Overview
This lecture explains Fischer and Haworth projection formulas, how to assign D and L enantiomers, and the importance of stereochemistry in sugars, amino acids, and drugs.
Fischer Projections & D/L Designation
- Fischer projections use intersecting lines: vertical lines for bonds going away, horizontal lines for bonds coming toward the viewer.
- In sugars, the chiral center farthest from the carbonyl group determines D/L configuration.
- In D-glyceraldehyde, the hydroxyl (OH) group is on the right; in L-glyceraldehyde, it is on the left.
- For glucose, the OH on carbon 5 determines the D (right side) or L (left side) form.
- Compounds with multiple chiral centers have D/L assigned by the highest numbered chiral carbon’s OH position.
- Diastereomers are stereoisomers that are not mirror images; enantiomers are mirror images.
Biological Relevance of Stereochemistry
- Most natural sugars are D isomers; most natural amino acids are L isomers in proteins.
- Many drugs (e.g., thalidomide, antihistamines, ibuprofen) exist as mixtures of enantiomers with different biological effects.
- Biological receptors often distinguish between enantiomers (e.g., scents in spearmint and caraway).
- Chemists work to separate or synthesize pure isomers for safer, more effective medications.
Haworth Projection Formulas
- Haworth projections represent cyclic forms of monosaccharides as two-dimensional rings.
- D-glucose cyclizes via a reaction between the carbonyl at C1 and the OH at C5, forming a six-membered ring (hemiacetal).
- The new chiral center formed at C1 is the anomeric carbon, allowing alpha and beta anomers.
- In D-form cyclic sugars, the CH2OH at the highest numbered carbon is above the ring; in L-form, it is below.
- Beta configuration: anomeric OH and CH2OH are on the same side; alpha configuration: they are on opposite sides.
- When alpha/beta is unspecified, a wavy line is used for the bond at the anomeric carbon.
Key Terms & Definitions
- Fischer Projection — two-dimensional representation of molecules showing chiral centers by intersecting lines.
- Haworth Projection — two-dimensional ring form of cyclic sugars showing spatial orientation.
- Enantiomer — mirror image stereoisomers not superimposable on each other.
- Diastereomer — stereoisomers not related as mirror images.
- Chiral Center — carbon atom bonded to four different groups.
- Anomeric Carbon — the new chiral center formed during cyclization of a sugar.
- Alpha/Beta Anomer — isomers formed at the anomeric center differing in the OH group's position.
Action Items / Next Steps
- Practice drawing Fischer and Haworth projections for glucose and glyceraldehyde.
- Review the assignment of D and L configurations in monosaccharides.
- Read about the biological effects of drug enantiomers for next class.