Abstract: Recently it has demonstrated found that sublimation, a subject almost completely neglected for a long time, provides a reliable explanation for the emergence and evolution of amino acid (AA) homochirality in nature. Slow partial sublimation of non-racemic mixtures of the proteinogenic AAs Ala, Leu, Pro, Phe and Val led to their considerable enantioenrichment.[1,2] Ala, Leu, Pro and Val can be deracemized by co-crystallization from aqueous solutions with enantiopure Asn, Asp, Glu, Thr and Ser followed by partial sublimation of the obtained mixtures.[3] In the same years unexpectedly it was found that some aliphatic AAs forming metastable racemic conglomerates (Val and Ile) are able to undergo quick high-temperature sublimation (HTS) at 430 °C with simultaneous amplification of their initial enantiomeric excess.[4,5] Enantioenrichment of other aliphatic AAs (Ala and Leu) via the HTS requires the presence of one of the conglomerate forming AA.[5] Later we revealed the phenomenon of HT deracemization at 500 °C of any aliphatic AAs (Ala, AABA, Ile, Leu, norLeu, norVal, tert-Leu, Val) independently of their racemate nature by means of sublimation in the mixtures with an enantiopure sublimable AA,[6] although decomposition is always a competitive pathway. A drastic synergistic effect was observed when the complexity of the system was increased and more racemic AAs was added to the initial mixtures. Based on the experimental results we put forward several important hypotheses on the obvious tendency of natural amino acids to homochiral self-organization[3] and on the “contagious” property of their homochirality observed experimentally under harsh non-selective conditions[6] what simulates realistic prebiotic environment. Continuing our research on HTS we studied thermally unstable proteinogenic AAs (Asn, Asp, Cys, Gly, Met, Sel, Ser, Thr, Trp). We observed that some of them (Cys, Ser, Thr) are able to give in low yields Ala and AABA.[7] HTS-decomposition of racemic or enantioenriched Ser gives racemic Ala among other products. The presence of one or several α-alkyl α-AAs (Ile, Leu, Val) allows the formation of Ala at a temperature (500 °C) where Ala was not observed starting from pure Ser. When Ser was heated in the presence of enantioenriched Val or Ile, Ala was obtained with significant enantiomeric excess with the same handedness than the starting Val or Ile, evidencing their role as an asymmetric inductor. HTS-decomposition of the thermally unstable Cys revealed the same behavior: formation of racemic Ala in the absence of another chiral AA and formation of enantioenriched Ala in the presence of enantioenriched Val or Ile. The main behavior of the sublimation reactions at high temperature of mixtures of AAs containing unstable derivatives and enantioenriched sublimable components is a common preservation of the sublimable α-alkyl α-AAs and an enantioenrichment of each. Among all the reactions considered as potentially prebiotic, the reiterative HTS of AAs has a particular place by its simplicity but mainly by a synergistic effect observed in complex mixtures leading to higher enantiomeric excess for each AA, this property being kept even in the presence of the many decomposition products of an unstable component.

Cite: Guillemin J. , Tarasevych A.V. , Vives T. , Snytnikov V.N.
Sublimation of Enantioenriched alpha-Amino Acids: A Path to Homochirality
XVIIIth International Conference on the Origin of Life 16-21 Jul 2017