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Amino Acids and Their Production during the Photolysis of Astrophysically relevant Ices

Amino acids are important biological molecules that serve as the basic molecular building blocks of proteins and enzymes used by all living things on Earth. All amino acids have a similar basic structure like that below, with different amino acids differing only in the specific structure that lies in the location labeled with the “R”.

Figure 1: The basic structure of all amino acids.

As two examples, here are schematics of the simple amino acids glycine (where R = H, a hydrogen atom) and alanine (where R = CH3, a methyl group).

Figure 2: The structures of the simple amino acids glycine (left) and alanine (right).

The chemical group at position “R” can be almost anything, but life on Earth uses slightly over 20 different amino acids. Other amino acids are possible, however. For example, over 70 different amino acids have been identified in meteorites. Why life uses only about 20 of all the possible amino acids is not presently clear.

One of the peculiar properties of amino acids is that they are “chiral” or “handed”. Because of the arrangement of chemical groups around the ‘central’ carbon atom of an amino acid, it is possible to make two versions of an amino acid that have identical chemical formulas but that are geometrically different, each being the mirror image of the other. The situation is somewhat similar to your two hands. They have the same ‘formula’ (Palm1Fingers4Thumb1), but no amount of rotating of either hand can get the two to perfectly superpose. In keeping with your hands, these two variants of an amino acid are often referred to as being ‘right’ and ‘left’ handed versions of the same molecule.

Figure 3: Amino acids come in right and left handed versions.

Our work has shown that when mixed molecular ices of the sorts we see in space are irradiated by UV photons, amino acids are one of the types of products that are created. So far we have only been able to detect the presence of some of the simpler amino acids in our photolysis residues. Below is a figure showing high-performance liquid chromatography data from a residue made by the UV photolysis of an ice containing H2O, CH3OH, CO, and NH3. Note the double peaks for serine and alanine, showing we have made both right and left handed versions of these molecules (glycine, where R = H, is not chiral).

Figure 4: UV photolysis of ices containing C, H, O, and N yield amino acids as products.

Extensive sets of experiments have shown that the production of amino ices during ice photolysis is a relatively robust process. Most ices that contain C, H, O, and N within their original molecular components will yield some amino acids when irradiated by ionizing radiation. More details about our work on amino acids can be found in the references listed at the end of this page.

This work shows that chemistry that occurs in space can lead to the production of amino acids, one of a number of materials critical to lie on Earth and a material whose presence likely played a key role in the origin of life on Earth. Since the process of ice photolysis is thought to be occurring wherever new stars and planets are formed, this implies amino acids may well be introduced to the surfaces of all newly formed planets.

Amino acids are known to be present in primitive meteorites and many of these amino acids show the presence of excess deuterium (heavy hydrogen). Excess deuterium is an indicator of the presence of extraterrestrial materials, clearly demonstrating that extraterrestrial organic amino acids exist, survive planetary accretion, and are delivered to the Earth’s surface. Thus, the abiotic production of amino acids in space, along with quinones, amphiphiles, hexamethylenetetramine, and nucleobases, may play a key role in the seeding of the surfaces of new planets with many of the building blocks needed to start life throughout the universe. It is interesting to note that, while meteorites are known to deliver over 70 varieties of amino acids to the Earth, modern life on the Earth uses only a fraction of these. Why living systems use the particular amino acids they do is not currently understood.

Elsila, J. E., Dworkin, J. D., Bernstein, M. P., & Sandford, S. A., 2007, "Mechanisms of Amino Acid Formation in Interstellar Ice Analogs", Astrophys. J., 660, 911-918.

Bernstein, M. P., Bauschlicher, C.W. Jr., & Sandford, S. A., 2004, "The Infrared Spectrum of Matrix Isolated Aminoacetonitrile, a Precursor to the Amino Acid Glycine", Advances in Space Research, 33, 40-43.

Bernstein, M. P., Ashbourn, S., Sandford, S. A., & Allamandola, L. J., 2004, "The Lifetimes of Nitriles (-C≡N) and Acids (-COOH) During Ultraviolet Photolysis and Their Survival in Space", Astrophys. J., 601, 365-370.

Bernstein, M. P., Dworkin, J. P., Sandford, S. A., Cooper, G. W., & Allamandola, L. J., 2002, "The Formation of Racemic Amino Acids by Ultraviolet Photolysis of Interstellar Ice Analogs", Nature, 416, 401-403.