Chapter 16 Carbohydrates Adel M. Awadallah Islamic University of Gaza Organic chemistry for medicine and biology students Chem 2311 Chapter 16 Carbohydrates By Prof. Dr. Adel M. Awadallah Islamic University of Gaza

Carbohydrates Carbohydrates are the most abundant organic molecules in nature The empiric formula for many of the simpler carbohydrates is (CH2O)n, hence the name “hydrate of carbon” Carbohydrates are classified as: Polyhydroxyaldehydes (aldoses) Polyhydroxyketones (Ketoses)

Monosaccharides

D,L-Sugars

Epimers are diastereomers that differ only in one stereogenic center. D-(-)-Erythrose and D-(-)-threose D-glucose , D-manose D-glucose, D-galactose

Anomeric carbons and the cyclic hemiacetal structure of Glucose Anomeric carbon is the carbon that forms the newstereogenic center

D-glucose is an aldohexose with the formula (C·H2O)6 D-glucose is an aldohexose with the formula (C·H2O)6. The red atoms highlight the aldehyde group, and the blue atoms highlight the asymmetric center furthest from the aldehyde; because this -OH is on the right of the Fischer projection, this is a D sugar. The α and β anomers of glucose. Note the position of the anomeric carbon (red or green) relative to the CH2OH group bound to carbon 5: they are either on the opposite sides (α), or the same side (β).

Mutarotation Mutarotation: is the change in optical rotation due to interconversion of anomers in solution. It is explained by the equilibrium

Pyranose and Furanose Structures Glucofuranoses are present to less than 1 % in glucose solutions

The Ketose D-fructose exists in solution mainly in two furanose forms

Problem: D-Erythrose cannot exist in pyranose forms, but furanose forms are possible. Explain. Draw the structure for α-D-erythrofuranose

Conformations of Pyranoses In β-D-Glucose all the larger substituents are at each ring carbon are equatorial, hence this form is preffered at equilibrium

Esters and Ethers from Monosaccharides

Reduction of Monosaccharides Aldoses are reduced to alditols Sorbitol is used commercially as a sweetener and sugar substituent

Oxidation of Monosaccharides Aloses are oxidized to aldonic acids A carbohydrate that reduces Ag+ or Cu2+ is called a reducing sugar

Aldoses are oxidized by aqueous nitric acid to dicarboxylic acids called aldaric acids

Formation of Glycosides from Monosaccharides

Mechanism of Glycoside Formation

Disaccharides (Maltose)

Cellobiose

Lactose

Sucrose

Sweeteners

Polysaccharides Starch and Glycogen Starch is the energy storing carbohydrate of plants Hydrolysis of starch Starch is composed of amylose (20%) and amylopectin In amylose, the glucose units (50 -300) are in continuous chain with 1,4 linkages

Amylopectin (300 – 5000 glucose units) is highly branched molecule Chains with consecutive 1,4 links average 25-30 units in length. These chains are connected by 1,6 linkages.

Glycogen Energy storing carbohydrate of anilmals 1,4 and 1,6 linked glucose units about 100,000 glucose units A branch every 8-12 glucose units Produced from glucose that is absorbed from the intestines into the blood Glycogen helps maintain the glucose balance in the body, by removing and storing excess glucose from ingested food and later supplying it to the blood

Cellulose Cellulose is an unbranched polymer of glucose joined by 1,4-β-glycosidic bonds (average 5000 units) Humans and other animals digest starch and glycogen Many bacteria contain β-glucosidase and can hydrolyze cellulose (found in Termites (النمل الأبيض) and Ruminants (الحيوانات المجترة ))

Cellulose acetate and cellulose nitrate

Chitin : is found in crustacean shells and insect exoskeletons Similar to cellulose, but at C2 acetylamino groupCH3CONH Pectines found in fruits and used in making jellies They linear polymers of D-galacturonic acid linked with 1,4-glycosidic bonds. D-galacturonic acidhas the same structure as D-galactose except that the C-6 primary alcohol group is replaced by a carboxyl group.