Interstellar methane as progenitor of amino acids?

With these research results, the team led by Weixin Huang at the University of Science and Technology of China (Hefei) has contributed to our fundamental understanding of the early development of molecules in the universe. "Gamma rays, high-energy photons commonly existing in cosmic rays and unstable isotope decay, provide external energy to drive chemical reactions of simple molecules in the icy mantles of interstellar dust and ice grains," states Huang. "This can result in more complex organic molecules, presumably starting from methane (CH₄), which is widely present throughout the interstellar medium."

Although higher pressures and temperatures reign on Earth and on planets in the so-called habitable zone, most studies of cosmic processes are only simulated under vacuum and at extremely low temperatures. In contrast, the Chinese team studied the reactions of methane at room temperature in the gas and aqueous phases under irradiation with a cobalt-60 emitter.

The composition of the products varies depending on the starting materials. Pure methane reacts -- with very low yield -- to give ethane, propane and hydrogen. The addition of oxygen increases the conversion, resulting mainly in CO₂ as well as CO, ethylene, and water. In the presence of water, aqueous methane reacts to give acetone and tertiary butyl alcohol; in the gas phase, it gives ethane and propane. When both water and oxygen are added, the reactions are strongly accelerated. In the aqueous phase, formaldehyde, acetic acid, and acetone are formed. If ammonia is also added, acetic acid forms glycine, an amino acid also found in space. "Under gamma radiation, glycine can be made from methane, oxygen, water, and ammonia, molecules that are found in large amounts in space," says Huang. The team developed a reaction scheme that explains the routes by which the individual products are formed. Oxygen (∙O₂⁻) and ∙OH radicals play an important role in this. The rates of these radical reaction mechanisms are not temperature-dependent and could thus also take place in space.

In addition, the team was able to demonstrate that various solid particles that are components of interstellar dust -- silicon dioxide, iron oxide, magnesium silicate, and graphene oxide -- change the product selectivity in different ways. The varied composition of interstellar dust may thus have contributed to the observed uneven distribution of molecules in space.

Silicon dioxide leads to a more selective conversion of methane to acetic acid. Says Huang, "because gamma radiation is an easily available, safe, and sustainable source of energy, this could be a new approach for using methane as a carbon source that can be efficiently converted into value-added products under mild conditions -- a long-standing challenge for industrial synthetic chemistry."