Carbohydrates

Contents Reactions of monosaccharides Oxidation Lactone Reduction Isomerization Schiff base formation Glycation Esterification Glycosidic bonds Dr. Nikhat Siddiqi

Reactions of Monosaccharides

Oxidation Aldoses 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. Dr. Nikhat Siddiqi

Oxidation Depending on the oxidizing agent used, the terminal aldehyde (or keto) or terminal alcohol or both the groups may be oxidized. Dr. Nikhat Siddiqi

Aldonic Acid The 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). Dr. Nikhat Siddiqi

Uronic Acid It 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 Dr. Nikhat Siddiqi

Oxidation of both aldehyde and CH2OH gives an aldaric acid. In case of glucose it is glycaric acid. Dr. Nikhat Siddiqi

Oxidation Dr. Nikhat Siddiqi

Lactone The 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. Dr. Nikhat Siddiqi

Oxidized forms of D-glucose Dr. Nikhat Siddiqi

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. Dr. Nikhat Siddiqi

Dr. Nikhat Siddiqi

Reactions of Momosaccharides Reduction

Reduction When treated with reducing agents such as sodium amalgam, the aldehyde or keto group of monosaccharide is reduced to corresponding alcohol. Dr. Nikhat Siddiqi

Reduction Reduction of aldehyde and ketone groups yields sugar alcohol called alditols. Reduction of D- glucose yields D- sorbitol Dr. Nikhat Siddiqi

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. Dr. Nikhat Siddiqi

Sugar Alcohols Dr. Nikhat Siddiqi

Isomerization

Isomerization After 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. Dr. Nikhat Siddiqi

Isomerization The 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. Dr. Nikhat Siddiqi

Tautomerization or enolization The 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. Dr. Nikhat Siddiqi

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. Dr. Nikhat Siddiqi

Schiff Base Formation

Schiff Base Formation The aldehyde and ketone groups of monosaccharides can also undergo Schiff base formation with amines or hydrazides, forming imines and hydrazones, respectively . Dr. Nikhat Siddiqi

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

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. Dr. Nikhat Siddiqi

Glycation

Glycation As 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. Dr. Nikhat Siddiqi

Glycation Glycation 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. Dr. Nikhat Siddiqi

Esterification

Esterification Alcohols 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. Dr. Nikhat Siddiqi

Esterification Sulphate 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. Dr. Nikhat Siddiqi

Glycosidic Bonds

O-Glycosidic Bonds Monosaccharides 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. Dr. Nikhat Siddiqi

O- Glycosidic Bond Dr. Nikhat Siddiqi

O-Glycosidic Bonds These 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. Dr. Nikhat Siddiqi

Glycosidic Bonds Glycosidic bonds formed between monosaccharide units are the basis for the formation of oligosaccharides and polysaccharides. Dr. Nikhat Siddiqi

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. Dr. Nikhat Siddiqi

The End