25th International Meeting on Organic Geochemistry IMOG 2011

O-25
The chemical structure of insoluble organic matter in
carbonaceous meteorites
Sylvie Derenne 1 , François Robert 2
1 BioEMCo CNRS / UPMC, Paris, France, 2 LMCM CNRS / MNHN, Paris, France (corresponding
author:sylvie.derenne@upmc.fr)
Carbonaceous meteorites are the most primitive
objects of the solar system. They exhibit significant
carbon contents and most of this carbon occurs as
insoluble organic matter (IOM), which might be the
first OM available on early Earth for life. Moreover,
this OM may contain specific extraterrestrial
signatures and should provide information on solar
system history. It is therefore of special interest to
decipher the chemical structure of this IOM. Through
the use of numerous complementary analytical tools,
key information was obtained on this structure at a
molecular level and a model of structure was built up.
IOM was isolated from the Murchison meteorite using
successive extractions and acid treatments (HCl, HF).
Its chemical structure was investigated through a
combination of various spectroscopic methods
(Fourier transform infra-red, solid-state 13 C and 15 N
NMR, electron paramagnetic resonance (EPR), X-ray
absorption near-edge spectroscopy), chemical (RuO4
oxidation) and thermal (pyrolysis) degradations and
high resolution transmission electron microscopy.
Taken together, these techniques provided a wealth
of qualitative and quantitative information. The IOM is
therefore based on a network of relatively small
polyaromatic units (comprising 10-15 benzene rings in
average), cross-linked by short, highly branched
aliphatic chains that may include ester and ether
functions. Nitrogen was shown to mainly occur as
heterocycles whereas sulphur is distributed (3/1)
between thiophene rings and aliphatic sulphides.
A statistical model is proposed for this molecular
structure, fitting with 11 parameters derived from the
aforementioned analyses.
EPR revealed an heterogeneous distribution of free
organic radicals and presence of diradicals, which can
be both considered as an extraterrestrial signature.
Moreover, these carbonaceous meteorites are known
to be highly enriched in deuterium but the precise
location and origin of this deuterium was still to
determine. D/H ratio was determined in individual
compounds released through RuO4 oxidation and
pyrolysis. These two techniques allow studying both
the aromatic units and the aliphatic linkages.
Deuterium distribution within these compounds
revealed no significant difference between aromatic
and aliphatic moieties but led to consider deuterium
location in the macromolecule and to distinguish three
types of H, namely aromatic, benzylic and aliphatic.
These types of H exhibit different deuterium
enrichment and the latter is related to the C-H bond
strength. Based on this relationship, we propose that
the IOM formed in a D-poor environment and was
then transported and further enriched through
exchange in a D-rich medium.
Very recently, nanoSIMS analyses revealed a high
spatial heterogeneity for deuterium in IOM and
pointed to the occurrence of hot-spots of D. Following
the same reasoning, such hot spots should
correspond to molecules with very weak C-H bonds.
Based on their abundance and heterogeneous
distribution, we proposed the organic free radicals to
be the hosts of these D enrichments. This was further
confirmed using pulsed EPR.
These results are difficult to reconcile with the usual
interpretation according to which high D/H ratios
represent survivals of interstellar grains. More likely,
the deuterium-enrichment process took place after the
formation of organic grains characterized by low D/H
ratios, through an isotopic exchange-reaction with Drich
gaseous molecules, such as H2D + or HD2 +. This
exchange reaction most likely took place in the diffuse
outer regions of the protoplanetary disk around the
young Sun..
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