Racemization

Enantiomers should also be distinguished from diastereomers which are a type of stereoisomer that have different molecular structures around a stereocenter and are not mirror images.

[3] In the solid state, racemic mixtures may have different physical properties from either of the pure enantiomers because of the differential intermolecular interactions (see Biological Significance section).

[4] In general, most biochemical reactions are stereoselective, so only one stereoisomer will produce the intended product while the other simply does not participate or can cause side-effects.

This is due to the fact that many biological molecules are chiral and thus the reactions between specific enantiomers produce pure stereoisomers.

However, bacteria produce D-amino acid residues that polymerize into short polypeptides which can be found in bacterial cell walls.

These polypeptides are less digestible by peptidases and are synthesized by bacterial enzymes instead of mRNA translation which would normally produce L-amino acids.

Likewise, the (S) stereoisomer is much more reactive than the (R) enantiomer in citalopram (Celexa), an antidepressant which inhibits serotonin reuptake, is active.

[14]: 373  An incoming group can approach from either side of the plane, so there is an equal probability that protonation back to the chiral ketone will produce either an R or an S form, resulting in a racemate.

In 1843, Louis Pasteur discovered optical activity in paratartaric, or racemic, acid found in grape wine.