Asymmetric amplification in amino acidsublimation involving racemic compound to conglomerate conversion

Abstract

A straightforward unprecedented sublimation protocol that reveals both conversion of a racemic compound into a racemic conglomerate and subsequent enantioenrichment has been developed for the proteinogenic amino acid valine. The phenomenon has been observed in closed and open systems, providing insight into asymmetric amplification mechanisms under presumably prebiotic conditions.Searching for new mechanisms of mirror-symmetry breaking at a molecular level is of primary importance to unveil the origin and evolution of single chirality on Earth. 1 According to long held views, some kind of highly efficient and inexorable asymmetric amplification process would have operated to generate any visible enantiomeric excess (ee) thus paving the way to biomolecular homochirality. 2 Much interest has emerged recently from abrasive grinding of a mixture of two enantiomorphous crystals that affords a complete transformation to a solid state of single chirality; one enantiomer completely disappears in an irreversible autocatalysis that nurtures the opposite one. 3,4 Also enantioenrichment can be achieved by using physical equilibria (liquidsolid, gas-solid, or even liquid-liquid). This involves selfdisproportionation or fractionation processes where enantioenriched mixtures lead ultimately to an unequal distribution of enantiomers between the different phases. 5 Enantioenrichment in gas-solid systems represents a less travelled path than enantioselection in a solution-solid mixture. Results so far are promising, 6-10 but the situation with some primeval amino acids is somewhat puzzling and interpretations largely depend on sample preparations.y Unfortunately, in these studies the collected sublimate that was investigated only represents a small fraction of the total amount of material. The underlying principle behind such fractionations lies in the distinctive nature of the intermolecular interactions between a racemic compound (D and L molecules crystallize in the same heterochiral crystal) and its enantiomorphic crystals. In general, the overwhelming majority of chiral molecules crystallize as racemic compounds. However, to perform an enantiomeric resolution, the formation of racemic conglomerates is a prerequisite (D and L molecules crystallize in different crystals). Therefore a lot of effort has been undertaken to convert racemic compounds in racemic conglomerates. So far, there have been successful reports about these conversions in the solution, 11,12 but the sublimation from the gas phase has hardly been explored to make racemic conglomerates.Here we demonstrate that the amino acid valine, which crystallizes as a racemic compound, is first converted via sublimation into a racemic conglomerate that upon further heating enriches in the major enantiomer. Valine was chosen for this study as this proteinogenic amino acid was found to be non-racemic in meteoritic samples and the enantiomeric excesses closely match the values obtained by asymmetric photolysis under simulated interstellar conditions. 13 Racemic compound cryst...

Molecular Chirality in Meteorites and Interstellar Ices, and the Chirality Experiment on Board the ESA Cometary Rosetta Mission

“…The sublimation protocol recently introduced by Viedma et al [66] reveals both the conversion of a racemic compound into a racemic conglomerate and subsequent enantioenrichment; it might open new horizons for the enantiomeric purification of previously overlooked amino acids. The sublimation protocol recently introduced by Viedma et al [66] reveals both the conversion of a racemic compound into a racemic conglomerate and subsequent enantioenrichment; it might open new horizons for the enantiomeric purification of previously overlooked amino acids.…”

On the Physical Basis of Asymmetry and Homochirality

“…After examining sublimation of various amino acids at high temperature, surprises came with valine. This substance, which crystallizes as a racemic compound, converts into a conglomerate phase by sublimation . This behavior is unprecedented in amino acids, although such a conversion has been apparently reported for a norbornenol derivative .…”

Homochirality of Organic Matter – Objective Law or Curious Incident?

“…Alternatively, high enantioenrichment of the remaining in‐solution portions of an initial nonracemic mixture can be achieved after the major part of the compound precipitates in the form of racemic crystals . Also, sublimation can affect conversion of a racemic crystal into a conglomerate, resulting in an eventual enantioenrichment of the product . Of a similar nature is the earlier‐considered enantioselective adsorption onto chiral quartz crystals.…”