose is seen in proteoglycans. g) D –lyxose : is constituent of lyxoflavin isolated from human heart muscle. 3) Amino sugars or hexosamines :Sugars containing amino group (NH 2) in their structure is called amino sugars. The amino sugars does not show any reducing property and wont form osazones. Two types of amino sugars are physiologically important. They are 1.

Glycosylamine

2.

Glycosamine (glycamine)

In case of glycosylamine glycosylamine the anomeric  –OH group is replaced by the the NH2 group. Eg:Eg:- Ribosylamine Ribosylamine , a derivative derivative of it is involved in the synthesis of purines. In case of glycosamine the alcoholic - OH group of  sugar molecule is replaced by the amino group. The amino group is added to the second carbon atom in case of  hexoses. Glucosamine & galactosamine are two naturally occurring glycosamine. The amino group in the sugar can be further acetylated to produce N - acetylated sugars such as N -acetyl-glucosamine -acetyl-glucosamine (GluNac) & N-acetyl-galactosamine (GalNac), etc. Glucosamine is seen in  Glucosamine

hyaluronic hyaluronic acid , heparin ,

blood group substance. Glucosamine is  Glucosamine

the chief organic constituent of 

cell wall of fungi & shells of crustacean (crabs, lobsters etc. It occurs as chitin made up of  repeated units of GluNac. So glucosamine glucosamine is called chitosamine.  Galactosamine occurs as GalNac in

chondroitin chondroitin

sulphates present in the cartilages, bones, tendons, and heart valve. So Galactosamine Galactosamine is called chondrosamine.  Certain antibiotics such as

erythromycin, erythromycin,

carbomycin, contain amino sugars. The amino

sugars are related to the antibiotic activity of  these drugs. Mannosamine is  Mannosamine

a constituent of glycoproteins. glycoproteins.

 D –mannosamine

condense with the pyruvic acid

to form a amino sugar acid called Neuraminic acid. It is found in the nature as N  –acetyl Neuraminic acid (NANA) known as Sialic acids. Neuraminic acid & sialic acids occur in no. of  muucopolysaccharides & in glycolipids like gangliosides. A no. of nitrogenous oligosaccharides which contain neuraminic acid are found in human milk. Certain bacterial cell walls contain muramic acid. Neuraminidase is the enzyme which hydrolyses to split NANA from the compound.  Muramic

acid is structurally a condension product

of D-Glucosamine & lactic acid.