Carbohydrate Chemistry – Chapter 3

Carbohydrate Chemistry – Chapter 3

Learning Objectives:

• Master the definition and classification of carbohydrates.

• Understand the chemical structure, isomeric forms, and chemical properties of monosaccharides (using glucose as an example).

• Understand the structure, properties, and roles of disaccharides and polysaccharides.

1. General Overview of Carbohydrate Chemistry

Carbohydrates, also known as saccharides, are widely distributed in plants and animals, playing a crucial role in structure and metabolism.

• Plants: Synthesize glucose from CO2 and H2O through photosynthesis and store it as starch.

• Animals: Obtain most of their carbohydrates from plants, with glucose being the most important and primary energy source.

• Glucose: Used to synthesize glycogen (storage form), ribose and deoxyribose (used in nucleic acid synthesis), galactose (synthesizes lactose in milk), glycolipids, glycoproteins, and proteoglycans.

1.1. Chemical Definition of Carbohydrates:

Carbohydrates are polyhydroxy aldehydes or polyhydroxy ketones and their derivatives.

1.2. Chemical Classification of Carbohydrates:

• Monosaccharides: The simplest carbohydrates, consisting of a single polyhydroxy aldehyde or polyhydroxy ketone unit. They cannot be hydrolyzed by acids into smaller units. They are also known as “oses” and can be considered the building blocks of carbohydrates. The most important monosaccharide in nature is D-glucose, a molecule with six carbon atoms.

• Oligosaccharides: Consist of 2-10 monosaccharide units linked together by osidic or glycosidic bonds. Examples: Maltose, Sucrose.

• Polysaccharides: Consist of many monosaccharides linked together (>10), forming either straight or branched chains. Two important polysaccharides in nature are starch and cellulose, both polymers of D-glucose.

2. Monosaccharides in Carbohydrates

• General Formula: (CH2O)n where n ? 3.

• Carbon Skeleton: A straight chain, except for the carbon bearing the carbonyl group, all other carbons carry -OH groups.

• Aldose: The carbonyl group is an aldehyde group.

• Ketose: The carbonyl group is a ketone group.

Classification Based on the Number of Carbons:

• n = 3: Triose

• n = 4: Tetrose

• n = 5: Pentose

• n = 6: Hexose

• n = 7: Heptose

General Properties of Monosaccharides:

• White crystalline substances, soluble in water, and sweet to taste.

2.1. D-aldoses and D-ketoses from 3C-6C:

D-aldoses:

• 3C: D-Glyceraldehyde

• 4C: D-Erythose, D-Threose

• 5C: D-ribose, D-arabinose, D-xylose, D-lyxose

• 6C: D-Allose, D-Altrose, D-Glucose, D-Mannose, D-Glucose, D-idose, D-galactose, D-Talose

D-ketoses:

• 3C: Dihydroxyacetone

• 4C: D-Erythrulose

• 5C: D-ribulose, D-xylulose

• 6C: D-Psicose, D-Sorbose, D-Fructose, D-Tagatose

2.3.1. Structure of Glucose:

• Linear Structure: L-glucose, D-glucose.

• Cyclic Structure: alpha-D-Glucopyranose, beta-D-glucopyranose.

• Chair Conformation: alpha-D-Glucosepyranose, beta-D-Glucosepyranose.

2.3.2. Isomeric Forms of Glucose:

• D-L Isomers: Based on the position of the OH group on the terminal carbon with the primary alcohol function (carbon number 5 of glucose). If the OH group is on the right side, the sugar belongs to the D series; if it’s on the left side, it belongs to the L series.

• Pyranose and Furanose Cyclic Structures: Glucose in solution exists as predominantly pyranose (over 99%).

• Alpha and Beta Isomers: Due to the cyclic structure of aldoses being a hemiacetal formed by the reaction of the aldehyde group with an alcohol group.

• Epimers: Isomers that differ in the position of the OH group on carbon 2 or 4 compared to glucose. Example: Mannose, Galactose.

• Aldose and Ketose Isomers: The carbonyl group can be an aldehyde (aldose) or a ketone (ketose).

2.3.3. Properties of Glucose:

• Reaction with Strong Inorganic Acids: Forms furfural.

• Reaction with Bases: Creates various isomers.

• Reducing Properties: Monosaccharides reduce Cu2+ to Cu+, forming a red precipitate.

• Reaction with Phenylhydrazine: Forms phenylhydrazone.

• Oxidation Reaction by Nitric Acid: Forms aldaric acid.

• Glycoside Formation: Forms O-glycosidic or N-glycosidic bonds.

• Ester Formation: Forms corresponding esters, most importantly phosphate esters.

2.4. Derivatives of Monosaccharides:

• Alcohol Derivatives: The carbonyl group of monosaccharides can be reduced to an alcohol function. Example: D-glucose is reduced to sorbitol.

• Acid Derivatives: Include aldonic acid, aldaric acid, and uronic acid.

• Amine Derivatives (osamines): Resulting from replacing the OH group on C2 with a D-NH2 group.

2.5. Some Commonly Encountered Monosaccharides with Important Roles:

• Pentose: Ribose, Ribulose, Arabinose, Xylose, Lyxose.

• Hexose: Glucose, Fructose, Galactose, Mannose.

3. DISACCHARIDES

• Formed by the linkage of two monosaccharides through a glycosidic bond.

• Maltose: Found in barley sprouts, brewer’s yeast, malt candy. Consists of two alpha-D-Glucopyranose molecules linked by an alpha-1,4-glycosidic bond.

• Lactose: Milk sugar, abundant in the milk of various animals. Consists of one alpha-D-glucopyranose molecule linked to one beta-D-galactopyranose molecule by a beta-1,4 bond.

• Sucrose (Saccharose): Cane sugar, abundant in sugarcane and sugar beets. Formed by the combination of one alpha-D-glucopyranose molecule with one beta-D-fructofuranose molecule through an alpha-beta-1,2 bond.

4. POLYSACCHARIDES

• Most carbohydrates found in nature are polysaccharides.

• Classification:

• Homopolysaccharides: Composed of a single type of monosaccharide.

• Heteropolysaccharides: Composed of various different types of monosaccharides.

• Storage Polysaccharides: Provide energy.

• Structural Polysaccharides: Structural components of cell structures.

4.1. Starch:

• The storage carbohydrate of plants, a primary carbohydrate source for humans.

• Structure: Composed of amylose and amylopectin.

• Hydrolysis: Forms dextrins and ultimately glucose.

4.2. Glycogen:

• The storage carbohydrate of animals, most abundant in the liver and muscles.

• Similar structure to amylopectin but with more branches and shorter branches.

4.3. Dextran:

• A homopolysaccharide of alpha-D-glucose, with a branched structure.

• Used as a plasma substitute.

4.4. Cellulose:

• The most abundant polysaccharide in nature, the primary component of plants.

• Structure: Consists of beta-D-glucose molecules linked together by beta-1,4 bonds.

• No Nutritional Value for Humans: The human body lacks the enzyme to hydrolyze beta-1-4-glycosidic bonds.

4.5. Chitin:

• A homopolysaccharide of N-acetyl-beta-D-glucosamine.

• Structure: Similar to cellulose, except the OH group on C2 is replaced by an acetyl amine group.

• Role: Forms the exoskeletons of arthropods and mollusks.

4.6. Pectin:

• A homopolysaccharide of D-galacturonic acid.

• Abundant in fruits, with economic value due to its gel-forming properties.

5. Complex Polysaccharides:

• Possess complex structures, widespread in nature, and play crucial biological roles.

5.1. Glycosaminoglycans and Proteoglycans:

• Glycosaminoglycans: Polysaccharides that play a vital role in the structure of vertebrates.

• Proteoglycans: Protein-carbohydrate structures called proteoglycans, attaching protein components to the glycosaminoglycan backbone.

5.2. Glycoproteins:

• Proteins with attached oligosaccharide or polysaccharide chains.

• Functions: Participate in the structural composition of tissues, cell membranes, components of mucus, hormones, immune components, and physiological fluids.

5.3. Bacterial Cell Wall Polysaccharides:

• Gram-positive bacteria: The cell wall is composed of multiple layers of polysaccharide-peptide complexes (peptidoglycan).

• Gram-negative bacteria: An outer lipid layer covers an inner peptidoglycan layer, and a second lipid layer lies on the inside.

• Lysozyme: Catalyzes the hydrolysis of glycosidic bonds between GlcNAc and MurNAc in the polysaccharide chain, leading to bacterial death due to the breakdown of the peptidoglycan layer.

Note: This article is just a summary of the content. Refer to additional resources for a deeper understanding of carbohydrate chemistry.