Lecture Notes on Carbohydrates: Stereochemistry and Nomenclature

Introduction

• Importance of stereochemistry in carbohydrates
• Humans digest D-sugars
• Focus on clarifying questions about carbohydrate stereochemistry and nomenclature

Stereochemistry: D and L Configuration

• D and L:
  • Refer to stereochemistry, not optical activity
  • Example: D-threose
    • Aldotetrose with aldehyde group and four carbons
    • Last chiral center hydroxyl group on the right = D-threose
    • Two chiral centers = 4 possible stereoisomers
    • D-threose rotates plane counterclockwise -> D-threose is D(-)-threose

Enantiomers and Diastereomers

• Enantiomers:

  • D and L configurations of a carbohydrate are enantiomers
  • Example: D-glucose and L-glucose
    • Differ at every chiral carbon
  • Enantiomers are complete mirror images

• Diastereomers:

  • D-aldohexoses as diastereomers
  • Not superimposable and not mirror images
  • Glucose and galactose differ at C4 -> Epimers

Epimers

• Epimers:
  • Diastereomers differing at one chiral center
  • Example: Glucose and galactose differ only at C4

Aldohexoses and Ketopentoses

• Aldohexoses:

  • Have 4 chiral centers -> 16 stereoisomers
  • 8 D-aldohexoses

• Ketopentoses:

  • Have 3 chiral centers
  • 8 stereoisomers (4 D, 4 L)

Common Monosaccharides

• Ribose:

  • All substituents on the right
  • Mnemonic: "Ribose is all right"

• Glucose:

  • D-glucose mnemonic: Middle finger gesture

• Mannose:

  • Mnemonic: "Man with a gun" gesture

• Galactose:

  • C4 epimer of glucose

• Fructose:

  • Ketose form of glucose

Summary

• Review the concepts of enantiomers, diastereomers, and epimers
• Understand and memorize common monosaccharides and mnemonics
• Practice identifying stereochemistry in carbohydrates