Unit 1: English Summary – Bioorganic and Medicinal Chemistry

Introduction to Carbohydrates: Carbohydrates are one of the four major biomolecules that play a crucial role in the biological functions of living organisms. They are primarily made up of carbon, hydrogen, and oxygen atoms and are one of the main sources of energy for living organisms. Carbohydrates are essential for metabolism and serve as the building blocks for several essential structures, including cell walls, energy storage, and signaling molecules.

In this section, we will explore the structure, classification, and various chemical properties of carbohydrates, as well as their significance in biological processes. By understanding the chemistry of carbohydrates, students will gain insights into how these biomolecules impact human health and are vital for industrial applications in sectors like food, beverage, and pharmaceuticals.

Classification of Carbohydrates:

Carbohydrates can be classified based on their complexity and the number of sugar units they contain:

1. Monosaccharides: These are the simplest form of carbohydrates and consist of a single sugar unit. They are classified based on the number of carbon atoms and the functional group present.

·       Trioses (3 carbon atoms): Example – Glyceraldehyde.

·       Tetroses (4 carbon atoms): Example – Erythrose.

·       Pentoses (5 carbon atoms): Example – Ribose (found in RNA).

·       Hexoses (6 carbon atoms): Example – Glucose, Fructose.

2. Disaccharides: Composed of two monosaccharide units linked together by a glycosidic bond. Examples include:

·       Sucrose (glucose + fructose)

·       Maltose (glucose + glucose)

·       Lactose (glucose + galactose)

3. Oligosaccharides: Carbohydrates containing 3-10 monosaccharide units. These are often found in cell recognition processes.
4. Polysaccharides: Large carbohydrates consisting of more than 10 monosaccharide units. Examples include starch, glycogen, and cellulose.

 

Reducing and Non-Reducing Sugars:

Carbohydrates can also be classified based on their ability to reduce other compounds, such as metal ions.

• Reducing Sugars: These are sugars that can reduce mild oxidizing agents such as copper (II) or silver (I) salts. They have a free aldehyde or ketone group that can undergo oxidation. Common reducing sugars include glucose, maltose, and lactose.
• Non-Reducing Sugars: These do not have a free aldehyde or ketone group. The glycosidic bond between the two sugar units prevents the sugar from undergoing oxidation. Sucrose is an example of a non-reducing sugar.

 

General Properties of Glucose and Fructose:

Glucose and fructose are two of the most important monosaccharides. They are structurally related but differ in their functional groups and properties.

·       Glucose:

·       Chemical formula: C6H12O6

·       A hexose monosaccharide with an aldehyde functional group.

·       It is a primary source of energy for cells and is present in the blood, where it is transported to cells via insulin.

·       It exists as a D-configuration and has both an open-chain and a cyclic structure.

·       Fructose:

·       Chemical formula: C6H12O6

·       A hexose monosaccharide with a ketone group.

·       Fructose is found in fruits, honey, and is metabolized in the liver. It is also a component of sucrose.

·       Like glucose, fructose also has both open-chain and cyclic forms, but it has a different structure due to its ketone group.

 

Open Chain Structure of Glucose and Fructose:

The open-chain structure of glucose and fructose plays a key role in their chemical reactivity.

• Glucose: The open-chain form of glucose has an aldehyde group at the end of the molecule, making it a reducing sugar. It can form a cyclic structure by reacting with its own hydroxyl group, forming a hemiacetal linkage.
• Fructose: Fructose has a ketone group, which can react with an alcohol group to form a hemiketal structure in the cyclic form. In solution, fructose exists as a mixture of several isomers.

 

Epimers, Mutarotation, and Anomers:

• Epimers: These are stereoisomers that differ in the configuration of only one carbon atom. In the case of glucose, its epimer is galactose, where the difference lies in the orientation of the hydroxyl group at carbon 4.
• Mutarotation: When a sugar like glucose or fructose is dissolved in water, it can undergo a process known as mutarotation. This involves the interconversion between the alpha (α) and beta (β) anomers of the sugar, changing the optical rotation of the solution. This occurs due to the equilibrium between the open-chain form and the cyclic form of the sugar.
• Anomers: Anomers are two possible cyclic forms of a sugar that differ in the configuration of the anomeric carbon (the carbon that was part of the carbonyl group in the open-chain form). In glucose, the two anomers are α-D-glucose and β-D-glucose, depending on the position of the hydroxyl group on the anomeric carbon.

 

Mechanism of Mutarotation:

Mutarotation occurs because glucose and other sugars are in equilibrium between their open-chain and cyclic forms. When a sugar solution is first prepared, it is in the open-chain form. Upon standing, the open-chain form cyclizes into either the α or β anomer. The interconversion between these forms leads to the change in optical rotation, and the system reaches a stable equilibrium between the two anomers.

 

Determination of Configuration of Glucose (Fischer’s Proof):

Fischer’s proof of the configuration of glucose is based on the study of its stereochemistry. By studying the reactions of glucose with reagents such as hydrogen cyanide and aldehyde groups, Fischer showed that glucose has the D-configuration. This was determined by comparing glucose to D-glyceraldehyde, the simplest sugar with the D-configuration.

Fischer also demonstrated that the glucose molecule can be represented in a Fischer projection, which provides a way to visualize the 3D structure of the sugar in a 2D format.

 

Cyclic Structure of Glucose (Haworth Projections):

The cyclic structure of glucose is formed when the hydroxyl group on carbon 5 reacts with the aldehyde group at carbon 1, forming a hemiacetal linkage. The resulting structure is a six-membered ring, known as a pyranose ring.

In Haworth projections, the glucose ring is drawn in a way that shows the spatial arrangement of the atoms in a flat, simplified manner. In this structure, the hydroxyl group on carbon 1 can be in either the axial or equatorial position, leading to the two anomers: α-D-glucose and β-D-glucose.

 

Cyclic Structure of Fructose:

Fructose, being a ketose, forms a five-membered ring known as a furanose ring. The reaction occurs between the hydroxyl group at carbon 5 and the ketone at carbon 2. This leads to the formation of a hemiketal linkage, which stabilizes the cyclic structure.

Similar to glucose, fructose also exhibits mutarotation, but the ring structure of fructose differs due to the ketone group at carbon 2.

 

Interconversions of Sugars:

Sugars can interconvert through various mechanisms:

• Ascending and Descending Sugar Series: In this process, aldoses can be converted to higher or lower sugars in the series. For example, glucose can be converted to higher aldoses like galactose, or to ketoses like fructose.
• Aldose to Ketose Conversion: Aldoses can be converted to ketoses through isomerization reactions. For instance, glucose can be isomerized to fructose under acidic conditions, a process catalyzed by enzymes like glucose isomerase.

 

Lobry de Bruyn-van Ekenstein Rearrangement:

This reaction involves the conversion of aldoses to ketoses and vice versa under basic conditions. The rearrangement occurs through an enediol intermediate, which facilitates the shift of the hydroxyl group and the formation of a new carbonyl group.

 

Kiliani-Fischer Method (Stepping Up of Aldoses):

This method involves elongating an aldose chain by one carbon atom. It is achieved by the reaction of an aldose with cyanide to form an intermediate aldosonitrile, which is then hydrolyzed to form a higher aldose.

 

Ruff’s and Wohl’s Methods (Stepping Down of Aldoses):

These methods involve the reduction of aldoses to lower sugars. In Ruff’s method, aldoses are oxidized to form acid derivatives, while in Wohl’s method, a sequence of reactions leads to the breakdown of aldoses into smaller sugars.

 

Linkage Between Monosaccharides and Structure of Disaccharides:

Disaccharides are formed when two monosaccharides are linked by a glycosidic bond. This bond is formed through a condensation reaction between the hydroxyl group of one monosaccharide and the anomeric carbon of another.

·       Sucrose: Composed of glucose and fructose linked by an α,β-glycosidic bond.

·       Maltose: Composed of two glucose molecules linked by an α-1,4 glycosidic bond.

·       Lactose: Composed of galactose and glucose linked by a β-1,4 glycosidic bond.

 

Conclusion:

Carbohydrates are fundamental biomolecules that are integral to the structure and function of living organisms. By understanding the classification, structure, and interconversion of sugars, as well as the mechanisms behind their various chemical reactions, students will be equipped with essential knowledge that connects chemistry to biology. This knowledge is crucial for future careers in food, beverage, and pharmaceutical industries, where carbohydrate chemistry plays a vital role in product development and the understanding of metabolic processes.