Formation of Methylamine and Ethylamine in Extraterrestrial Ices and Their Role as Fundamental Building Blocks of Proteinogenic α-amino Acids

Abstract

The -CH-NH₂ moiety represents the fundamental building block of all proteinogenic amino acids, with the cyclic amino acid proline being a special case (-CH-NH- in proline). Exploiting a chemical retrosynthesis, we reveal that methylamine (CH₃NH₂) and/or ethylamine (CH₃CH₂NH₂) are essential precursors in the formation of each proteinogenic amino acid. In the present study we elucidate the abiotic formation of methylamine and ethylamine from ammonia (NH₃) and methane (CH₄) ices exposed to secondary electrons generated by energetic cosmic radiation in cometary and interstellar model ices. Our experiments show that methylamine and ethylamine are crucial reaction products in irradiated ices composed of ammonia and methane. Using isotopic substitution studies we further obtain valuable information on the specific reaction pathways toward methylamine. The very recent identification of methylamine and ethylamine together with glycine in the coma of 67P/Churyumov-Gerasimenko underlines their potential to the extraterrestrial formation of amino acids.