Lec 7

Carbohydrates Learning Goals

  • Understand the names and structures (including stereochemistry) of common sugars.

  • Explore open-chain and ring forms of monosaccharides, including their stereochemistry.

  • Learn about glycosidic linkage.

  • Review Fischer and Haworth projections.

  • Examine modifications of carbohydrates.

Introduction to Carbohydrates

  • Monomers: Basic building blocks of carbohydrates.

  • Common terms: Enantiomers, epimers, anomers are crucial for understanding stereoisomerism in sugars.

Monosaccharides

  • Definition: Simple sugars that consist of polyhydroxy aldehydes or ketones.

Types of Monosaccharides

  • Trioses (n = 3):

    • Examples:

    • D-Glyceraldehyde (an aldotriose)

    • Dihydroxyacetone (a ketotriose)

Stereoisomerism

Enantiomers

  • Carbon chain has mirror images.

  • Example: D-Glyceraldehyde vs. L-Glyceraldehyde.

    • C2 is the chiral carbon.

Epimers and Diastereomers

  • Epimers: Isomers that differ at one chiral center.

    • E.g., D-glucose and D-mannose vs. D-galactose.

  • Diastereomers: Isomers that differ at two or more chiral centers.

    • E.g., D-threose vs. D-erythrose.

Tetroses

  • n = 4:

    • Aldoses: Possess two chiral carbons, leading to 22 possible isomers.

    • Ketoses: One chiral carbon, leading to 21 possible isomers.

Pentoses

  • n = 5:

    • Aldoses: Examples include D-Ribose, D-Arabinose, D-Xylose, D-Lyxose.

    • Three chiral carbons result in 23 isomers.

    • Ketoses: D-Ribulose and D-Xylulose have two asymmetric carbons, yielding 22 isomers.

Hexoses

  • n = 6:

    • Aldoses: Includes D-Allose, D-Altrose, D-Glucose, D-Mannose, etc.

    • Total 24 isomers from 4 asymmetric carbons.

    • Ketoses: D-Fructose and D-Sorbose have 3 asymmetric carbons yielding 23 isomers.

Cyclization of Monosaccharides

  • Monosaccharides can undergo intramolecular reactions to form stable rings (hemiacetals/hemiketals).

  • Ring formation: Pentoses and hexoses typically form 5-membered (furanoses) or 6-membered (pyranoses) rings.

Anomeric Carbon

  • In cyclic forms, the new asymmetric center created at C1 for aldoses (anomeric carbon) differentiates between anomers (α and β forms).

Example of Cyclization

  • D-Fructose:

    • Open-chain form displayed leads to a new C2 as the anomeric carbon.

    • Forms α-D-Fructofuranose and β-D-Fructofuranose.

Fischer vs. Haworth Projections

  • Fischer projections depict linear forms; Haworth projections illustrate cyclic forms.

  • In Fischer, -OH groups on the right translate to down in Haworth projections.

Isomer Interconversion

  • Different forms of glucose (α-furanose, β-furanose, etc.) demonstrate very low proportions in equilibrium states.

Sugar Modifications

  • Various modifications impact sugar functionality, including:

    1. Phosphate Esters (e.g., glyceraldehyde-3-phosphate, glucose-1-phosphate).

    2. Glycosides: Hemiactals react with alcohols to form acetals through glycosidic linkages.

    3. Oxidative and Reduction Reactions: Converting aldoses to acids or alcohols, affecting their chemical reactivity.

Distinguishing Features of Disaccharides

  • Monomers linked by glycosidic bonds with structural differences.

  • Maltose: Glucosylglucose (α(1→4)).

  • Sucrose: Glucosylfructoside (α(1→β2)).

Structural Polymers

  • Cellulose: Linear polymers of β-D-Glc, structural in cell walls.

  • Starch: Composed of amylose and amylopectin as storage homopolymers.

Glycoproteins

  • Created from proteins and saccharide chains (O-linked and N-linked glycans).

  • Functions include recognition, interaction with microbes, and indicating blood groups (ganglioside composition).

Carbohydrates Learning Goals Revisited

  • Review outcomes such as stereochemistry, structures, ring forms, glycosidic linkages, and modifications for comprehensive understanding of carbohydrate biochemistry.