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.
Publications:
Bailey, J. 2000, Astronomical Sources of Circularly Polarized
Light and the Origin of Homochirality, Orig. Life. Evol Biosphere
in
press. preprint
pdf
Bailey, J. 2000, Circular Polarization and the Origin
of Biomolecular Homochirality, Presented at Bioastronomy 99,
Hawaii, A.S.P. Conf Series in press. preprint
pdf
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, Presented at Bioastronomy
99, Hawaii, A.S.P. Conf Series in press. preprint
Bailey, J., 2000, Chirality and the Origin of Life, Acta
Astronautica, 46, 627-631. preprint
pdf
Chrysostomou, A., Gledhill, T.M., Menard, F., Hough, J.H., Tamura, M.
and Bailey, J., 2000, Polarimetry of Young Stellar Objects - III.
Circular Polarimetry of OMC-1, Mon. Not. R. Astron. Soc. 312,
103-115. preprint
pdf
Bailey, J., Chrysostomou, A., Hough, J.H., Gledhill, T.M., McCall, A.,
Clark, S., Menard, F. and Tamura, M., 1998, Circular Polarization
in Star Forming Regions: Implications for Biomolecular Homochirality,
Science,
281,
672-674, preprint
pdf
figure
1 (colour eps)
Other Links:
AAO press release
Polarized
Life (Scientific American)
ABC
News report
The Sinister
Cosmos (Scientific American)
Life
From the Stars (Ad Astra)
Space
Industry New Report