Polymérisation des énantiomères d'un acide α-aminé : Stéréosélection et amplification de l'asymétrie

5. Internationales Makromolekulares Symposium

Abstract

Proteins of higher living systems are built only with amino acids the configuration of which is L. However the synthesis of these building blocks in abiotic conditions leads to the formation of a racemic mixture. 

Two questions then arise. Is the L configuration due to a random event or to interaction with an asymmetric forces field? At which state of the evolution became one configuration prevalent ? 

Due to their ordered structures poly-DL-peptides, a probable inter-mediairy step of the evolution, appear as an inter sting system for amplification of asymmetry and enrichment of the medium in one of the enantiomer (supposing a slight initial dissymmetry in the chemical composition). 

Two kinds of experiments are reported, carried out with polymers of benzylglutamate and derivatives of the same aminoacid as monomers. First an unsuccessfull attempt was made to use the asymmetric right-handed helical backbone to induce a reversal of the configuration od D-residues introduced in it. Second, initiation of polymerisation of L- or D-amino acid derivatives by preformed helical poly-DL-peptides with a favourite helical sense but with residues of different (configuration at the active end. The qualitative results show that the configuration of the last residue is not the most important factor for the initial rate of polymerisation. 

Finally, the role of the different structures, extended-β or α-helix and the newly described π_OL-helix is evoked in correspondance with the structure of natural peptides as gramicidin A.