Amino acids in comets and meteorites: stability under gamma radiation and preservation of the enantiomeric excess

Amino acids in Solar system bodies may have played a key role in the chemistry that led to the origin of life on Earth. We present laboratory studies testing the stability of amino acids against high energy radiation. All the 20 chiral amino acids in the levo form found in the proteins of the current terrestrial biochemistry were irradiated in the solid state with γ-radiation to a dose of 3.2 MGy, which is the dose equivalent to that produced by radionuclide decay in comets and asteroids in 1.05 x 10 9 yr. For each amino acid the radiolysis degree and the radioracemization degree were measured by differential scanning calorimetry and by optical rotatory dispersion spectroscopy. From these measurements, a radiolysis rate constant k dsc and a radioracemization rate constant k rac were determined for each amino acid and extrapolated to a dose of 14 MGy, which corresponds to the expected total dose delivered by natural radionuclide decay to all the organic molecules present in comets and asteroids in 4.6 x 10 9 yr, the age of the Solar system. It is shown that all the amino acids studied can survive a radiation dose of 14 MGy, although certain fractions of them are lost as a result of radiolytic processes. Similarly, the radioracemization process accompanying the radiolysis does not extinguish the initial enantiomeric enrichment. Knowledge of the radiolysis and radioracemization rate constants may permit the calculation of the original concentrations of the amino acids at the time of the formation of the Solar system, starting from the concentration found today in carbonaceous chondrites. For some amino acids the concentration in the pre-solar nebula could have been up to 6 times higher than that currently observed in meteorites. The preservation of an original enatiomeric excess is also expected. This study adds experimental support to the suggestion that amino acids were formed in the interstellar medium and in chiral excess and then were incorporated into comets and asteroids at the epoch of Solar system formation.