Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
The system is a method of specifying relative configuration of carbohydrates and amino acids by comparing them to the reference molecule, Glyceraldehyde.
The standard reference molecule is Glyceraldehyde (), which has one chiral center. In the Fischer projection, if the group on the chiral carbon is on the right, it is the -isomer; if on the left, it is the -isomer.
For monosaccharides with multiple chiral centers (like Glucose), the or configuration is determined by the position of the group on the highest-numbered chiral carbon (the penultimate carbon).
In amino acids, the configuration is determined by the position of the amino group () on the -carbon. If the group is on the left in the Fischer projection (with at the top), it is an -amino acid. Most naturally occurring amino acids are -isomers.
It is crucial to distinguish between (configuration) and or (optical rotation). and indicate the spatial arrangement of atoms, whereas and indicate the direction in which the molecule rotates plane-polarized light.
In the Fischer projection, the most oxidized carbon (like or ) is always placed at the top (C1).
📐Formulae
💡Examples
Problem 1:
Identify whether the following structure of Glucose represents the or configuration: The group on is positioned on the right side in the Fischer projection.
Solution:
The structure represents -Glucose.
Explanation:
According to the system rules for carbohydrates, the configuration is determined by the chiral carbon furthest from the aldehyde group (the penultimate carbon, which is in Glucose). Since the group on is on the right, it corresponds to the configuration of -Glyceraldehyde, making it -Glucose.
Problem 2:
Given an amino acid where the group is on the left side of the chiral -carbon in a Fischer projection (with the group at the top), assign the configuration.
Solution:
-configuration.
Explanation:
For amino acids, if the group is located on the left side of the vertical line in the Fischer projection (where the carboxyl group is at the top), the molecule is assigned the -configuration. This is characteristic of almost all proteinogenic amino acids.