Web address:
http://www.sciencedaily.com/releases/2011/01/
110107145634.htm
Origin of Life on Earth: 'Natural' Asymmetry of Biological Molecules May Have
Come from Space
enlarge
The circularly polarized ultraviolet light (UV-CPL) beam on the DESIRS beamline
at the SOLEIL synchrotron facility, shown as it passes through a gas filter
filled with xenon. (Credit: © Thomas Lannes)
ScienceDaily (Jan. 7, 2011) Certain molecules do exist in two forms which are
symmetrical mirror images of each other: they are known as chiral molecules. On
Earth, the chiral molecules of life, especially amino acids and sugars, exist
in only one form, either left-handed or right-handed. Why is it that life has
initially chosen one form over the other?
A consortium bringing together several French teams led by Louis d'Hendecourt
(1), CNRS senior researcher at the Institut d'astrophysique spatiale
(Université Paris-Sud 11 / CNRS), has for the first time obtained an excess of
left-handed molecules (and then an excess of right-handedones) under conditions
that reproduce those found in interstellar space. This result therefore
supports the hypothesis that the asymmetry of biological molecules on Earth has
a cosmic origin. The researchers also suggest that the solar nebula formed in a
region of massive stars.
This work has just been published online on the web site of The Astrophysical
Journal Letters. The experiment was carried out at the SOLEIL synchrotron
facility in collaboration with the Laboratoire de chimie des molécules
bioactives et des arômes (Université de Nice/CNRS) and with the support of CNES.
Chiral molecules are molecules that can exist in two forms (enantiomers) which
are symmetrical mirror images of each other, one left-handed and the other
right-handed. For instance, our hands are chiral since they come in two forms,
the left hand and the right hand, that are symmetrical with their mirror image
but not super imposable on it. Biological molecules are mostly chiral, with
some forms being favored over others. For instance, the amino acids that make
up proteins only exist in one of their two enantiomeric forms, the left-handed
(L) form. On the other hand, the sugars present in the DNA of living organisms
are solely right-handed (D). This property that organic molecules have of
existing in living organisms in only one of their two structural forms is
called homochirality.
What is the origin of such asymmetry in biological material? There are two
competing hypotheses. One postulates that life originated from a mixture
containing 50% of one enantiomer and 50% of the other (known as a racemic
mixture), and that homochirality progressively emerged during the course of
evolution. The other hypothesis suggests that asymmetry leading to
homochirality preceded the appearance of life and was of cosmic origin. This is
supported by the detection of L excesses in certain amino acids extracted from
primitive meteorites. According to this scenario, these amino acids were
synthesized non-racemically in interstellar space and delivered to Earth by
cometary grains and meteorites.
To lend more weight to this hypothesis, the researchers first reproduced
analogs of interstellar and cometary ices in the laboratory (2). The novel
aspect of their experiment was that, using the DESIRS beamline at the SOLEIL
synchrotron facility, the ices were subjected to circularly polarized
ultraviolet radiation (UV-CPL) (3), which is supposed to mimic the conditions
encountered in some space environments. When the ices were warmed up, an
organic residue was produced. A detailed analysis of this mixture revealed that
it contained a significant enantiomeric excess in one chiral amino acid,
alanine. The excess, which was over 1.3%, is comparable to that measured in
primitive meteorites. The researchers thus succeeded in producing, under
interstellar conditions, asymmetrical molecules of life from a mixture that did
not contain chiral substances. This is the first time that a scenario that
explains the origin of this asymmetry has been demonstrated using an experiment
that reproduces an entirely natural synthesis.
This result reinforces the hypothesis that the origin of homochirality is
prebiotic and cosmic, in other words genuinely interstellar. According to this
scenario, the delivery of extraterrestrial organic material containing an
enantiomeric excess synthesized by an asymmetrical astrophysical process (in
this case, UV-CPL radiation) is the cause of the asymmetry of life's molecules
on Earth. This material may even have formed outside the solar system. Finally,
the solar nebula may have formed in regions of massive star formation. In such
regions, infrared radiation circularly polarized in the same direction has been
observed.
These findings imply that the selection of a single enantiomer for the
molecules of life observed on Earth is not the result of chance but rather of a
deterministic physical mechanism.
Notes:
(1) 2003 CNRS Silver Medal
(2) In the 1980s, Louis d'Hendecourt developed a technique to generate
interstellar ice analogs in the laboratory.
(3) The Orion nebula produces circularly polarized light at levels of 17% in
the infrared. It is calculated that it also radiates in the ultraviolet,
radiation which is able to break the (strong) covalent bonds betweens the atoms
of ice molecules.
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Story Source:
The above story is reprinted (with editorial adaptations by ScienceDaily
staff) from materials provided by CNRS (Délégation Paris Michel-Ange), via
AlphaGalileo.
Journal Reference:
1. Pierre de Marcellus, Cornelia Meinert, Michel Nuevo, Jean-Jacques
Filippi, Grégoire Danger, Dominique Deboffle, Laurent Nahon, Louis Le Sergeant
d'Hendecourt, Uwe J. Meierhenrich. Non-racemic amino acid production by
ultraviolet irradiation of achiral interstellar ice analogs with circularly
polarized light. The Astrophysical Journal, 2011; 727 (2): L27 DOI:
10.1088/2041-8205/727/2/L27
Need to cite this story in your essay, paper, or report? Use one of the
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MLA
CNRS (Délégation Paris Michel-Ange) (2011, January 7). Origin of life on Earth:
'Natural' asymmetry of biological molecules may have come from space.
ScienceDaily. Retrieved January 9, 2011, from http://www.sciencedaily.com
/releases/2011/01/110107145634.htm
Note: If no author is given, the source is cited instead.
Disclaimer: Views expressed in this article do not necessarily reflect those of
ScienceDaily or its staff.
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