Paris, 17 Oct 2002
Our bodies contain proteins that are made of smaller molecules that can be either left- or right-handed, depending upon their structure. Regardless of which hand we use to write, however, all human beings are 'left-handed' at the molecular level. Life on Earth uses the left-handed variety and no one knows how this preference crept into living systems. In 2012, ESA's Rosetta lander will land on a comet to investigate, among other things, if the origin of this preference lies in the stars.
Living cells use tiny organic molecules (called amino acids) to build proteins in the same way as children build things out of Lego bricks. Most amino acids come in two mirror-image varieties, right- and left-handed. The arrangement of the thumb and four fingers on a left hand is the mirror image of the arrangement on the right. In amino acids, the arrangement of the atoms determines whether the molecule is left- or right-handed.
Uwe Meierhenrich, at the University of Bremen, Germany, thinks that the Earth's early supply of amino acids came from space, carried by comets. He is part of a European team who reproduced the way organic molecules form in space, to try to understand what the Rosetta lander might find on Comet Wirtanen in 2012.
At a laboratory in Leiden, The Netherlands, they lowered the temperature of a chamber to -261°C, pumped out the air, and injected a rarefied mixture of molecules known to exist in space: water, ammonia, and simple carbon molecules. These molecules froze onto artificial dust grains inside the chamber. They then shone an ultraviolet lamp onto the samples, to simulate starlight. "Our aim was to simulate interstellar conditions as accurately as possible. We did not adapt the conditions to produce amino acids," says Meierhenrich.
Using a version of Rosetta's Cometary Sampling and Composition (COSAC) experiment, they found newly formed right- and left-handed amino acids in equal quantities. Earthly life, however, uses only left-handed amino acids. Experiments during the 1950s showed that adding right-handed amino acids to proteins stopped the proteins from growing. Was Earth supplied with more left-handed amino acids than right-handed ones in the beginning, allowing life to begin? One famous study has suggested this. Meierhenrich says, "The excess (of left-handed amino acids) found in the Murcheson meteorite is really small and some of the analyses are controversial. Rosetta will give us a much clearer picture."
Some scientists think the key to creating an excess of left over right is in the type of ultraviolet light that shines on the amino acids. When ultraviolet starlight strikes dust grains, it can begin to twist - either clockwise or anticlockwise. Depending on the direction in which it is twisting, it destroys one handedness of amino acid more than the other. In 1998, an international group of astronomers discovered large amounts of 'twisty' light occurring naturally in the dusty cocoons of some young stars, where planets might be forming. "I think this is the most probable origin of the excesses," says Meierhenrich who is now using a Paris Laboratory where scientists can twist ultraviolet light to recreate these conditions.
We may well find that Rosetta's findings on Comet Wirtanen reliably indicate that our molecular left-handedness is indeed a legacy of the stars.
European Space Agency
http://www.esa.int/export/esaCP/index.ht ml