Chirality and the Origin of Life
The origin of the homochirality of biological molecules (the use in living organisms of only left-handed or L-amino acids and right-handed or D-sugars) has puzzled scientists since the chirality of molecules was discovered by Louis Pasteur more than 150 years ago. Recently it has been discovered that an excess of L-amino acids is present in the Murchison and Murray meteorites indicating that a preference for L-amino acids existed in solar system material before there was life on Earth. This supports an idea, first proposed by Rubenstein et al. (1983, Nature 306, 118), for an extraterrestrial origin for homochirality.
In this model the action of circular polarized light on interstellar chiral molecules introduced a left handed excess into molecules in the material from which the solar system formed. Some of this organic material then finds its way onto Earth via impacts of comets, meteorites and dust particles during the heavy bombardment phase in the first few hundred million years of the solar system. These molecules were then part of the prebiotic material available for the origin of life, and tipped the scales for life to develop with L-amino acids and D-sugars.
Rubenstein et al. originally proposed that synchrotron radiation from neutron stars in supernova remnants would be a suitable source of the required UV circularly polarized light. However, this interpretation is not supported by theory or observation which show that the circular polarization of these sources is very low. Our observations with the Anglo-Australian Telescope (at left) have shown suprisingly high circular polarizations (the red and white regions in the image) in the infrared light from reflection nebulae in the star forming regions Orion OMC1 and NGC 6334. Although we can only observe these regions at infrared wavelengths which can penetrate the thick dust clouds in which they are embedded, we predict that circular polarization should also be present at the ultraviolet wavelengths needed for asymmetric photolysis of molecules such as amino acids. We suggest that our own solar system formed in such a region of high circular polarization, leading to the excess of L-amino acids which we see in meteorites and to the homochirality of biological molecules. It is possible that without such a process operating it would not be possible for life to start. This may have implications for the frequency of occurrence of life in the universe.
Lucas, P.W., Hough, J.H., Bailey, J., Chrysostomou, A., Gledhill, T.M., McCall, A., 2005, UV Circular Polarisation in Star Formation Regions: The Origin of Homochirality?, Orig Life Evol. Biosphere, 35, 29-60 (ADS)
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Menard, F., Chrysostomou, A., Gledhill, T.M., Hough, J.H., and Bailey, J., 2000, High circular polarization in the star forming region NGC 6334: implications for biomolecular homochirality, in Bioastronomy 99: A new era in the search for Life, A.S.P. Conf Series, 213, 355 (ADS)
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