carbohydrates. contents reactions of monosaccharides oxidation lactone reduction isomerization...

Carbohydrates

Dr. Nikhat Siddiqi 2

Contents

Reactions of monosaccharidesOxidationLactoneReduction IsomerizationSchiff base formationGlycationEsterificationGlycosidic bonds

Reactions of Monosaccharides


OxidationAldoses and ketoses were historically

referred to as reducing sugars because they responded positively in a chemical test that effected oxidation of their aldehyde and hydroxyketone functionalities, respectively.

In the presence of oxidizing agents, metal ions such as Cu2+ and certain enzymes, monosaccharides readily undergo oxidation reaction.

http://www.ncbi.nlm.nih.gov/bookshelf/?book=glyco2&part=glossary&rendertype=def-item&id=glossary.gl1-d6




OxidationDepending on the oxidizing agent

used, the terminal aldehyde (or keto) or terminal alcohol or both the groups may be oxidized.


Aldonic AcidThe carboxylic acid formed by

oxidation of the aldehyde in an aldose is referred to as a glyconic acid (e.g., gluconic acid is the oxidation product of glucose).



Uronic AcidIt is also possible to oxidize the

hydroxy groups of monosaccharides, most notably the terminal OH group (i.e., C-6 of glucose).

In this reaction, a glycuronic acid is produced



Oxidation of both aldehyde and CH2OH gives an aldaric acid.

In case of glucose it is glycaric acid.


Oxidation


LactoneThe carbonyl groups in both

aldonic and uronic acids can react with an OH group in the same molcule to form a cyclic ester known as lactone.

Lactones are commonly found in nature. For example ascorbic aid (vit C) is a lactone derivative of D-glucuronic acid.


Oxidized forms of D-glucose


Glucose reacts with oxidizing agents such as cupric ion (Cu2+) (Benedict’s reagent)because the open-chain form has a free aldehyde group that is readily oxidized.

This reaction occurs only with sugars that an revert to the open chain form, all monosaccharides are reducing sugars.

Reactions of MomosaccharidesReduction


ReductionWhen treated with reducing

agents such as sodium amalgam, the aldehyde or keto group of monosaccharide is reduced to corresponding alcohol.


Reduction Reduction of

aldehyde and ketone groups yields sugar alcohol called alditols.

Reduction of D-glucose yields D-sorbitol


Sugar alcohols, also known as polyols, polyhydric alcohols, or polyalcohols.

For example, glucitol, also known as sorbitol, has the same linear structure as the chain form of glucose, but the aldehyde (-CHO) group is replaced with a -CH2OH group.

Other common sugar alcohols include the monosaccharides erythritol and xylitol and the disaccharides lactitol and maltitol.

Sugar alcohols have about half the calories of sugars and are frequently used in low-calorie or "sugar-free" products.


Sugar Alcohols

Isomerization


IsomerizationAfter several hours an alkaline

solution of D-glucose also contains D-mannose and D-fructose.

It involves an intramolecular shift of a hydrogn atom and relocation of a double bond.

The intermediate is called as enediol.


IsomerizationThe reversible

transformtion of glucose to fructose is an aldose-ketose interconversion.

Because the configuration at a single asymetric carbon changes, the conversion of glucose to mannose is epimerization.


Tautomerization or enolizationThe process of

shifting a hydrogen atom from one carbon to another to produce enediols is known as tautomerization.

Sugars possessing anomeric carbon undergo tautomerization in alkaline solution.


When glucose is kept in alkaline solution for several hours, it undergoes isomerization to form D-fructose and D-mannose.

This results in the formation of a common inetrmidiate namely enediol for all the three sugars.

Schiff Base Formation


Schiff Base FormationThe aldehyde

and ketone groups of monosaccharides can also undergo Schiff base formation with amines or hydrazides, forming imines and hydrazones, respectively .




Conjugation of a monosaccharide to an amino group by formation of an imine.



This reaction is often used to conjugate the monosaccharide to proteins (via their lysine residues).

The imines formed with amino groups are not stable to water and are typically reduced with sodium cyanoborohydride (NaCNBH3) in a process termed reductive amination.


Glycation


GlycationAs aldehydes, reducing sugars can

also form Schiff bases with amino groups of the lysine residues in proteins.

This nonenzymatic process that links glycans to proteins is termed “glycation” and is distinct from “glycosylation,” which involves the formation of a glyco-sidic bond between the sugar and protein.







GlycationGlycation products can undergo further

reactions that lead to the formation of protein cross-links, and these can have pathogenic consequences (i.e., they are immunogenic and change the properties of the protein).

Glycation products of glucose accumulate at higher levels in diabetics than in healthy individuals because of elevated blood glucose levels. These modified proteins are thought to underlie some of the pathologies associated with diabetes.



Esterification


EsterificationAlcohols readily form esters when

reacted with acids, anhydrides, or acyl halides.

The most important types of sugar esters that occur in nature are

(1) phosphate esters (including diphosphate esters),

(2) acyl esters (with acetic acid or fatty acids), and

(3) sulfate esters.


EsterificationSulphate esters of carbohydrate

moleules are found predominantly in proteoglycan components of connective tissue.

Sulphate esters are charged so they bind large amounts of water and small ions.

They also participate in forming salt bridges between carbohydrate chains.

Glycosidic Bonds


O-Glycosidic BondsMonosaccharides can be

modified by reaction with alcohols and amines to form adducts. For example, d-glucose will react 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.


O- Glycosidic Bond


O-Glycosidic BondsThese two

glucopyranosides 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.


Glycosidic BondsGlycosidic bonds formed between

monosaccharide units are the basis for the formation of oligosaccharides and polysaccharides.


N-glycosidic bond.The anomeric

carbon atom of a sugar can be linked to the nitrogen atom of an amine to form an N-glycosidic bond.

The End

carbohydrates. contents reactions of monosaccharides oxidation lactone reduction isomerization...

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