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Anal Chem 1999 Sep 15;71(18):4000-6
Microfabricated capillary electrophoresis amino acid chirality analyzer for
extraterrestrial exploration.
Hutt LD, Glavin DP, Bada JL, Mathies RA
Department of Chemistry, University of California, Berkeley 94720, USA.
Chiral separations of fluorescein isothiocyanate-labeled amino acids have
been
performed on a microfabricated capillary electrophoresis chip to explore the
feasibility of using such devices to analyze for extinct or extant life signs in
extraterrestrial environments. The test system consists of a folded
electrophoresis channel (19.0 cm long x 150 microns wide x 20 microns deep) that
was photolithographically fabricated in a 10-cm-diameter glass wafer sandwich,
coupled to a laser-excited confocal fluorescence detection apparatus providing
subattomole sensitivity. Using a sodium dodecyl sulfate/gamma-cyclodextrin pH
10.0 carbonate electrophoresis buffer and a separation voltage of 550 V/cm at 10
degrees C, baseline resolution was observed for Val, Ala, Glu, and Asp
enantiomers and Gly in only 4 min. Enantiomeric ratios were determined for amino
acids extracted from the Murchison meteorite, and these values closely matched
values determined by HPLC. These results demonstrate the feasibility of using
microfabricated lab-on-a-chip systems to analyze extraterrestrial samples for
amino acids.
PMID: 10500487, UI: 99430274
Orig Life Evol Biosph 1999 May;29(3):329-32
Extraterrestrial handedness: a reply.
Bonner WA, Rubenstein E, Brown GS
Department of Chemistry, Stanford University, CA 94305, USA.
Recent investigations of stable isotope ratios of amino acids from the
Murchison
meteorite have shown them to be of unambiguous extraterrestrial origin, and
examinations of their enantiomeric compositions, where terrestrial contamination
can be excluded, have found a consistent excess of L-enantiomers. One
explanation for this observation has been the asymmetric photolysis of racemic
extraterrestrial amino acids by circularly polarized light (CPL) in the
synchrotron radiation from orbiting electrons around the pulsar remnants of
supernovae. Mason (1997) has attempted to discredit this mechanism on the
grounds that circular dichroism (CD) bands for optically active molecules
alternate in sign and sum to zero over the entire spectrum, and hence
enantioselective photochemical reactions cannot be induced by broad band CPL. We
submit arguments disputing this conclusion and present reasons for expecting
that broad band CPL synchrotron radiation would be quite capable of inducing
asymmetric photolysis, particularly in aliphatic amino acids.
PMID: 10465720, UI: 99392672
Orig Life Evol Biosph 1999 Mar;29(2):215-9
The extraterrestrial origin of the homochirality of
biomolecules--rebuttal to
a
critique.
Bonner WA, Greenberg JM, Rubenstein E
Department of Chemistry and School of Medicine, Stanford University,
Stanford,
CA 94305, USA.
Having concluded that abiotic terrestrial mechanisms would have been
ineffectual
for the origin of terrestrial homochirality, we have proposed an alternative
extraterrestrial scenario involving stereoselective ultraviolet photolysis of
the racemic constituents of interstellar grain mantles by circularly polarized
synchrotron radiation from neutron stars, followed by terrestrial accretion of
the resulting chiral molecules via cometary impact. Recently L. Keszthelyi
(1995) has reviewed a number of our arguments and advanced several erroneous
calculations and conclusions purporting to negate them. We offer here points of
rebuttal to Keszthelyi's criticisms, and support our inferences with recent data
regarding indigenous enantiomeric excesses of L-amino acids in the Murchison
meteorite.
PMID: 10391773, UI: 99317968
Orig Life Evol Biosph 1998 Oct;28(4-6):385-412
Carbonaceous micrometeorites and the origin of
life.
Maurette M
C.S.N.S.M., Batiment 104, 91405, Orsay-Campus, France.
Giant micrometeorites (sizes ranging from approximately 50 to 500
micrometers),
such as those that were first recovered from clean pre-industrial Antarctic ices
in December 1987, represent by far the dominant source of extraterrestrial
carbonaceous material accreted by the Earth's surface, about 50,000 times the
amount delivered by meteorites (sizes > or = a few cm). They correspond to
large
interplanetary dust particles that survived unexpectedly well their
hypervelocity impact with the Earth's atmosphere, contrary to predictions of
theoretical models of such impacts. They are related to relatively rare groups
of carbonaceous chondrites (approximately 2% of the meteorite falls) and not to
the most abundant meteorites (oridinary chondrites and differentiated
micrometeorites). About 80% of them appear to be highly unequilibrated
fine-grained assemblages of mineral grains, where an abundant carbonaceous
component is closely associated on a scale of < or = 0.1 micron to both
hydrous
and anhydrous minerals, including potential catalysts. These observations
suggest that micrometeorites could have functioned as individual microscopic
chemical reactors to contribute to the synthesis of prebiotic molecules on the
early Earth, about 4 billion years ago. The recent identification of some of
their complex organics (amino acids and polycyclic aromatic hydrocarbons), and
the observation that they behave as very efficient 'cosmochromatographs',
further support this 'early carbonaceous micrometeorite' scenario. Future
prospects include identifying the host phases (probably ferrihydrite) of their
complex organics, evaluating their catalytic activity, and assessing whether
synergetic interactions between micrometeorites and favorable zones of the early
Earth (such as submarine hydrothermal vents) accelerated and/or modified such
synthesis.
Publication Types:
Review
Review, tutorial
PMID: 10357645, UI: 99283858
EXS 1998;85:159-88
Homochirality and life.
Bonner WA
Department of Chemistry, Stanford University, CA 94305, USA.
After clarifying the frequently misused term homochirality, the crucial
importance of homochirality and chiral purity in the development and maintenance
of the essential biopolymers of life--proteins and nucleic acids--is discussed.
The harsh and forbidding prebiotic environment during the era of cometary impact
after formation of the Earth approximately 4.5 Gyr ago is described, after which
the most important abiotic mechanisms proposed historically for the genesis of
chiral molecules on the primitive Earth are enumerated. Random and determinate
terrestrial mechanisms are each evaluated with regard to the environmental
restraints imposed during the impact era, and it is concluded that all such
mechanisms would be inapplicable and implausible in the realistic prebiotic
environment. To circumvent these limitations, an extended hypothesis is
presented describing an extraterrestrial source of homochiral terrestrial
molecules. Illustrated in Figure 2, this scenario involves the partial
asymmetric photolysis of the racemic constituents of organic mantles on
interstellar dust grains by the circularly polarized ultraviolet components of
the synchrotron radiation emanating from the neutron star remnants of
super-novae. The resulting homochiral constituents with low enanantiomeric
excesses (e.e.s) so produced in the organic mantles are subsequently conveyed to
Earth either by direct accumulation or, more likely, after coalescence into
comets or asteroids, followed by repetitive impingement during the impact era.
Finally, the low e.e.s of the extraterrestrial homochiral molecules so
introduced are amplified by terrestrial autocatalytic or polymerization
mechanisms into a state of chiral purity, then are ultimately concentrated and
protected by sequestration in the interiors of spontaneously formed
protocellular vesicles--there to await further chemical evolution toward the
biomolecules of life. Recent observations of the excess of L-over D-amino acids
in the Murchison meteorite are cited as validation for the early stages of the
proposed hypothesis.
Publication Types:
Review
Review, tutorial
PMID: 9949874, UI: 99135033
Extremophiles 1998 Aug;2(3):313-9
Life on Mars: chemical arguments and clues from Martian meteorites.
Brack A, Pillinger CT
Centre de Biophysique Moleculaire, CNRS, Orleans, France. brack@cnrs-orleans.fr
Primitive terrestrial life-defined as a chemical system able to transfer its
molecular information via self-replication and to evolve-probably originated
from the evolution of reduced organic molecules in liquid water. Several sources
have been proposed for the prebiotic organic molecules: terrestrial primitive
atmosphere (methane or carbon dioxide), deep-sea hydrothermal systems, and
extraterrestrial meteoritic and cometary dust grains. The study of carbonaceous
chondrites, which contain up to 5% by weight of organic matter, has allowed
close examination of the delivery of extraterrestrial organic material. Eight
proteinaceous amino acids have been identified in the Murchison meteorite among
more than 70 amino acids. Engel reported that L-alanine was surprisingly more
abundant than D-alanine in the Murchison meteorite. Cronin also found excesses
of L-enantiomers for nonprotein amino acids. A large collection of
micrometeorites has been recently extracted from Antarctic old blue ice. In the
50- to 100-micron size range, carbonaceous micrometeorites represent 80% of the
samples and contain 2% of carbon, on average. They might have brought more
carbon than that involved in the present surficial biomass. The early histories
of Mars and Earth clearly show similarities. Liquid water was once stable on the
surface of Mars, attesting the presence of an atmosphere capable of
deccelerating C-rich micrometeorites. Therefore, primitive life may have
developed on Mars as well and fossilized microorganisms may still be present in
the near subsurface. The Viking missions to Mars in 1976 did not find evidence
of either contemporary or past life, but the mass spectrometer on the lander
aeroshell determined the atmospheric composition, which has allowed a family of
meteorites to be identified as Martian. Although these samples are essentially
volcanic in origin, it has been recognized that some of them contain carbonate
inclusions and even veins that have a carbon isotopic composition indicative of
an origin from Martian atmospheric carbon dioxide. The oxygen isotopic
composition of these carbonate deposits allows calculation of the temperature
regime existing during formation from a fluid that dissolved the carbon dioxide.
As the composition of the fluid is unknown, only a temperature range can be
estimated, but this falls between 0 degree and 90 degrees C, which would seem
entirely appropriate for life processes. It was such carbonate veins that were
found to host putative microfossils. Irrespective of the existence of features
that could be considered to be fossils, carbonate-rich portions of Martian
meteorites tend to have material, at more than 1000 ppm, that combusts at a low
temperature; i.e., it is an organic form of carbon. Unfortunately, this organic
matter does not have a diagnostic isotopic signature so it cannot be
unambiguously said to be indigenous to the samples. However, many circumstantial
arguments can be made to the effect that it is cogenetic with the carbonate and
hence Martian. If it could be proved that the organic matter was preterrestrial,
then the isotopic fractionation between it and the carbon is in the right sense
for a biological origin.
Publication Types:
Review
Review, tutorial
PMID: 9783179, UI: 98456508
Orig Life Evol Biosph 1998 Oct;28(4-6):413-24
A search for extraterrestrial amino acids in carbonaceous Antarctic
micrometeorites.
Brinton KL, Engrand C, Glavin DP, Bada JL, Maurette M
Scripps Institution of Oceanography, University of California at San Diego
92093, USA.
Antarctic micrometeorites (AMMs) in the 100-400 microns size range are the
dominant mass fraction of extraterrestrial material accreted by the Earth today.
A high performance liquid chromatography (HPLC) based technique exploited at the
limits of sensitivity has been used to search for the extraterrestrial amino
acids alpha-aminoisobutyric acid (AIB) and isovaline in AMMs. Five samples, each
containing about 30 to 35 grains, were analyzed. All the samples possess a
terrestrial amino acid component, indicated by the excess of the L-enantiomers
of common protein amino acids. In only one sample (A91) was AIB found to be
present at a level significantly above the background blanks. The concentration
of AIB (approximately 280 ppm), and the AIB/isovaline ratio (> or = 10), in
this
sample are both much higher than in CM chondrites. The apparently large
variation in the AIB concentrations of the samples suggests that AIB may be
concentrated in rare subset of micrometeorites. Because the AIB/isovaline ratio
in sample A91 is much larger than in CM chondrites, the synthesis of amino acids
in the micrometeorite parent bodies might have involved a different process
requiring an HCN-rich environment, such as that found in comets. If the present
day characteristics of the meteorite and micrometeorite fluxes can be
extrapolated back in time, then the flux of large carbonaceous micrometeorites
could have contributed to the inventory of prebiotic molecules on the early
Earth.
PMID: 9742723, UI: 98415148
Nature 1997 Oct 23;389(6653):804
Extraterrestrial handedness.
Mason SF
Comment on: Nature 1997 Sep 18;389(6648):265-8
PMID: 9349811, UI: 98007967
Nature 1997 Sep 18;389(6648):265-8
Isotopic evidence for extraterrestrial
non-racemic amino acids in the
Murchison
meteorite.
Engel MH, Macko SA
School of Geology and Geophysics, The University of Oklahoma, Norman 73019,
USA.
AB1635@OU.EDU
Many amino acids contain an asymmetric centre, occurring as laevorotatory, L,
or
dextrorotatory, D, compounds. It is generally assumed that abiotic synthesis of
amino acids on the early Earth resulted in racemic mixtures (L- and
D-enantiomers in equal abundance). But the origin of life required, owing to
conformational constraints, the almost exclusive selection of either L- or
D-enantiomers, and the question of why living systems on the Earth consist of
L-enantiomers rather than D-enantiomers is unresolved. A substantial fraction of
the organic compounds on the early Earth may have been derived from comet and
meteorite impacts. It has been reported previously that amino acids in the
Murchison meteorite exhibit an excess of L-enantiomers, raising the possibility
that a similar excess was present in the initial inventory of organic compounds
on the Earth. The stable carbon isotope compositions of individual amino acids
in Murchison support an extraterrestrial origin -- rather than a terrestrial
overprint of biological amino acids-although reservations have persisted. Here
we show that individual amino-acid enantiomers from Murchison are enriched in
15N relative to their terrestrial counterparts, so confirming an
extraterrestrial source for an L-enantiomer excess in the Solar System that may
predate the origin of life on the Earth.
Comments:
Comment in: Nature 1997 Sep 18;389(6648):234-5
Comment in: Nature 1997 Oct 23;389(6653):804
Comment in: Nature 1998 Jul 16;394(6690):236
PMID: 9305838, UI: 97449369
Nature 1997 Sep 18;389(6648):234-5
Origins of life. A left-handed solar system?
Chyba CF
Comment on: Nature 1997 Sep 18;389(6648):265-8
PMID: 9305833, UI: 97449364
Orig Life 1982 Sep;12(3):307-10
Formation of amino acids from models of Titan and more oxidized atmospheres.
Ishigami M, Kinjo M, Nagano K, Hattori Y
Protein and non-protein amino acids were synthesized following hydrolysis of
products obtained by high frequency discharge techniques applied to model
atmospheres consisting of N2 as a nitrogen source together with CH4 and/or CO2
as a carbon source. Highest yields were obtained in the absence of CO2 and from
mixtures rich in CH4. Amino acids would indeed be expected on the frozen surface
of Titan with its CH4-N2 atmosphere.
PMID: 6819504, UI: 83142817
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