non-superimposable

Meaning: Two structures are superimposable if rotating or moving one in space makes every atom coincide with the other. A chiral molecule cannot be overlaid on its mirror image, so the pair are enantiomers. Chirality usually comes from a stereocenter, a carbon with four different groups. Enantiomers share melting point, solubility and other physical properties in an achiral environment, but they rotate plane-polarized light in opposite directions and fit differently into chiral sites such as enzyme active sites. In biochemistry, the α carbon of every standard amino acid except glycine is a stereocenter, and proteins are built almost entirely from L-amino acids. Most biological sugars are the D form.

Equation: maximum number of stereoisomers = 2ⁿ, where n is the number of stereocenters (dimensionless). Meso compounds make the actual number smaller.

On the MCAT: You may be shown Fischer projections or wedge-and-dash drawings and asked whether two molecules are identical, enantiomers or diastereomers. Enantiomers have the opposite configuration at every stereocenter. A passage may also explain why an enzyme acts on one enantiomer and not the other.

Common confusion: Non-superimposable does not by itself mean enantiomers. Diastereomers are also non-superimposable on each other, but they are not mirror images, because they differ at some stereocenters and not others. Enantiomers are specifically non-superimposable mirror images. Also, D or L describes configuration, not the direction (+ or −) in which light is rotated.