Formation of Astrobiologically Important Molecules in Extraterrestrial Ices

The origin of life on Earth was likely to have been assisted by the availability of a wide variety of prebiotic molecules such as sugars, amino acids, and lipids. One of the key questions is how these molecules were originally formed, and with the recent discoveries of increasingly complex molecules in the interstellar medium suggests that these molecules need not have formed on the Earth, but could have been readily available from the material that formed the solar system. The recent identification of the simplest carbon hydrate, glycolaldehyde, and the simplest amino acid, glycine, in extraterrestrial space are significant milestones from an astrobiological viewpoint.

Glycolaldehyde represents the first member of monosaccharide sugar and denotes an important biomarker, which can react to glycolaldehyde phosphates and complex sugars such as ribose. The latter is the building block of RNA carrying the genetic information of the living organism. However, despite its fundamental importance in astrochemistry and biology, the formation of this molecule and its isomers acetic acid and methyl formate in extraterrestrial environments is unresolved. Here we simulate the harsh conditions of interstellar space in laboratory experiments and investigate synthetic routes how this important molecules glycolaldehyde and glycine can be formed.

A detailed retro synthesis utilizing concepts of non-equilibrium radical chemistry in low temperature ices was utilized to identify potential precursor mixtures of ice how both molecules can be formed. These are CO₂/CH₄ and CO/CH₃OH ices to form glycolaldehyde, acetic acid, and methyl formate and CO₂/CH₃NH₂ to synthesize interstellar glycine. Indeed, we were able to identify all three C₂H₄O₂ isomers in electron-irradiated ice samples. Glycine was also identified. Note that glycine does not exist in its neutral form. Due to the methylamine matrix, we found only deprotonated species, i.e. the H₂CNH₃COO- anion and its isomer.

W.M. Keck Laboratory in Astrochemistry Surface Scattering Machine