  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 gamma 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 gamma radiation to a dose of 3.2 MGy which is the dose equivalent to that
derived by radionuclide decay in comets and asteroids in 1.05x109 years. For
each amino acid 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 4.6x109 years, the age of
the Solar System. It is shown that all the amino acids studied can survive a
radiation dose of 14 MGy in significant quantity although part of them are lost
in radiolytic processes. Similarly, also the radioracemization process
accompanying the radiolysis does not extinguish the chirality. 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.
