Radiolysis and radioracemization of 20 amino acids from the beginning of the Solar System

Abstract

A series of chiral amino acids in the levo form used in the current terrestrial biochemistry were irradiated in the solid and dry state with γ-radiation to a dose of 3.2 MGy which is the dose equivalent to that derived by radionuclide decay in comets and asteroids in 1.05 × 109 years at a depth >20 m. For each amino acids, the radiolysis degree and the radioracemization degree was measured, respectively by differential scanning calorimetry (DSC) and by optical rotatory dispersion (ORD) 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 the natural radionuclides decay to all the organic molecules present in comets and asteroids in 4.6 × 109 years, the age of the Solar System. It is shown that all the amino acids studied survive a radiation dose of 3.2 MGy although part of them is lost in radiolytic processes. Similarly also the radioracemization process accompanying the radiolysis does not extinguish the initial enantiomeric enrichment. Even the extrapolation to 14 MGy corresponding to 4.6 × 109 years shows the partial survival of all amino acids studied and their 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. Based on these results, it is not at all a surprise that amino acids have been found in meteorites and in measureable chiral excess. Furthermore, the amino acids showing the best level of radiolysis and radioracemization resistance are just those commonly found in enantiomeric enrichment in meteorites.