Life on Earth, as we know it today, is inseparable from homochirality: L-aminoacids, D-sugars and DNA are demonstrative enough examples. Though no chiral molecule has been identified so far in the interstellar medium, the presence of biological chiral molecules and of a measurable enantiomeric excess in the organic matter of well-defined families of meteorites (carbonaceous chondrites) is a strong support to the hypothesis of an extra-terrestrial formation of prebiotic elements, then delivered to the Earth by meteorites and comet infalls. However, the origin of this enantiomeric excess is still to be explained. Selective adsorption could be part of the answer.A comparative study of the adsorption of the R and S enantiomers of a-alanine and lactic acid on a chiral surface of hydroxylated a-quartz was carried out using numerical simulation techniques. Structurally different adsorption sites were found with quite different adsorption energies for the same molecule-surface couple, raising the problem of considering adsorption as a local property or as a global response characteristic of the whole surface. To solve this dilemma we propose a two-step statistical approach.A first screening is performed to detect the possible adsorption sites on the hydroxylated surface using semi-empirical density functional tight binding theory (DFTB+). Then, a statistical treatment is done, based on the occurrence of each adsorption site whose energy is recalculated using first principle periodic DFT. The a-quartz is found poorly selective in the present cases, but nevertheless it shows that, even if the adsorption energies are close, R-alanine and S-lactic acid are preferentially adsorbed.For this mechanism of selective adsorption to be efficient, it seems necessary to find more complex natural surfaces whose geometrical characteristics/defects could be able to generate stronger interactions and above all, more interactions specific of each enantiomer.

Enhancing homochirality by enantio-selective adsorption: A statistical approach

248th National Meeting (2014)

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