Chirality in prebiotic molecules and the phenomenon of photo- and radioracemization

Abstract

The abiotic origin of some biologically active molecules and macromolecules have been reviewed. Particular emphasis has been dedicated to the formation of aminoacids and about the possible mechanisms for their asymmetric synthesis and accumulation. The phenomenon of racemization has been explained in detail and then applied in the context of the photo- and radioracemization. Apparent and true photo- and radioracemization have been distinguished. These phenomena play against the preservation of chirality of asymmetric molecules in meteorites and in space. This fact may explain why low enantiomeric excess have been found in meteorites and limited only to certain aminoacids quite uncommon in terrestrial environments.It can be hypothesized that chiral molecules are formed in space for instance under the CPL photolysis and then they are slowly and unselectively degraded and radioracemized by cosmic rays and other high energy radiations. However, the experiment on the radioracemization of certain terpenes has shown that the radioracemization rate is not so dramatically high to cause the complete racemization of the mixture even at very high radiation doses. The radioracemization reaction appears not selective at all, therefore the enantiomeric excess observed only in α-methyl amino acids in meteorites must be attributed to water-thermally driven methamorphism which is instead able to cause selective racemization on α-H amino acids but not on α-methyl amino acids.