In the lab, two chemical products are normally made. The product is a racemic mixture, meaning that it is composed of molecules which are structurally identical, but which are mirror images of each other. Mysteriously, in biological processes, only one of these mirror images is made.
The pair of mirror molecules are called enantiomers, and arise from the chirality of the molecule. If a molecule is chiral, its mirror image is not superimposable onto the original molecule. A reaction gives one or both mirror images, which are labelled right handed (D enantiomers) or left handed (L enantiomers). In nature, normally only the right handed version of any sugars, and the left handed enantiomer of any amino acids is made, allowing the reactions within the body to be so selective.
But where does this homochirality come from? Analysis of compounds found on meteorites may hold the answer. Meteorites are pieces of rock or metal which have fallen to the Earth’s surface from outer space. They are likely to have been the main way in which the building blocks for life were initially delivered to Earth billions of years ago. George Cooper and Andro Rios, from NASA’s Ames Research Center in California recently analysed samples from carbonaceous meteorites 4.5 billion years old. They show the earliest chemistry in the solar system and have revealed an excess of right handed sugar enantiomers on the surface. It’s reasonable to guess that the asymmetry found in nature is likely to have come from space.
It’s unknown why this excess exists on the meteorites. Scientists have suggested that it may be due to circularly polarised light, electromagnetic waves with the direction of the electric field rotating, present when the molecules were synthesised. The next step is to test this, by using circular polarised light in simulation experiments to see if this is what could have caused it. Scientists also want to test more samples, maybe those collected from future space missions like the OSIRIS-Rex mission to asteroid 101955 Bennu, which launches in September.
George Cooper and Andro C. Rios, Proceedings of the National Academy of Sciences of the United States of America, 22nd April 2016, Enantiomer excesses of rare and common sugar derivatives in carbonaceous meteorites, http://www.pnas.org/content/early/2016/05/25/1603030113 (accessed 01/06/2016)
Emma Stoye, Chemistry World, 1st June 2016, Meteorite compounds hint at origins of life’s asymmetry, http://www.rsc.org/chemistryworld/2016/06/meteorite-origin-life-asymmetry-homochirality-sugar (accessed 01/06/2016)