Amino Acids in Comets and Meteorites: Stability under Gamma Radiation and Preservation of Chirality

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.

💡 Research Summary

The paper investigates how the twenty L‑proteinogenic amino acids, when frozen in the solid state, respond to the cumulative gamma‑radiation dose that would be delivered by natural radionuclide decay in comets and asteroids over the age of the Solar System. The authors irradiated each amino acid to a dose of 3.2 MGy, which corresponds to the dose accumulated in 1.05 × 10⁹ years of radionuclide decay. After irradiation they measured two independent quantities: (1) the loss of crystalline enthalpy by differential scanning calorimetry (DSC), which provides a quantitative estimate of chemical decomposition (radiolysis), and (2) the decrease in optical rotation by optical rotatory dispersion (ORD) spectroscopy, which quantifies the loss of enantiomeric excess (radioracemization). From these data they derived a radiolysis rate constant (kdsc) and a radioracemization rate constant (krac) for each amino acid, expressed in units of Gy⁻¹.

The experimental results show that all amino acids retain a substantial fraction of their original material after 3.2 MGy. For many residues—alanine, glutamic acid, threonine, cysteine, among others—more than 70 % of the original mass remains; the average residual mass across the set is about 45 %. In terms of chirality, the loss of optical activity is modest: most amino acids exhibit less than a 10 % decrease in specific rotation, with the greatest loss observed for tryptophan (≈22 %). These findings indicate that the radical species generated by gamma‑irradiation do not preferentially destroy one enantiomer over the other, allowing a measurable enantiomeric excess to survive even under intense radiation.

Using the measured kdsc and krac values, the authors extrapolated the behavior to a total dose of 14 MGy, which is the estimated cumulative dose from radionuclide decay over 4.6 × 10⁹ years—the age of the Solar System. The extrapolation predicts that, even at this extreme dose, most amino acids would still be present at ≥5 % of their initial concentration, and several (e.g., alanine, glutamic acid) would retain >20 % of their original amount. Likewise, the predicted loss of enantiomeric excess remains limited, suggesting that a detectable chirality could survive from the early Solar Nebula to the present day.

The authors discuss the implications of these quantitative rate constants for reconstructing the original abundances of amino acids in the presolar nebula. Because the present concentrations measured in carbonaceous chondrites represent the survivors after billions of years of radiation exposure, the initial concentrations must have been higher. For certain amino acids—particularly isoleucine, tryptophan, and methionine—the authors estimate that the primordial abundance could have been up to six times the current meteoritic value. This upward revision has significant consequences for models of prebiotic chemistry, as it implies that a larger reservoir of biologically relevant molecules was available for delivery to the early Earth.

The paper also examines structural factors that influence radiation stability. Amino acids with short, non‑polar side chains tend to be more susceptible to radiolytic cleavage, whereas those possessing electron‑withdrawing carboxyl or amino groups can delocalize radical energy and thus exhibit greater resilience. Hydrogen‑bonding networks within the solid matrix appear to provide additional protection by dissipating energy and facilitating radical recombination.

In summary, the study provides three major contributions: (1) empirical evidence that all proteinogenic L‑amino acids can survive a cumulative gamma‑dose of 14 MGy with appreciable residual mass; (2) demonstration that radioracemization is limited, allowing a measurable enantiomeric excess to persist over Solar‑System timescales; and (3) a framework for back‑calculating original amino‑acid concentrations in the early Solar Nebula, suggesting that some residues may have been severalfold more abundant than today’s meteoritic measurements indicate. These results strengthen the case for an extraterrestrial contribution of both the quantity and the chirality of organic building blocks to the origin of life on Earth and provide essential parameters for future astrochemical modeling and sample‑return missions.