Life is handed. Science has known that ever since the time of Louis Pasteur tweezing apart crystals of naturally produced tartaric acid into left-handed and right-handed forms. The DNA double helix tends to spiral only one way in nature, the amino acids encoded by the genes and used to build proteins are almost invariably left-handed, so too are the proteins themselves and often the molecules with which they interact.
Why this is so, however, remains a mystery. This has led to scientists coming up with some quite bizarre theories to explain life’s handedness, from simple thermodynamics explanations to the effects of synchrotron light on the organic molecules that seeded life on Earth.
Sandra Pizzarello
On one hand, it would seem that nature could simply choose either chiral form of any molecule to play with. Many molecules can exist in two so-called handed or chiral forms – each, known as an enantiomer, is a non-superimposable mirror image of its counterpart – like a pair of hands. To the chemist this handedness in nature is more than academic.
One handed form of a molecule will react with another chiral molecule differently from the opposite enantiomer, just as a left hand will only react with a left-handed glove. One enantiomer of the molecule limonene tastes of oranges, the other lemons. Similarly, S-(+)-carvone smells like caraway while its mirror image, R-(–)-carvone, smells of spearmint.
To the pharmaceutical scientist this difference can mean the difference between an effective drug and an ineffective one, or worse still one with serious side effects. One enantiomer of the painkiller ibuprofen is three times stronger than its mirror image. Similarly, thalidomide the infamous morning sickness drug is effective in one chiral form, the other causes serious birth defects. Unfortunately, the two forms interchange in the body and so a safe form of thalidomide for morning sickness is not possible.
Now, Sandra Pizzarello of Arizona State University has discovered that some of the possible molecules that acted as precursors of life on Earth were present in one handed form at much higher levels than previously thought.
Writing in the Proceedings of the National Academy of Sciences, Pizzarello and colleagues Yongsong Huang (ASU) and Marcelo Alexandre (Brown University) have studied the organic compounds, including amino acids, found in a special group of asteroid fragments that fell to Earth as a meteorite and are almost as old as the Solar System itself at 4.5 billion years.
The mystery of life’s handedness, says Pizzarello, parallels in many ways the questions that surround the origin of life on Earth.
Previously, Pizzarello and ASU professor emeritus John Cronin had found a chiral bias among the amino acids from the Murchison meteorite, which landed in Australia in 1969. Just as with amino acids in living things there were more left-handed amino acids than right. Pizzarello’s new study is of an exceptionally pristine meteorite found in Antarctic ice.
Thanks to the pristine nature of this meteorite, we were able to demonstrate that other extraterrestrial amino acids carry the left-handed excesses in meteorites and, above all, that these excesses appear to signify that their precursor molecules, the aldehydes, also carried such excesses, Pizzarello explains, In other words, a molecular trait that defines life seems to have broader distribution as well as a long cosmic lineage.
Further reading
Proc. Natl Acad. Sci., 2008, in press;
http://dx.doi.org/10.1073/pnas.0709909105
Suggested searches
chirality
meteorites