The D- and L- system classifies molecules based on what characteristic?

The D- and L- system classifies molecules primarily based on their optical activity, specifically how they rotate plane-polarized light. This system is particularly relevant for carbohydrates and amino acids, where the designation of a molecule as D or L refers to the configuration of the molecule relative to glyceraldehyde, a simple sugar.

In this context, D- and L- refer to the orientation of the hydroxyl (-OH) group on the penultimate carbon atom (the second-to-last carbon in the chain). For D-configuration, the hydroxyl group is on the right side in the Fischer projection, while for L-configuration, it is on the left. This classification system does not depend on the size of the molecule, its chemical bonding, or its physical appearance, but instead focuses on how the specific structure of the molecule affects its interaction with polarized light, a key characteristic defining optical activity. Thus, the correct answer highlights the importance of optical activity in classifying molecules within this system.