Chapter_10 Flash

Chapter 10: Carbohydrates

10.1 Introduction to Carbohydrates

  • Carbohydrates are classified into three main classes:

    • Monosaccharides: Simple sugars that consist of single units.

      • Chemically classified as polyhydroxy aldehydes or ketones (compounds with a carbonyl group).

    • Oligosaccharides: Short carbohydrate chains consisting of 2-10 monosaccharide units linked by glycosidic bonds.

    • Polysaccharides: Long carbohydrate polymers containing many units (20+) generally through glycosidic bonds.

10.2 Monosaccharides

  • Monosaccharides include:

    • Trioses: 3 carbon sugars (e.g., glyceraldehyde)

    • Pentoses: 5 carbon sugars (e.g., ribose, deoxyribose)

    • Hexoses: 6 carbon sugars (e.g., glucose (aldehyde) and fructose (ketone))

  • The structure depends on the carbon arrangement and presence of asymmetric carbons, which lead to the creation of isomers.

10.3 Asymmetric Carbons and Isomerism

  • Chiral Centers: Asymmetric carbons can exist as two isomers (enantiomers) that are mirror images of each other.

  • Fischer Projection: Provides a method to represent 3D structures of glucose and other carbohydrates. The D and L configurations are determined based on the orientation of the hydroxyl group at the last asymmetric carbon.

10.4 Classification of Isomers

  • Isomers have the same molecular formula but differ in structural arrangement.

    • Constitutional Isomers: Different connectivity in atoms.

    • Stereoisomers: Same connectivity but differ in spatial arrangement.

    • Enantiomers: Stereoisomers that are non-superimposable mirror images.

    • Diastereomers: Stereoisomers that are not mirror images of each other.

      • Epimers: A subtype of diastereomers that differ at only one specific carbon.

      • Anomers: A specific type of epimer differing at the anomeric carbon in cyclic forms.

10.5 Cyclic Structures of Monosaccharides

  • Monosaccharides can cyclize due to interactions between aldehyde/ketone and alcohol functional groups, forming cyclic structures like

    • Pyranose: 6-membered ring (as in glucose)

    • Furanose: 5-membered ring (as in fructose)

  • The anomeric carbon becomes asymmetric upon cyclization, leading to alpha and beta configurations of the sugar.

10.6 Chair Conformation of Cyclic Sugars

  • The chair conformation is more stable for cyclic sugars than flat structures due to electron repulsion and size of bulky substituents.

  • Substituents can occupy either axial positions (pointing up and down) or equatorial positions (pointing sideways). Equatorial positions provide higher stability as they allow more space and avoid steric hindrance.

  • Stability:

    • Beta-D-Glucopyranose is generally more stable due to most hydroxyl groups being in equatorial positions compared to alpha-D-Glucopyranose.

10.7 Reducing and Non-reducing Sugars

  • Reducing Sugars have free aldehyde or ketone groups and can reduce other substances (e.g., D-glucose).

  • Alterations at the anomeric carbon (e.g., glycosidic bond formation) can lead to sugars losing their reducing properties.

  • Glycosidic Bonds form when two monosaccharides react, resulting in disaccharides and oligosaccharides.

    • Example: Maltose (formed from two glucose units) has an alpha-1,4-glycosidic bond.

10.8 Formation and Types of Glycosidic Bonds

  • Glycosidic bonds are classified as O-linked (through oxygen), N-linked (through nitrogen), etc.

  • Reactive hydroxyl groups can form covalent bonds with other molecules leading to various glycosides.

  • Oligosaccharides are composed of monosaccharides linked via glycosidic bonds, essential for polysaccharides and various biological functions.

In the context of sugars, particularly reducing sugars, the concepts of reduction and oxidation can be illustrated as follows:

  1. Reducing Sugars: These are sugars that can act as reducing agents because they contain a free aldehyde or ketone group. For example, D-glucose can donate electrons to other substances, thereby reducing them. When glucose is oxidized, it is converted into gluconic acid.

  2. Oxidation Process: During oxidation, the aldehyde group (−CHO) of the glucose molecule is converted to a carboxylic acid group (−COOH). This process results in an increase in the oxidation state of the sugar.

  3. Reduction Process: Conversely, if a sugar is reduced, it may involve the conversion of a carbonyl group into an alcohol group. This is typically seen in the reduction of sugars to sugar alcohols (e.g., converting glucose to sorbitol).

  4. Glycosidic Bonds: When monosaccharides undergo glycosylation (forming glycosidic bonds), the reaction can also involve redox processes. The formation of these bonds can lead to changes in the reducing properties of sugars. Once the anomeric carbon forms a glycosidic bond, the sugar typically becomes non-reducing, as the free aldehyde or ketone group is no longer available to undergo further redox reactions.

  5. Biological Relevance: In biochemical pathways, the oxidation of glucose during cellular respiration provides energy, while the reduction of other compounds is crucial in biosynthetic pathways.