Lecture on Carbohydrates

Introduction to Carbohydrates

• Carbohydrates and sugars are the same class of molecules.
• Named as hydrates of carbon, with carbon atoms bearing a hydrogen and a hydroxyl group.
• Small carbohydrates are known as sugars and end with "ose" (e.g., glucose, sucrose).

Monosaccharides

• Monomeric units that form polysaccharides.
• Named based on the number of carbon atoms (3-6 carbon atoms).
  • Triose: 3-carbon monosaccharide
  • Tetrose: 4-carbon monosaccharide
  • Pentose: 5-carbon monosaccharide
  • Hexose: 6-carbon monosaccharide
• Contain either an aldehyde or ketone functional group (aldose or ketone).
• Chiral Centers: Carbons with hydrogen and hydroxyl groups.

Fischer Projections

• A method to visualize molecules.
• Horizontal lines: wedge bonds, Vertical lines: dash bonds.
• Aldehyde or ketone at the top of linear monosaccharides.
• Stereochemistry reported by the orientation of the hydroxyl group on the chiral center farthest from the carbonyl.
  • D-Sugar: Hydroxyl points right.
  • L-Sugar: Hydroxyl points left.
• Derived from glyceraldehyde (aldotriose).

D and L Sugars

• Nature prefers D-sugars.
• Stereoisomers: Molecules with multiple chiral centers have 2^N stereoisomers.
  • Example: 16 aldohexoses (8 and their L enantiomers).
• Swapping groups on chiral center results in inversion.

Cyclic Form of Monosaccharides

• Exist in equilibrium between linear and cyclic forms.
• Cyclization occurs through intramolecular hemiacetal formation.
• Anomers: Different stereoisomers formed during cyclization.
  • Alpha anomer: New hydroxyl down.
  • Beta anomer: New hydroxyl up.

Haworth Projections

• Used to depict cyclic monosaccharides.
• Anomeric carbon on the right, CH2OH group opposite.
  • Alpha anomer: Hydroxyl group down.
  • Beta anomer: Hydroxyl group up.

Pyranose and Furanose

• Pyranose: Six-membered ring from cyclization.
• Furanose: Five-membered ring if hydroxyl on carbon four attacks.
• Equilibrium exists between alpha and beta forms.

Mutarotation

• Transition between alpha and beta anomers.
• Glucose: Beta anomer favored due to equatorial position of hydroxyl.
• Equilibrium favors beta in glucose (about 2:1).
• Some sugars (e.g., mannose) prefer alpha anomer due to the anomeric effect.

Conclusion

• Monosaccharides and their properties lead to the formation of polysaccharides.
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