Carbohydrates: Monosaccharides

Introduction

• Focus on monosaccharides (monomers of carbohydrates).
• Future discussions on derivatives of monosaccharides.
• Roles of carbohydrates:
  • Energy storage (e.g., glycogen in animals, starch in plants).
  • Structural components (e.g., cellulose, chitin).
  • Cell type recognition (carbohydrate markers).
  • Components of DNA, RNA, co-factors, and other molecules.

Basic Terminology

• Monosaccharides: Simple sugars, cannot be broken down into simpler sugars under mild conditions. Basic building blocks.
• Oligosaccharides: Few monosaccharides, typically 2-12 units.
• Polysaccharides: Polymers of monosaccharides, usually hundreds of units.

Types of Monosaccharides

• Can be represented in open or closed chain forms.
• General structure: 3 to 6 carbons with at least one oxygen.
• Aldose (Aldehyde group):
  • Alahh functional group.
  • Sugar with aldehyde group.
• Ketose (Ketone group):
  • Carbonyl group between two carbons.
  • Sugar with ketone group.
• Sugars have an 'o' ending.
  • "Al" prefix denotes aldose.
  • "Ket" prefix denotes ketose.

Stereochemistry

• Stereogenic carbon: Carbon with four different groups, leading to stereochemistry.
• Stereochemistry naming based on D and L:
  • D-sugar: OH group on the right.
  • L-sugar: OH group on the left.

Definitions

• Enantiomers: Mirror images with opposite stereochemistry at every chiral carbon.
  • Example: D-glucose and L-glucose.
• Diastereomers: Stereoisomers with at least one chiral center the same and one different.
  • Example: D-mannose and D-galactose are diastereomers.
• Epimers: Differ at only one chiral center.
  • Example: D-glucose and D-mannose.
• Anomers: Differ at the anomeric carbon, found in cyclic forms.

Cyclic Forms of Monosaccharides

• Cyclization: Formation of ring structures from open chains.
  • Aldose forms hemiacetal.
  • Ketose forms hemiketal.
• Ring structures:
  • Furanoses: Five-membered rings.
  • Pyranoses: Six-membered rings.
• Anomeric carbon stereochemistry:
  • Alpha (α): OH group down.
  • Beta (β): OH group up.
• Common sugars like hexoses often form these rings (e.g., D-glucose).

Ring Formation Process

• Haworth Projection: Way of representing the cyclic structure of a sugar.
  • Positions of groups in the open chain determine their orientation in the ring.
  • Right-side groups in open chain -> Down in cyclic form.
  • Left-side groups in open chain -> Up in cyclic form.
• D and L designation affects the position of CH2OH in the ring.

Step-by-Step Ring Closure

1. D-glucose example: Label carbons 1-5.
2. Oxygen from carbon 5 attaches, forming the ring.
3. Right-side groups -> Down positions.
4. Left-side groups -> Up positions.
5. Carbon 5's CH2OH group position:
   • Up for D-sugar.
   • Down for L-sugar.
6. Top attack -> Alpha anomer.
   • OH group down.
7. Bottom attack -> Beta anomer.
   • OH group up.