Exam 3 Lecture 5

Carbohydrates serve as information storage molecules and play a vital role in mediating cell-cell interactions.

Monosaccharides are the simplest form of carbohydrates.
They are defined as aldehydes or ketones that contain two or more hydroxyl groups (–OH).
Naming:
- If the monosaccharide contains an aldehyde, it is termed aldose.
- If it contains a ketone, it is called ketose.
- The names of monosaccharides are also based on the number of carbon atoms present.

Simplest monosaccharides: trioses (three carbon atoms).
- Examples:
  - Glyceraldehyde (aldose)
  - Dihydroxyacetone (ketose; derived from structure of acetone with two hydroxyl groups)
Importance:
- Glyceraldehyde and dihydroxyacetone are frequently referenced in metabolic pathways.

Fischer projections are commonly used to depict the structures of monosaccharides.
- Vertical lines: Carbon atoms with substituents behind the plane.
- Horizontal lines: Functional groups in front of the plane, depicted as an embrace shape.

The configuration of monosaccharides (D or L) is determined by the position of the hydroxyl group attached to the terminal carbon:
- L Glyceraldehyde: Hydroxyl group points to the left.
- D Glyceraldehyde: Hydroxyl group points to the right.
D stereochemistry is predominant in naturally occurring carbohydrates.

Enantiomers: Non-superimposable mirror images (D and L forms).
Diastereomers: Stereoisomers that are not mirror images.
- Key type of diastereomer: Epimer
  - An epimer differs from another sugar only at a single chiral carbon.

D Galactose is the C4 epimer of D Glucose.
- Structure drawn by switching configuration at carbon four.
D Mannose is the epimer of D Glucose at C2.

Reaction of an aldehyde with an alcohol yields a hemiacetal.
Reaction of a ketone with an alcohol yields a hemiketal.
For sugars with five or more carbons, these reactions can occur intramolecularly.

Pyranose: Six-membered ring formation (similar to pyridine).
- Nucleophilic attack occurs from the hydroxyl group off carbon five.
Furanose: Five-membered ring formation (similar to furan).
- Nucleophilic attack occurs from the hydroxyl group off carbon four.

Example: D Fructose, a hexose ketose, forms a five-membered ring (furanose).
- Hydroxyl at C2 attacks the ketone.
The formation mechanism does not require acid or base; it occurs in neutral conditions.

In D sugars:
- Alpha configuration: Hydroxyl group below the plane at the anomeric carbon (C1).
- Beta configuration: Hydroxyl group above the plane at the anomeric carbon (C1).
In L sugars, the designation of alpha and beta is reversed.

Nucleophilic attack occurs through rotation into a configuration where the hydroxyl group is positioned appropriately.
Stereochemistry at the newly formed chiral carbon is designated as R or S, maintaining consistency in cyclic forms.

Sugars can exist in open-chain and cyclic forms, with reversible reactions allowing for interconversions.
Example of interconversion:
- Between alpha-D-glucopyranose and beta-D-glucopyranose via free rotation about the C1-C2 bond.

Identify sugars as either aldose or ketose, determine pentose or hexose, and classify as D or L sugar, as well as alpha or beta based on stereochemistry at the anomeric carbon (two bonds to oxygen).
- Example Q1: Aldose or Ketose, Pentose or Hexose?
- Correct approach: Identify attached groups and analyze Fischer projections.
Key point: Most sugars exist in varying configurations, and students should become proficient at drawing Fischer projections to validate their answers.