A. Draw the Fischer projection standard orientation for L-glucose and D-glucose:
The Fischer projection is a two-dimensional representation of a molecule that shows its stereochemistry. For glucose, we can represent both L-glucose and D-glucose in their standard orientations.
- L-Glucose:
HOCH2 | H-C-OH | HO-C-H | H-C-OH | C=O - D-Glucose:
HOCH2 | H-C-OH | H-C-OH | HO-C-H | C=O
B. What is the isomeric relationship for the two structures you drew?
L-glucose and D-glucose are enantiomers. This means they are non-superimposable mirror images of each other. Their molecular formulas are identical, but their arrangements in three-dimensional space differ, which leads to different optical activities. Enantiomers have different properties in chiral environments, such as biological systems, where one may be metabolized differently than the other.
C. What do the D and L designations mean?
The D and L designations refer to the configuration of the chiral carbon furthest from the carbonyl group (C1 in the Fischer projection). The ‘D’ designation indicates that the hydroxyl group (-OH) on this carbon is on the right side of the Fischer projection, while the ‘L’ designation indicates that it is on the left side. This system is based on the original identification of the optical activity of sugars in relation to glyceraldehyde, where D-glyceraldehyde had the -OH group on the right and L-glyceraldehyde had it on the left. The distinction helps in categorizing sugars into two families, which is crucial for understanding their behavior in biological processes.