Carbohydrate Stereochemistry and Nomenclature

Introduction

• Focus on stereochemistry of carbohydrates, particularly the last chiral center.
• Importance: Biological function and human enzymatic digestion of D sugars.

D and L Nomenclature

• D and L indicate stereochemistry, not optical activity.
• Example: D-threose
  • Aldehyde functional group, 4 carbons (aldotetrose).
  • Last chiral center's hydroxyl group on the right side → D-threose.
  • Two chiral centers → 4 possible stereoisomers.
  • D-threose rotates plane-like counterclockwise (D(-)threose).

Enantiomers and Diastereomers

• Enantiomers: D and L configurations differ at every chiral carbon.
  • Example: D-glucose vs L-glucose (aldohexose, 6 carbons).
• Diastereomers: Not mirror images, differ at some but not all chiral centers.
  • D-aldohexoses are diastereomers of each other.
  • Example: D-allose vs D-glucose differ at one chiral carbon.

Epimers

• Epimers: Diastereomers differing at one chiral center.
  • Example: Glucose and Galactose differ at C4 carbon.

Stereoisomers Calculation

• Aldohexoses have 4 chiral centers → 16 stereoisomers (2^4).
  • Half are D (OH on right side), half are L.
  • 8 D-aldohexoses and 8 L-aldohexoses.

Common Monosaccharides

• Ribose: Aldopentose, hydroxyl groups all on right.
  • Mnemonic: "all right."
• D-Glucose: Aldohexose, resembles middle finger gesture.
• D-Mannose: Two extended fingers (pointer and middle) resemble a man holding a gun.
  • Mnemonic: "man with a gun."
• D-Galactose: C4 epimer of glucose.
• D-Fructose: Ketose version of glucose.

Summary

• Understanding carbohydrate stereochemistry is essential for grasping their biological roles and classifications.
• Key terms: Enantiomers, Diastereomers, Epimers.
• Mnemonics aid in memorizing common monosaccharides.