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 γ radiation photolysis. All the 20 chiral amino acids in the levo form used in the proteins of the current terrestrial biochemistry have been 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 1.05x10 9 years. For each amino acids the radiolysis degree and the radioracemization degree was measured by differential scanning calorimetry (DSC) and by optical rotatory dispersion (ORD) spectroscopy. From these measurements a radiolysis rate constant kdsc and a radioracemization rate constant krac have been determined for each amino acid and extrapolated to a dose of 14 MGy which corresponds to the expected total dose delivered by the natural radionuclides decay to all the organic molecules present in comets and asteroids in 4.6x10 9 years, the age of the Solar System. It is shown that all the amino acids studied can survive a radiation dose of 14 MGy although part of them are lost in radiolytic processes. Similarly also the radioracemization process accompanying the radiolysis does not extinguish the initial enantiomeric enrichment. The knowledge of the radiolysis and radioracemization rate constants may permit the calculation of the original concentration of the amino acids at the times of the formation of the Solar System starting from the concentration found today in carbonaceous chondrites. For some amino acids the concentration in the presolar nebula could have been up to 6 times higher than currently observed in meteorites. It is also expected the preservation of an original enatiomeric excess. 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 in comets and asteroids at the epoch of the Solar System formation.