Amino acids, among other uses, are the building blocks of proteins, and form one of the basic classes of important biological compounds. Amino acids can be constructed in two forms, left-handed and right-handed. Life uses only left-handed amino acids, and one question biologists have asked is why life uses left-handed rather than right-handed amino acids.
Researchers examining the Tagish Lake meteorite that fell over Canada in 2000 have found that the meteorite contains four times as much of the left-handed amino acid aspartic acid than it does of the right-handed version, while it only has a slight excess of the left-handed form of the amino acid alanine. The researchers note that if the amino acids in the meteorite were created by Earth-based organisms, there should be an overwhelming dominance of left-handed forms in all types amino acids. The difference between the two, they say, is a sign that the amino acids in the meteorite originated not in terrestrial biology, but in inorganic chemical processes in space. They confirmed this by analyzing the carbon isotopes in the amino acids, which also match the isotope distribution expected in a a space-based rather than a terrestrial chemical process.
The team concludes that this analysis provides evidence that space-based chemical processes can lead to a predominance of left-handed over right-handed forms of some amino acids in meteoroids. Previous studies have shown that effects such as polarized light can influence chemical reactions in such a way as to produce more of the left-handed than right-handed forms. The team also notes that some crystallization processes that begin with a small imbalance can, by serving as the template for further chemical production, amplify small imbalances into large ones—they suggest this explains why the imbalance in aspartic acid is greater than that in alanine in the Tagish Lake meteorite.
If early life on Earth made use of organic molecules delivered to the Earth from meteorites, the discovery that chemical processes in space generate predominantly left-handed forms of amino acids helps explain how it is that life on Earth todayuses left-handed amino acids exclusively.
(NASA)
Image: NASA