Tim Thompson

For the most part, classical thinking on abiogenesis assumes that prebiotic chemistry was of essentially terrestrial origin. However, in the last decade or so, there has been increasing attention to the possibility that the prebiotic building blocks of life did not form on the earth, but rather were delivered to it from space.

But you might think that only transfers the problem to a more difficult environment. If such molecules don't readily form on a nice warm earth, how do they form in the frozen depths of space?

Most people don't realize that cold space is in fact a very chemically active place. Hundreds, if not by now thousands, of chemical reactions have been cataloged, all of which occur at temperatures below 100K, and as low as 10K, in gas phase, and on grain surfaces, in the interstellar medium. As of October 24, 2001, <a href="http://www.cv.nrao.edu/~awootten/allmols.html" target="_blank">there are 122 molecules known to exist</a> in the interstellar medium, or in comets. There are no definitive detections of amino acids, though I think there is one tenetative.

Could amino acids form in space? As the two papers reported below, from this week's issue of <a href="http://www.nature.com" target="_blank">Nature</a>, the answer may well be "yes".

In the first paper (Bernstein et al.), the authors created an analog of interstellar ice grains, by depositing a vapor on a substrate at a temperature of 15K, forming ices primarily of amorphous H2O, with varying mixtures of NH3, CH3OH and HCN., at a pressure of 10^-8 Torr (1.33 x 10^-8 millibar), all conditions representative of the interstellar medium. The ice was exposed to UV radiation, once again at intensities characteristic of the interstellar medium. Isotopic labeling, and control experiments without UV irradiation were used to determine that detected amino acids were not the result of contamination.

The results were that after irradiation, the products included N-formal glycine, cycloserine (4-amino-3-isoxazolidinone) and glycerol, before hydrolysis. After hydrolysis, glycine, alanine, serine, glycerol, ethanolamine and glyceric acid were all detected, with tentative detections of amino malonic acid and hydantoins. The mixture was racemic (equal amounts of right & left handed), which also indicates against contamination, since contamination would likely be 100% left handed, or close to it.

In the second paper (G.M. Muñoz Caro et al.), the authors did much the same as had the previous group. They created an analog interstellar ice, and exposed it to UV. This time the ice consisted of H2O, CH3OH, NH3, CO and CO2, with H2O at twice the abundance of the other molecules (by molecular composition). This is a slightly different ice mixture than the first group used. Temperature and pressure were similar, in this case 12K and 10^-7 millibar.

In this second case, the authors retreived 16 different amino acids, including glycint, alanine, valine, proline, serine, and aspartic acid. Isotopic tagging was used to show absence of contamination, and the products were also racemic, as in the previous example.

These two papers together represent the first experimental demonstrations that amino acids can form, under conditions that replicate those of the interstellar medium. We already know that amino acids are found in meteorites, and these experiments suggest that those amino acids could have formed in interstellar ice grain mantles exposed to stellar ultraviolet. Or, they may have formed as by products of ice grain chemicals, in the meteorites, after the meteorite warmed a bit. In either case, this is important new evidence to support the proposition that at least some of the prebiotic chemicals that give rise to abiogenesis, did not form on earth, but came here in comets & asteroids, or even as interplanetary dust.

Racemic amino acids from the ultraviolet photolysis of interstellar ice analogues
<a href="http://www.nature.com" target="_blank">Nature</a> 416: 401-403, March 28, 2000

Max P. Bernstein, et al.

Abstract: The delivery of extraterrestrial organic molecules to Earth by meteorites may have been important for the origin and early evolution of life. Indigenous amino acids have been found in meteorites - over 70 in the Murchison meteorite alone. Although it has been generally accepted that the meteoritic amino acids formed in liquid water on a parent body, the water in the Murchison meteorite is depleted in deuterium relative to the indigenous organic acids. Moreover, the meteoritical evidence for an excess of laevo-rotatory amino acids is hard to understand in the context of liquid-water reactions on meteorite parent bodies. Here we report a laboratory demonstration that glycine, alanine and serine naturally form from ultraviolet photolysis of the analogues of icy interstellar grains. Such amino acids would naturally have a deuterium excess similar to that seen in interstellar molecular clouds, and the formation process could also result in enantiomeric excesses if the incident radiation is circularly polarized. These results suggest that at least some meteoritic amino acids are the result of interstellar photochemistry, rather than formation in liquid water on an early Solar System body.

Amino acids from ultraviolet irradiation of interstellar ice analogues
<a href="http://www.nature.com" target="_blank">Nature</a> 416: 403-406, March 28, 2000

G.M. Muñoz Caro, et al.

Abstract: Amino acids are the essential molecular components of living organisms on Earth, but the proposed mechanisms for their spontaneous generation have been unable to account for their presence in Earth's early history. The delivery of extraterrestrial organic compounds has been proposed as an alternative to generation on Earth, and some amino acids have been found in several meteorites. Here we report the detection of amino acids in the room-temperature residue of an interstellar ice analogue that was ultraviolet-irradiated in a high vacuum at 12 K. We identified 16 amino acids; the chiral ones showed enantiomeric separation. Some of the identified amino acids are also found in meteorites. Our results demonstrate that the spontaneous generation of amino acids in the interstellar medium is possible, supporting the suggestion that prebiotic molecules could have been delivered to the early Earth by cometary dust, meteorites or interplanetary dust particles.

Just to make the story complete, earlier studies on the stability of amino acid molecules in space are encouraging. Once formed, they are subject to fairly rapid destruction by the same UV that made it possible, unless they are protected in ice mantles on interstellar grains, or in a dense cloud protected from UV (The photostability of Amino Acids in Space, P. Ehrenfreund et al., <a href="http://www.journals.uchicago.edu/ApJ/about.html" target="_blank">Astrophysical Journal</a> Letters 550: L95-L99, March 20, 2001). There is also evidence, as suggested in both papers, that if the UV impacting the ice is circularly polarized, the result could be a non-racemic product. There is some experimental evidence to support this view (Mechanism of pH-dependent photolysis of aliphatic amino acids and enantiomeric enrichment of racemic leucine by circularly polarized light, H. Nishino et al., <a href="http://pubs.acs.org/journals/orlef7/" target="_blank">Organic Letters</a> 3(6): 921-924, March 22, 2001), and it is also evident that the necessary environment can be found in space (Astronomical sources of circularly polarized light and the origin of homochirality, J. Bailey, <a href="http://www.kluweronline.com/issn/0169-6149/contents" target="_blank">Origins of Life and Evolution of the Biosphere</a> 31(1-2): 167-183, Feb-Apr, 2001).

The "salting" of earth, with prebiotic amino acids from space, appears to be a sound scientific proposal.

Coragyps

I like it! Ain't chemistry wonderful?

Dr.GH

Thanks for the links. Gary

Baloo

The one thing about the theory that doesn't ring true to me: it seems like an organic soup formed by terrestrial processes would be much, much larger and would therefore have a much better chance of producing the "original self-replicator" than a "small soup" formed around an impact crater of an extraterrestrial object. Then again, my understanding of abiogenesis is fairly out-dated...