Amino acids in our ice experiments, and in meteorites

Amino acids are very important biological molecules. Amino acids are the basic molecular components of our proteins and enzymes and thus to a great extent all living things are made of amino acids. This is an image of a small amino acid called alanine that is very common in your body. Click on the picture for a larger image. Click here to learn more about this structure and what it means.

It turns out that although we think of amino acids as being bio-molecules, they are also found in meteorites. We and many other space scientists are very interested in the presence of amino acids and related molecules in meteorites, for a number of reasons. First, it seems that molecules from space helped to make the Earth habitable, and may have even had something to do with the evolution or even origin of life, so we want to know all about these life-like molecules in meteorites so we can better understand the chemical environment on the early Earth at the time of the rise of life. Second, if we are to find Life elsewhere in the Solar System we need some molecules that we can use as indicators of Life. If amino acids and related compounds are commonly delivered to the surface of dead planets by meteorites, then we need to know which ones and how much so that we are not fooled into thinking there might be life where there is really only meteorite debris.

We know that at least some of these amino acids in meteorites are genuine extraterrestrial molecules, and not contamination. The strongest such evidence is the fact that these molecules are enriched in heavy isotopes. For example, they often carry heavy hydrogen atoms (having an extra neutron also called Deuterium) in place of normal hydrogen. They also are sometimes have carbon atoms that have an extra neutron (¹³C, said Carbon thirteen). These isotopes do exist on Earth, but very rarely, so the amount (and the fact that they are found in meteorites) indicates that these molecules formed in space. Another evidence pointing these amino acids being extraterrestrial is their chirality, or handedness. Many amino acids (like alanine, shown above) can exist in non identical left and right handed forms, but Life on earth only uses the left. In A non-biological synthesis like the Strecker reaction usually gives you equal parts fo left and right, and that (roughly) is what is seen in meteorites. For more information on this chirality/handedness thing follow this link to our amino acid chirality page. OK, so these amino acids are extraterrestrial, but how did they form?

It has always been assumed that these amino acids formed by the Strecker reaction, in which three simple molecules (ammonia, an aldehyde, and hydrogen cyanide) get together in liquid water. Reactions in water in space are not so crazy as it might sound at first, since there is evidence that one can have liquid water at the center of a large asteroid or comet, and these are believed to be where such meteorites come from, so its reasonable. However, there is also evidence that the liquid water that percolated through what became the Murchison meteorite, for example, was depleted of deuterium, so if these molecules formed in this liquid water how did they manage to emerge deuterium enriched?

We think that its also possible that some of the amino acids that are seen in meteorites came from reactions in the primorial interstellar ice grains from which the Solar System formed. (follow this link, to learn more about these cold environments). In our experiments reported in Nature in 2002, we describe how we simulated the Ultraviolet photolysis of interstellar ice grains and formed the amino acids glycine, alanine, and serine, (as well as related hydroxy acids and alcohols like glycerol that are also seen in meteorites). We think that the deuterium enrichment of these small amino acids in meteorites argues for a low temperature ice synthesis since, deuterium is typically enriched in very low temperature environments. Please follow the link below to the PDF to read the entire text of our Nature article. Or you can just read the press release and popular press coverage of this article.

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