A universal chemical rule states: «Synthesis of chiral compounds from achiral reagents always yields the racemic modification» and «optically inactive reagents yield optically inactive products».

To resolve a racemate another homochiral compound is required: actually, (R) and (S) compounds have identical properties except when they interact with other chiral phenomena. E.g., the products of the reaction of the (R) and (S) enantiomers with an (S’) substance generates R-S’ and S-S’ compounds (named diasteroisomers) which are not mirror images and thus differ in physical properties such as solubility.

Asymmetric autocatalysis (Franck 1953): the amplification of an e.e. by means of a chemical reaction in which the product is a chiral compound and one enantiomer catalyzes its own formation while inhibiting the formation of the opposite enantiomer (e.g. Soai reaction, 1995)

Mechanisms for synthesis of D-AAs

Inversion of a-C of the corresponding L-AA (racemases or epimerases)

  • PLP-dependent enzymes
  • cofactor independent enzymes
  • multi-step processes

N-acyl-amino acid racemase + N-acyl-D-AA-deacylase

– Stereospecific amination of the corresponding a-keto acid (D-AA-amino transferases)

  • The substrate specificity of the second
  • Half-reaction determines the D-AA formed

Mechanism for D-AA synthesis by a PLP-dependent racemase


Loredano Pollegioni, Università degli Studi dell'Insubria