Carbs 2

Introduction to Carbohydrates

  • Course Info
      - CHEM 2713
      - Instructor: Dr. Amitabh Jha
      - Institution: Acadia University

Overview of Carbohydrate Chemistry

  • Classification of carbohydrates

  • Nomenclature of carbohydrates

  • Main types of sugars

  • Structural representations of sugars
      - Fischer projections
      - Haworth projections
      - Chair projections

  • Derivatives of monosaccharides

Monosaccharides

  • General Formula: Often represented as (CH₂O)n where n >= 3

  • Common examples: Glucose, Fructose

Structural Representations

Fischer Projections

  • Linear representation demonstrating the configuration of substituents at each chiral center.

  • The carbon numbering in Fischer projections begins with the carbonyl carbon.

Haworth Projections

  • Cyclic forms of sugars illustrated using a hexagonal or pentagonal structure.

  • Represents the stereochemistry of the sugar in a more realistic manner than Fischer.

  • Example:
      - Haworth structure of D-glucose
      - Characteristics of anomers

Chair Projections

  • Provide a more spatially accurate depiction of the sugar ring's structure compared to Haworth projections.

  • Useful in visualizing substituent orientation and steric hindrance.

Cyclization of Monosaccharides

  • Sugars with five or six carbons are typically found in cyclic forms.

Formation of Hemiacetals and Hemiketals

  • Mechanism:
      - Aldehyde or ketone functional groups react with hydroxyl groups.
      - Reaction occurs in aqueous solution, leading to ring formation.
      - Instability leads to potential reversion to aldehyde or ketone forms.

Cyclization of Monosaccharides

  • Intramolecular Reaction:
      - Aldehydes and ketones in five or six-carbon sugars react with alcohol groups on neighboring carbons.
      - This forms either:
        - Hemiacetal: In aldoses (e.g., glucose)
        - Hemiketal: In ketoses (e.g., fructose)

  • Nomenclature for Cyclic Structures:
      - Five-membered rings are termed furanoses (resembles furan).
      - Six-membered rings are termed pyranoses (resembles pyran).

Anomers

  • Definition:
      - Result from cyclization leading to two distinct forms (α-anomer and β-anomer) of monosaccharides.

  • Importance of Anomeric Carbon:
      - The carbon at position 1 (or anomeric carbon) controls the stereochemistry, affecting properties and reactivity of sugars.

Stereochemistry

Diastereomers and Anomers

  • Cyclization results in two forms with different spatial configurations due to possible arrangements of the hydroxyl group on the anomeric carbon.
      - α-Anomer: Hydroxyl group is below the plane in Haworth projection.
      - β-Anomer: Hydroxyl group is above the plane in Haworth projection.

Mutarotation

  • Definition:
      - Spontaneous interconversion between α and β forms in solution, leading to an equilibrium mixture.

  • Rotation Change:
      - E.g., pure α-D-glucopyranose in solution changes specific rotation from +112° to +52.5°.
      - E.g., pure β-D-glucopyranose in solution changes specific rotation from +18.7° to +52.5°.

Carbohydrate Reactions

Types of Reactions

  • Monosaccharides undergo various chemical reactions:
      - Oxidation:
        - Aldehyde oxidation yields aldonic acids, CH₂OH oxidation yields uronic acids, and oxidation of both leads to aldaric acids.
      - Reduction:
        - Formation of sugar alcohols (e.g., D-glucitol from D-glucose) via reduction processes.
      - Isomerization:
        - Monosaccharides can change formats (e.g., D-glucose can isomerize to D-mannose or D-fructose).
      - Esterification:
        - Hydroxyl groups can interact with acids to form esters, modifying sugar properties significantly.
      - Glycoside Formation:
        - Hemiacetals can react with alcohols to form glycosides (acetal/ketal forms), where linkage is glycocidic.

Monosaccharide Derivatives

Uronic Acids

  • Formed by the oxidation of sugars and play important roles in metabolism (e.g., D-glucuronic acid).

Deoxy Sugars

  • Sugars with hydroxyl groups replaced by hydrogen (e.g., 2-Deoxy-β-D-ribose in DNA).

Glycosylation

  • Process:
      - Catalyzed by glycosyltransferases to attach sugars to proteins/lipids, creating N- or O-glycosidic bonds.
      - Example: Glycation in the Maillard reaction involves non-enzymatic reactions of sugars with proteins, affecting food chemistry and nutritional values.

Summary of Key Concepts

  • Monosaccharides: Basic structural units with polyhydroxy functionalities.

  • Cyclization: Characteristic for sugars with functional groups leading to stable cyclic forms (hemiacetals/hemiketals).

  • Structural Representations: Fischer, Haworth, and chair forms provide insight into sugar configuration.

  • Reactivity: Sugars undergo a series of reactions typical of their functional groups, leading to diverse biochemical pathways and products.