The stereochemistry of amino acids in the Murchison meteorite

Abstract
Precambrian Research 106 (2001) 35–45
Review
The stereochemistry of amino acids in the Murchison
meteorite
M.H. Engel a, *, S.A. Macko b
a School of Geology & Geophysics, Uni�ersity of Oklahoma, Norman, OK 73019, USA
b Department of En�ironmental Sciences, Uni�ersity of Virginia, Charlottes�ille, VA 22903, USA
The questions of how, where and when life originated in our solar system remain largely unanswered. Some
advances have been made with respect to abiotic synthesis of the key molecules deemed essential for the construction
of a living cell. In particular, a variety of plausible mechanisms have been suggested for the synthesis of amino acids,
the building blocks of peptides and proteins. Laboratory simulation experiments result in the synthesis of racemic
amino acids. However, life as we know it is based almost exclusively on L-enantiomers rather than racemic mixtures
of D- andL-enantiomers. A partial solution to this problem may be that the L-enantiomer excess essential for life’s
origin on Earth was introduced from elsewhere in the solar system. For the past 20 years we have investigated the
stereochemistry of amino acids in stones of the Murchison meteorite. Many of the common protein amino acids in
Murchison are not racemic (L-enantiomer excess) and, based on their overall distribution and respective stable isotope
compositions, do not appear to be artifacts of terrestrial contaminants (i.e. L-amino acids) introduced subsequent to
impact. We hypothesize that comet and meteorite impacts during the early stages of Earth’s formation provided at
least some of the essential components with the correct stereochemistry for the origin of life. © 2001 Elsevier Science
B.V. All rights reserved.
Keywords: Amino acids; Stable isotopes; Murchison meteorite; Stereochemistry; Racemization
1. Introduction
For as far back in time as the terrestrial rock
record extends, it appears that microbial life may
have existed on Earth. Pflug and colleagues (e.g.
Pflug and Heinz, 1997 and references therein)
provided possible microscopic and spectroscopic
* Corresponding author.
0301-9268/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0301-9268(00)00123-6
www.elsevier.com/locate/precamres
evidence for primitive cells in banded ironstones
of the 3.8 Ga old Isua supracrustal belt, southwest
Greenland. These findings have been the
topic of some debate, because amino acids and
hydrocarbons that could be extracted from these
rocks appeared to be modern contaminants (e.g.
Nagy et al., 1981). It should be noted, however,
that stable carbon isotope analyses of carbonaceous
materials (e.g. Schidlowski, 1993 and refer-

36
ences therein) and, more recently, of organic inclusions
in apatite crystals (Mojzsis et al., 1996;
Nutman et al., 1997), provide evidence that some
of the carbonaceous material in Isua as well as
perhaps in slightly older rocks in the region (Mojzsis
et al., 1996), is possibly of ancient biolgical
origin. This is because the � 13 C values for these
samples exhibit the depletion in 13 C that is typical
for most organisms on Earth to this day (Schidlowski,
1993).
If the currently known rock record of Earth
reflects an essentially uninterrupted history of biological
activity, serious limitations are placed on
(1) constraining an appropriate temporal window
for life’s origin on Earth and (2) elucidating the
origins and availability of the organic compound
inventory that must have preceded life. Debate
continues with respect to the extent to which the
Earth’s earliest Archean atmosphere/hydrosphere
was oxidizing (e.g. Watanabe et al., 1997), which
constrains the feasibility of terrestrial, abiotic organic
synthesis at the air/sea interface. The importance
of alternative, deep ocean environments
such as hydrothermal vents for abiotic synthesis
of organic compounds (e.g. Hennet et al., 1992)
and an assessment of their relative stabilities (e.g.
Shock, 1990; Qian et al., 1993; Helgeson et al.,
1998 and references therein) and implications for
life’s origin (e.g. Baross and Hoffman, 1985; Russell
et al., 1989; Corliss, 1990) continue to be
intensely explored. However, hypotheses concerning
abiotic organic synthesis during the earliest
Archean remain untestable until significantly
older terrestrial sediments or metasediments are
subsequently discovered that predate life. Whilst
remnants of 3230 Ma hydrothermal vents have
recently been discovered (de Ronde et al., 1997),
there is no doubt that living systems were already
operational by this time. Finally, increasing evidence
that the Earth is likely to have experienced
several major episodes of impact events during the
first �0.5 Ga of its history (Chyba et al., 1990;
Chyba and Sagan, 1992) must to some extent
limit the likelihood for survival of early life forms
had they evolved during this time interval.
Given the environmental constraints and time
constraints for the abiotic synthesis of organic
compounds on Earth and the prospects for their
M.H. Engel, S.A. Macko / Precambrian Research 106 (2001) 35–45
subsequent assimilation as a viable cell, it is perhaps
not surprising that alternative scenarios have
been suggested concerning the introduction of
life’s precursors to Earth (e.g. Chyba et al., 1990;
Chyba and Sagan, 1992; Greenberg, 1997) or life
itself (Wickramasinghe et al., 1997) from extraterrestrial
sources. The occurrence of remanents of
past extraterrestrial life and their introduction to
Earth via impact events also continues to be
explored (e.g. McKay et al., 1996). Whilst sources
and modes of synthesis remain uncertain, there is
ample evidence for the occurrence of organic
compounds in extraterrestrial materials that have
impacted Earth at various times. In particular, the
carbonaceous meteorites provide important insights
with respect to the synthesis of amino acids
at approximately the time of formation of the
solar system (Engel et al., 1993; Cronin and
Chang, 1993). Organic-rich carbonaceous meteorites
are rare occurrences with few having been
collected (�36) at the time of their observed falls
(Nagy, 1975). With the exception of additional
meteorites and micrometeorites (e.g. Maurette,
1998) collected from the ice in Antarctica, environmental
conditions on the dynamic Earth are
not conducive for the preservation of meteorites
that may have fallen on land in the geologic past.
The fact that anything introduced to Earth is
immediately susceptible to contamination by terrestrial
organic matter presents a formidable challenge
with respect to distinguishing indigenous
organic compounds from terrestrial overprints.
This is particularly true for compounds such as
amino acids, that are ubiquitous to living systems.
What follows is a summary of what is currently
known about the distribution and stereochemistry
of amino acids in carbonaceous meteorites, focusing
on the common amino acids that are the
building blocks of peptides and proteins in living
systems. A hypothesis is discussed for distinguishing
terrestrial vs extraterrestrial amino acids based
on their stable isotope compositions.
2. Amino acid distributions
Since the beginning of the nineteenth century
there have only been approx. 36 observed falls of

carbonaceous meteorites for which stones were
collected. In only a few cases were the stones
archived for subsequent study. It is interesting to
note that since the falls of Pueblito de Allende
and Murchison in 1969, no observed falls of
carbonaceous meteorites have been reported. This
30 year hiatus is the longest interval without an
observed fall since the fall of Alais in 1806. For
the past 39 years, numerous investigations of the
distributions of amino acids in carbonaceous meteorites
have been undertaken, most of which
have indicated the presence of common protein
amino acids. Caution must be exercised when
attempting to interpret these data sets, as the
precautions taken for sample storage have varied
over the decades and laboratory protocols for
sample handling, extraction and analysis have
been substantially modified with the passage of
time. Thus, the precision and accuracy of amino
acid abundances in meteorites relative to background
impurities (e.g. in solvents, acids and water
used for sample extractions), in particular for
analyses performed prior to 1970, are uncertain.
Urey (1966), Hayes (1967) and Nagy (1975) noted
many of the challenges encountered when attempting
amino acid analyses at trace levels, and,
subsequent to these reports, attempts were made
(and continue to be made) to improve methods
for sample handling and analysis.
Given the antiquity of most observed falls of
carbonaceous meteorites, it is not surprising that
attention has focused on the Murchison meteorite
that fell in Australia in 1969, and on carbonaceous
meteorites that have been recovered from
the Antarctic ice sheet over the past few decades.
However, the pristine nature of stones recovered
from Antarctica requires further investigation,
owing to their extended residence times and thus
the possibility of contamination from amino acids
in the ice (Engel and Macko, 1997a; Bada et al.,
1998).
As summarized by Shock and Schulte (1990),
the relative abundances of amino acids in
Murchison meteorite stones have been fairly consistent
from laboratory to laboratory. Differences
in absolute abundances may in part reflect the
variety of extraction procedures and analytical
methods that have been employed (e.g. Shock and
M.H. Engel, S.A. Macko / Precambrian Research 106 (2001) 35–45 37
Schulte, 1990). We have analyzed several
Murchison stones, all of which were initially
archived at the Field Museum, Chicago, IL, USA.
The abundances of common amino acid constituents
in the acid hydrolyzed water extracts of
two of these stones are shown in Table 1. For
comparison, amino acid abundances are also
shown for the same extract of two stones (one of
which was a portion of the same stone analyzed
by Engel and Nagy, 1982) reported by Cronin and
Pizzarello (1983). The distributions of the common
�-amino acids are fairly consistent from
stone to stone. The distributions of common
amino acids in unhydrolyzed water extracts of
Murchison stones are also similar, although absolute
abundances are less than in the hydrolyzed
water extracts (Table 2). An interesting characteristic
of Murchison is the fact that several amino
acids that are common to biological systems are
either present in trace amounts (e.g. serine,
threonine) or are below current detection limits
(e.g. tyrosine, phenylalanine, methionine, cysteine,
lysine, histidine, arginine). The absence of these
amino acids supports the hypothesis that the interiors
of these stones have received minimal, if any
input of amino acids from exogenous sources
subsequent to impact (Engel and Nagy, 1982,
1983).
It is also notable that the absolute abundances
of several amino acids in the hydrolyzed water
extract of Murchison appear to have decreased
with the passage of time subsequent to impact.
This is particularly the case with respect to �aminoisobutyric
acid and isovaline (Table 1), nonprotein
amino acids that have rarely been
reported as constituents of biological systems, although
they may apparently form from hydantoin
precursors originating from coal gasification (Olson,
1992) and/or industrial waste (Mita and Shimoyama,
1998). It is presently unknown whether
the apparent decrease in absolute abundances of
�-aminoisobutyric acid and isovaline are a consequence
of their instability (Cronin and Pizzarello,
1983) or, alternatively, reflect the possibility that
amino acid distributions and abundances actually
vary from stone to stone. However, it appears
that the concentrations of �-aminoisobutyric acid
and isovaline have not decreased with the passage

38
M.H. Engel, S.A. Macko / Precambrian Research 106 (2001) 35–45
Table 1
Concentrations (nmol g −1 ) of principal amino acids in hydrolyzed water extracts of the Murchison meteorite
Amino acid Engel and Nagy (1982) a Cronin and Pizzarello (1983) b Engel and Macko (1997b) c
A B
Common amino acids
Glutamic acid 18.2
15.4 23.5 10.8
Aspartic acid
8.5 4.9 5.1 4.7
n.r. d Proline 13.5
n.r. n.r.
Glycine
45.8 43.7 96.0 24.5
�-Alanine 13.1
8.8 15.2 12.8
Leucine
1.9 �1.8 �3.9 2.5
Sarcosine 4.7
n.r. n.r. n.r.
Alanine
15.3 19.4 43.3 10.4
Valine
Exotic amino acids
8.6
4.7 10.2 n.r.
�-Aminoisobutyric acid 107.8 145.3
100.2 20.1
Isovaline 23.6 53.4
27.7 8.0e a Amino acid concentrations determined by GC.
b Amino acid concentrations determined by HPLC. A: Values for a portion of the same stone analyzed by Engel and Nagy (1982).
B: Values for a stone from the ASU collection.
c Amino acid concentrations determined by HPLC.
d4n.r.; not reported.
e Value for isovaline includes a contribution from valine that co-eluted during HPLC analysis.
of time in the unhydrolyzed water extracts of
Murchison (Table 2), implying that it may be the
precursors of these amino acids rather than the
amino acids themselves that are unstable.
To date, the only really anomalous amino acid
distribution reported for a Murchison stone was
one that was archived at the Smithsonian Institution
(Cronin and Pizzarello, 1983; Shock and
Schulte, 1990). The anomalously high levels of
common protein amino acids in this stone are
assumed to be a consequence of terrestrial contamination.
The higher abundances of serine and
Table 2
Concentrations (nmol g −1 ) of principal amino acids in unhydrolyzed water extracts of the Murchison meteorite
Amino acid
Cronin (1976a) a Cronin (1976b) 2 Silfer (1991) a
Common amino acids
Glutamic acid 3.5 1.9
4.6
Aspartic acid 1.0
1.2
3.9
Glycine 31.0 25.5
28.1
�-Alanine 6.9
5.7
8.1
Leucine
0.9
1.2 1.6
Isoleucine 0.8 1.4
1.8
Alanine
Exotic amino acids
17.1 15.9 12.9
�-Aminoisobutyric acid
19.9 15.0
16.0
6.0b 4.6b 7.5b Isovaline
�-Amino-n-butyric acid 5.3
4.6
3.7
a Concentration determined by HPLC.
b Concentration includes contribution from valine that co-eluted during analysis.

threonine in this stone, common constituents of
fingerprints (Nagy, 1975), indicate the possibility
of contamination during sample handling.
3. Amino acid stereochemistry
The conventional wisdom has been that amino
acids in extraterrestrial materials were synthesized
via abiotic mechanisms. To the best of our knowledge,
all laboratory experiments designed to simulate
natural, abiotic synthesis in our solar system
have resulted in racemic mixtures of amino acids
(e.g. Khare et al., 1986; Hennet et al., 1992; Engel
et al., 1995). It should be noted, however, that
whilst there have been hundreds of abiotic syntheses
attempted since the initial, pioneering experiments
of Miller (1953), in very few instances have
attempts been made to document the stereochemistry
of the amino acid products (e.g. Khare et al.,
1986; Hennet et al., 1992; Engel et al., 1995).
It has generally been assumed that since laboratory
simulation experiments result in the synthesis
of racemic amino acids, amino acids in extraterrestrial
materials that contain one or more chiral
centers should also be racemic. Any indication of
an L-enantiomer excess has generally been attributed
to contamination from the modern terrestrial
biosphere (Bada et al., 1983, 1998).
However, as in the case of simulation experiments,
it is important to note that very few chromatograms
have ever been published that
document the stereochemistry of amino acids in
carbonaceous meteorites (e.g. Kvenvolden et al.,
1970; Engel and Nagy, 1982; Cronin and Pizzarello,
1997; Engel and Macko, 1997b).
Kvenvolden et al. (1970) published the first gas
chromatogram of amino acids in the Murchison
meteorite. They concluded that alanine was approximately
racemic. However, limited column
resolution and the lack of capability for selected
ion monitoring at that time did not permit the
comprehensive assessment of the stereochemistry
of most of the common amino acids. Subsequent
to this initial finding, Engel and Nagy (1982)
reported that whilst alanine was the most highly
racemized amino acid in the acid hydrolyzed water
extract of the Murchison meteorite, in was not
M.H. Engel, S.A. Macko / Precambrian Research 106 (2001) 35–45 39
racemic and exhibited an excess of the L-enantiomer.
They reported that the other common
protein amino acids were even less racemized.
Whilst Bada et al. (1983) suggested that the only
plausible explanation for these results was contamination,
Engel and Nagy (1983) argued that if
the stone had been contaminated by terrestrial
biota, then the other amino acids that are common
to all living systems (e.g. cysteine, methionine,
tyrosine, phenylalanine, etc.) should also have
been present. Also, if contamination was sufficient
to perturb all of the protein amino acid D/L
values, then the abundances for the common
amino acids present in this stone should not have
been identical to those of most other Murchison
stones (e.g. Table 1 and Table 2). Furthermore, it
would be expected that if surficial contamination
had occurred, this would have been removed by
the water extraction process. However, sequential
acid digestion of the meteorite powder subsequent
to water extraction led to the recovery of amino
acids that were even less racemized (Table 3). Our
previous attempts at culturing microbial communities
on Murchison substrates were singularly
unsuccessful (Engel and Nagy, 1983). However,
the Murchison meteorite fall was 30 years ago
and it is evident that storage protocols for the
stones have varied. As indicated above, at least
one stone archived at the Smithsonian Institution
appears to have been contaminated. More recently,
Steele et al. (1999) have reported bacterial
and fungal contaminants on a small, exterior sample
of a Murchison stone. Thus, appropriate caution
should be exercised when selecting
appropriate stones for future analyses.
More recently, we have determined the stereochemistry
for amino acids in the hydrolyzed water
extracts of two additional Murchison stones, and
our results support the initial findings of Engel
and Nagy (1982). All of the common amino acids
exhibit a moderate to strong excess of the L-enantiomer
(Table 3). As in the initial reports of Engel
(1980) and Engel and Nagy (1982), mass spectrometry
has been employed to document peak
purities and selected ion monitoring (SIM) has
been used to confirm the amino acid D/L values
(Engel et al., 1990; Engel and Macko, 1997b).
Pizzarello and Cronin (1998) have suggested that

40
M.H. Engel, S.A. Macko / Precambrian Research 106 (2001) 35–45
Table 3
Amino acid D/L values in various extracts of Murchison meteorite stones
Amino acid Engel et al., (1990) b
Engel and Nagy (1982) a Engel and Macko (1997b) c
A
B
Alanine 0.60 0.31 0.85 0.50
Glutamic acid 0.30 0.18 0.57 0.30
Aspartic acid
0.30 0.13 0.22
0.20
Proline 0.3
0.11 N.D. d N.D.
Leucine 0.17 0.03
N.D.
N.D.
Valine
N.D. N.D. 0.63
N.D.
a A: hydrolyzed water extract; B: acid digestion of residual stone that had been previously water extracted.
b Unhydrolyzed water extract.
c Hydrolyzed water extract.
d N.D.: not determined.
the L-enantiomer excess for one of the protein
amino acids, i.e. alanine, is a consequence of
co-elution of an exotic component. However, we
have not observed this to be the case in our
Murchison stones, either by mass spectrometry
(SIM) or isotopically (see below). Moreover, it
would seem highly improbable that such an explanation
could account for the L-excess observed for
all of the other protein amino acids in Murchison.
This would require co-elutions of exotic components
of identical mass spectra and stable carbon
and nitrogen isotopic compositions exclusively
with the L-enantiomers rather than the D-enantiomers.
For example, selected major ions for
glutamic acid and aspartic acid in the acid hydrolyzed
water extract of the Murchison stone
most recently analyzed (Engel and Macko, 1997b)
are shown in Fig. 1a and 1b. Clearly, both amino
acids exhibit an L-enantiomer excess and, as in the
case of our previous findings (Engel and Nagy,
1982; Engel et al., 1990; Engel and Macko,
1997b), the mass spectrometric methods employed
preclude that this is a consequence of co-elution
of unknown components with the respective Lenantiomers.
All ions unique to glutamic acid
(Fig. 1a) and to aspartic acid (Fig. 1b) provide the
same D/L values, respectively. Cronin and Pizzarello
(1997) have also reported an apparent
L-enantiomer excess for several non-protein
amino acids in the Murchison meteorite.
As discussed above, the amino acid distributions
in these stones argues against contamination
as a likely explanation for this L-enantiomer excess.
However, this in itself is not sufficient to
prove beyond all doubt that the L-enantiomer
excess is extraterrestrial and original to the stone.
Given that these findings are the first indications
of extraterrestrial optical activity, a new approach
involving stable isotope geochemistry at the
molecular level was developed as an independent
validation of the indigeneity of these compounds.
4. Stable isotope compositions of amino acid
enantiomers
Engel and Macko (1984, 1986) initially hypothesized
that it should be possible to verify that the
D- and L-enantiomers of an amino acid were
derived from a common source by determining
their respective stable carbon and nitrogen isotope
compositions. Given that abiotic synthesis results
in racemic mixtures of amino acids, with the
pathways for D-amino acid and L-amino acid
synthesis presumably being indentical, the respective
enantiomers of an amino acid should have the
same stable isotope compositions. For any systems
in which amino acids initially consist of an
excess of one enantiomer (e.g. almost exclusively
the L-enantiomer for biological systems on Earth),
with the other enantiomer subsequently forming
by racemization, the respective enantiomers for a
given amino acid should be isotopically identical.
This is a result of the fact that amino acids retain

M.H. Engel, S.A. Macko / Precambrian Research 106 (2001) 35–45 41
Fig. 1. (a) Selected ions (m/z 152, 180, 198 and 226) of glutamic acid in the hydrolyzed water extract of a Murchison meteorite stone
analyzed by Engel and Macko (1997b). Details of the analytical methods are reported in Engel and Macko (1997b). (b) Selected ions
(m/z 184, 185 and 212) of aspartic acid in the hydrolyzed water extract of a Murchison meteorite stone analyzed by Engel and
Macko (1997b). Details of the analytical methods are reported in Engel and Macko (1997b).

42
their stable carbon and nitrogen isotopic integrity
during racemization, irrespective of whether the
initial enantiomer is of the D- or L-configuration
(Engel and Macko, 1984, 1986). It is well-documented
(e.g. Epstein et al., 1987) that the bulk
amino acid fraction of the Murchison meteorite is
enriched in 13 C and 15 N relative to any known
common biological materials on Earth. The foundation
for our investigations has been that if it
were possible to measure the stable carbon and
nitrogen isotope compositions of the individual Dand
L-enantiomers of amino acids in the
Murchison meteorite, it would be possible to determine
if the L-enantiomer excess was extraterrestrial
in origin or a consequence of terrestrial
contamination. If the latter were the case, it
would be expected that the L-enantiomer of an
amino acid would be depleted in 13 C and 15 N
relative to its respective D-enantiomer, owing to
partial input of terrestrial L-amino acids that were
depleted in these heavier isotopes relative to the
extraterrestrial components.
We have developed methods for the direct determination
of the stable carbon and nitrogen
isotope compositions of the D- andL-enantiomers
of amino acids at natural abundance levels using
Table 4
Stable carbon and nitrogen isotope values of amino acids in
the Murchison meteorite
Amino acid � 13 C(‰,PDB) a � 15 N(‰,atm N 2) b
�-Aminoisobutyric
acid
+5 +184
Isovaline +17
+66
N.D. c
Sarcosine +129
Glycine +22 +37
�-Alanine N.D. +61
D-Glutamic acid N.D. +60
L-Glutamic acid +6 +58
D-Alanine +27.7
+60
L-Alanine +26.1
+57
L-Leucine N.D. +60
D,L-Proline N.D. +50
D,L-Aspartic acid N.D. +61
a 13 � C values are for amino acids in an unhydrolyzed water
extract (Engel et al., 1990).
b 15 � N values are for amino acids in a hydrolyzed water
extract (Engel and Macko, 1997b).
c N.D.; not determined.
M.H. Engel, S.A. Macko / Precambrian Research 106 (2001) 35–45
gas chromatography/combustion/isotope ratio
mass spectrometry (GC/C/IRMS; Silfer et al.,
1991, 1994; Macko et al., 1997). The test of our
hypothesis has focused on the Murchison meteorite.
Our findings confirm an extraterrestrial origin
for the amino acids. All of the Murchison amino
acids analyzed to date are enriched in 13 C and 15 N
relative to terrestrial biogenic amino acids implying,
as previously suggested (Epstein et al., 1987;
Engel et al., 1990; Cronin and Chang, 1993; Pizzarello
et al., 1994; Engel and Macko, 1997b),
possible interstellar sources for the precursors of
these compounds. For amino acids for which the
respective D- and L-enantiomers could be isotopically
characterized, the � 13 Cand� 15 N values were
virtually indistinguishible (Table 4), thus providing
independent confirmation for the indigeneity
of both stereoisomers.
Can the stable isotope compositions of the individual
amino acids in Murchison be used to reconstruct
their synthetic pathways? In theory, this
should be possible, but it is important to recognize
that the field of molecular isotope geochemistry
is relatively new and thus data sets for
terrestrial and extraterrestrial organic compounds
are quite limited. In general, the stable carbon
isotope compositions of amino acids formed via
abiotic chemical reactions exhibit an apparent
kinetic isotope effect, tending to become depleted
in 13 C with increasing chain length, implying that
higher molecular weight structures may be forming
from lower molecular weight precursors (e.g.
Engel et al., 1995). There is an indication that the
Murchison amino acids (Table 4) exhibit this
effect with respect to stable carbon isotope composition.
The trend for � 15 N values is, however,
different. �-Aminoisobutyric acid is substantially
enriched in 15 N, glycine is depleted, and the remaining
amino acids are all, with the exception of
proline, close to +60‰, implying a common,
homogenous source (Table 4). Unlike carbon, all
of the Murchison amino acids analyzed to date
(Table 4) contain only a single nitrogen atom. A
kinetic isotope effect similar to that observed for
carbon would, therefore, not be expected. To the
best of our knowledge, stable nitrogen isotope
values for a series of amino acids synthesized via
abiotic pathways have not yet been reported.

Thus, it is not possible to say with any certainty
whether this Murchison data set is consistent for
such pathways or not. What is apparent, however,
is that the nitrogen source(s) for �-aminoisobutyric
acid and sarcosine are distinct. In addition to
being a common product of abiotic synthesis (Engel
et al., 1995), glycine is a common product of
the decomposition of other �-amino acids. The
more depleted � 15 N value for glycine in
Murchison (Table 4) may reflect contributions of
isotopically depleted glycine resulting from kinetic
isotope effects associated with the decomposition
of other amino acids subsequent to their initial
synthesis. Alternatively, perhaps glycine, a relatively
simple structure, formed in interstellar space
from isotopically distinct precursors prior to the
synthesis and subsequent diagenesis of the more
complex amino acids on the parent body of
Murchison. Preliminary evidence suggests that
glycine may exist in the interstellar medium
(Snyder, 1997).
Assuming that the L-amino acid excess in
Murchison is authentic, the question of how this
came to be will not likely be easily resolved.
Whilst laboratory simulations of abiotic synthesis
of amino acids result in racemic mixtures, most
experiments to date have been nothing more than
variations on the theme initially conceived by
Miller (1953). It could be that extraterrestrial
organic materials, such as those encountered in
meteorites, form through stereoselective pathways
that have yet to be discovered. Alternatively, it
has been suggested that post-synthetic, diagenetic
phenomena such as the exposure of amino acids
to circularly polarized light emitted by neutron
stars (e.g. Bonner, 1991; Bailey et al., 1998) may
account for the preferential enantiomer destruction.
An alternative scenario is that the
Murchison meteorite contains residual compounds
from once living systems that have partially
racemized subsequent to death as they do on
Earth. It is possible for amino acids of great
antiquity to be non-racemic. Diagenetic condensation
reactions, even in the presence of water
(which is requisite for racemization), are known
to greatly diminish and, in some cases, actually
stop the amino acid racemization (Rafalska et al.,
1991). Yet, under such circumstances, amino acids
M.H. Engel, S.A. Macko / Precambrian Research 106 (2001) 35–45 43
sufficiently retain their structural integrity to the
extent that they can be recovered by the simple
extraction methods commonly used for the analysis
of carbonaceous meteorites (Engel et al., 1990;
Engel and Macko, 1997b). Do the stable isotope
values for amino acids in the Murchison meteorite
reflect the range of values typically associated
with biosynthetic processes (e.g. Macko et al.,
1987; Engel et al., 1994) on Earth? Unfortunately,
the answer to this question awaits the acquisition
of larger data sets than are presently available.
Certainly, it can be concluded, however, that the
starting materials from which the Murchison
amino acids were synthesized were isotopically
distinct from those typical of Earth.
5. Summary
Interior samples of three Murchison meteorite
stones were analyzed for their amino acid distributions,
stereochemistry and stable isotope compositions.
All of the common �-amino acids are
not racemic, exhibiting an excess of the L-enantiomer.
The absence of many amino acids (e.g.
phenylalanine, tyrosine, lysine, histidine, arginine,
etc.) that are common protein constituents of all
terrestrial living systems indicates that these samples
have not been contaminated by microbiota or
handling. The 13 C and 15 N enrichment of the
Murchison amino acids further demonstrates their
extraterrestrial origins. The essentially identical
� 13 C and � 15 N values for the D- and L-enantiomers
of individual amino acids provides an
independent confirmation of their indigeneity. An
explanation for the L-enantiomer excess is an
important topic for future research, as the answer
to this question may be the key as to why all
living systems on Earth are based on the biosynthesis
of proteins containing exclusively L-enantiomers
rather than D-enantiomers.
Acknowledgements
We thank the National Science Foundation
(Geology & Paleontology) for continued support
of our molecular isotope research. We thank An-

44
dre Brack, Everett K. Gibson Jr., and Frances
Westall for their helpful comments and suggestions.
This paper is dedicated to the memory of
Bartholomew Nagy, a free spirit and independent
thinker who was never awed by conventional
wisdom or constrained by what other people
thought. He let his data, however unpopular at
times, speak for itself, and we will continue to do
the same.
References
Bada, J.L., Cronin, J.R., Ho, M.-S., Kvenvolden, K.A., Lawless,
G.J., Miller, S.L., et al., 1983. On the reported optical
activity of amino acids in the Murchison meteorite. Nature
301, 494–496.
Bada, J.L., Glavin, D.P., McDonald, G.D., Becker, L., 1998.
A search for endogenous amino acids in martian meteorite
ALH84001. Science 279, 362–365.
Bailey, J., Chrysostomou, A., Hough, J.H., Gledhill, T.M.,
McCall, A., Clark, S., et al., 1998. Circular polarization in
star-formation regions: Implications for biomolecular homochirality.
Science 281, 672–674.
Baross, J.A., Hoffman, S.E., 1985. Submarine hydrothermal
vents and associated gradient environments as sites for the
origin and evolution of life. Origins of Life 15, 327–345.
Bonner, W.A., 1991. The origin and amplification of
biomolecular chirality. Origins of Life 21, 59–111.
Chyba, C.F., Sagan, C., 1992. Endogenous production, exogenous
delivery and impact-shock synthesis of organic
molecules: an inventory for the origins of life. Nature 355,
125–132.
Chyba, C.F., Thomas, P.J., Brookshaw, L., Sagan, C., 1990.
Cometary delivery of organic molecules to the early Earth.
Science 249, 366–373.
Corliss, J.B., 1990. Hot springs and the origin of life. Nature
347, 624.
Cronin, J.R., 1976a. Acid-labile amino acid precursors in the
Murchison meteorite. I. Chromatographic fractionation.
Origins of Life 7, 337–342.
Cronin, J.R., 1976b. Acid-labile amino acid precursors in the
Murchison meteorite. II. A search for peptides and amino
acyl amides. Origins of Life 7, 343–348.
Cronin, J.R., Pizzarello, S., 1983. Amino acids in meteorites.
Adv. Space Res. 3, 5–18.
Cronin, J.R., Chang, S., 1993. Organic matter in meteorites:
Molecular and isotopic analyses of the Murchison meteorite.
In: Greenberg, J.M., et al. (Eds.), The Chemistry of
Life’s Origins. Kluwer Academic, The Netherlands, pp.
209–258.
Cronin, J.R., Pizzarello, S., 1997. Enantiomeric excesses in
meteoric amino acids. Science 275, 951–955.
Engel, M.H., 1980. Ph.D. Thesis, Univrsity of Arizona, Tucson,
AZ.
M.H. Engel, S.A. Macko / Precambrian Research 106 (2001) 35–45
Engel, M.H., Nagy, B., 1982. Distribution and enantiomeric
composition of amino acids in the Murchison meteorite.
Nature 296, 837–840.
Engel, M.H., Nagy, B., 1983. Author’s reply. Nature 301,
496–497.
Engel, M.H., Macko, S.A., 1984. The separation of amino
acid enantiomers for stable nitrogen and carbon isotope
analyses. Anal. Chem. 56, 2598–2600.
Engel, M.H., Macko, S.A., 1986. Stable isotope evaluation of
the origins of amino acids in fossils. Nature 323, 531–533.
Engel, M.H., Macko, S.A., 1997a. Stable isotope analysis of
amino acid enantiomers in the Murchison meteorite at
natural abundance levels. Proc. SPIE 3111, 82–85.
Engel, M.H., Macko, S.A., 1997b. Isotopic evidence for extraterrestrial
non-racemic amino acids in the Murchison
meteorite. Nature 389, 265–268.
Engel, M.H., Macko, S.A., Nagy, B., 1993. The organic
geochemistry of carbonaceous meteorites: amino acids and
stable isotopes. In: Engel, M.H., Macko, S.A. (Eds.), Organic
Geochemistry, Principles and Applications. Plenum,
New York, pp. 685–695.
Engel, M.H., Goodfriend, G.A., Qian, Y., Macko, S.A., 1994.
Indigeneity of organic matter in fossils: A test using stable
isotope analysis of amino acid enantiomers in fossil mollusc
shells. Proc. Natl. Acad. Sci. USA 91, 10475–10478.
Engel, M.H., Macko, S.A., Qian, Y., Silfer, J.A., 1995. Stable
isotope analysis at the molecular level: A new approach for
determining the origins of amino acids in the Murchison
meteorite. Adv. Space Res. 15, 99–106.
Engel, M.H., Macko, S.A., Silfer, J.A., 1990. Carbon isotope
composition of individual amino acids in the Murchison
meteorite. Nature 348, 47–49.
Epstein, S., Krishnamurthy, R.V., Cronin, J.R., Pizzarello, S.,
Yuen, G.U., 1987. Unusual stable isotope ratios in amino
acid and carboxylic acid extracts from the Murchison
meteorite. Nature 326, 477–479.
Greenberg, J.M., 1997. Prebiotic chiral molecules created in
interstellar dust and preserved in comets, comet dust and
meteorites: an exogenous source of life’s origins. Proc.
SPIE 3111, 226–237.
Hayes, J.M., 1967. Organic constituents of meteorites — A
review. Geochim. Cosmochim. Acta 31, 1395–1440.
Helgeson, H.C., Owens, C.E., Knox, A.M., Richard, L., 1998.
Calculation of the standard molal thermodynamic properties
of crystalline, liquid, and gas organic molecules at high
temperatures and pressures. Geochim. Cosmochim. Acta
62, 985–1081.
Hennet, R.J.-C., Holm, N.G., Engel, M.H., 1992. Abiotic
synthesis of amino acids under hydrothermal conditions
and the origin of life: a perpetual phenomenon? Naturwissenschaften
79, 361–365.
Khare, B.N., Sagan, C., Ogino, H., Nagy, B., Er, C., Schram,
K.H., et al., 1986. Amino acids derived from Titan tholins.
Icarus 68, 176–184.
Kvenvolden, K.A., Lawless, J., Pering, K., Peterson, E., Flores,
J., Ponnamperuma, C., et al., 1970. Evidence for
extraterrestrial amino acids and hydrocarbons in the
Murchison meteorite. Nature 228, 923–926.

Macko, S.A., Estep, M.L.F., Hare, P.E., Hoering, T.C., 1987.
Isotopic fractionation of nitrogen and carbon in the synthesis
of amino acids by microorganisms. Isotope Geosci.
65, 79–92.
Macko, S.A., Uhle, M.E., Engel, M.H., Andrusevich, V.,
1997. Stable nitrogen isotope analysis of amino acid enantiomers
by gas chromatography/combustion/isotope ratio
mass spectrometry. Anal. Chem. 69, 926–929.
Maurette, M., 1998. Carbonaceous micrometeorites and the
origin of life. Origins of Life and Evolution of the Biosphere
28, 385–412.
McKay, D.S., Gibson Jr, E.K., Thomas-Keprta, K.L., Vali,
H., Romanek, C.S., Clemett, S.J., et al., 1996. Search for
past life on Mars: possible relic biogenic activity in Martian
meteorite ALH84001. Science 273, 924–930.
Miller, S.L., 1953. Production of amino acids under possible
primitive earth conditions. Science 117, 528–529.
Mita, H., Shimoyama, A., 1998. �-Aminoisobutyric acid and
isovaline in Tokyo Bay sediments. Geochim. Cosmochim.
Acta 62, 47–50.
Mojzsis, S., Arrhenius, G., McKeegan, K.D., Harrison, T.M.,
Nutman, A.P., Friend, C.R.L., 1996. Evidence for life on
Earth before 3,800 million years ago. Nature 385, 55–59.
Nagy, B., 1975. Carbonaceous Meteorites. Elsevier,
Amsterdam.
Nagy, B., Engel, M.H., Zumberge, J.E., Ogino, H., Chang,
S.Y., 1981. Origins of amino acids and hydrocarbons in the
3,800 MYR old Isua rocks, southwestern Greenland. Nature
289, 53–56.
Nutman, A.P., Mojzsis, S.J., Friend, C.R.L., 1997. Recognition
of �3850 Ma water-lain sediments in West Greenland
and their significance for the early Archaean Earth.
Geochim. Cosmochim. Acta 61, 2475–2484.
Olson, E.S., 1992. Amino acids from coal gasification? Nature
357, 202.
Pflug, H.D., Heinz, B., 1997. Analysis of fossil organic nanostructures:
terrestrial and extraterrestrial. Proc. SPIE 3111,
86–97.
Pizzarello, S., Cronin, J.R., 1998. Alanine enantiomers in the
Murchison meteorite. Nature 394, 236.
Pizzarello, S., Feng, X., Epstein, S., Cronin, J.R., 1994. Isotopic
analyses of nitrogeneous compounds from the
Murchison meteorite: ammonia, amines, amino acids and
polar hydrocarbons. Geochim. Cosmochim. Acta 58,
5579–5587.
Qian, Y., Engel, M.H., Macko, S.A., Carpenter, S., Deming,
J.W., 1993. Kinetics of peptide hydrolysis and amino acid
decomposition at high temperature. Geochim. Cosmochim.
Acta 57, 3281–3293.
M.H. Engel, S.A. Macko / Precambrian Research 106 (2001) 35–45 45
.
Rafalska, J.K., Engel, M.H., Lanier, W.P., 1991. Retardation
of racemization rates of amino acids incorporated into
melanoidins. Geochim. Cosmochim. Acta 55, 3669–
3675.
de Ronde, C.E.J., Channer, D.M., Faure, K., Bray, C.J.,
Spooner, E.T.C., 1997. Fluid chemistry of Archean
seafloor hydrothermal vents: implications for the composition
of circa 3.2 Ga seawater. Geochim. Cosmochim. Acta
61, 4025–4042.
Russell, M.J., Hall, A.J., Turner, D., 1989. In vitro growth of
iron sulphide chimneys: possible culture chambers for
origin-of-life experiments. Terra Res. 1, 238–241.
Schidlowski, M., 1993. The initiation of biological processes
on Earth: Summaries of empirical evidence. In: Engel,
M.H., Macko, S.A. (Eds.), Organic Geochemistry, Principles
and Applications. Plenum, New York, pp. 639–655.
Shock, E.L., 1990. Geochemical constraints on the origin of
organic compounds in hydrothermal systems. Origins of
Life 20, 331–367.
Shock, E.L., Schulte, M.D., 1990. Summary and implications
of reported amino acid concentrations in the Murchison
meteorite. Geochim. Cosmochim. Acta 54, 3159–3173.
Silfer, J.A., 1991. Ph.D. Thesis, University of Oklahoma, OK.
Silfer, J.A., Engel, M.H., Macko, S.A., Jumeau, E.J., 1991.
Stable carbon isotope analysis of amino acid enantiomers
by conventional isotope ratio mass spectrometry and combined
gas chromatrography/isotope ratio mass spectrometry.
Anal. Chem. 63, 370–374.
Silfer, J.A., Qian, Y., Macko, S.A., Engel, M.H., 1994. Stable
carbon isotope compositions of individual amino acid
enantiomers in mollusc shell by GC/C/IRMS. Org.
Geochem. 21, 603–609.
Snyder, L.E., 1997. Detection of large interstellar molecules
with radio interferometers. Proc. SPIE 3111, 296–304.
Steele, A., Westall, F., McKay, D.S., 1999. Contamination of
the Murchison meteorite. In: Proceedings of the 30th Lunar
and Planetary Science Conference. LPI Publication,
Houston, TX.
Urey, H.C., 1966. Biological material in meteorites: a review.
Science 151, 157–166.
Watanabe, Y., Naraoka, H., Wronkiewicz, D.J., Condie,
K.C., Ohmoto, H., 1997. Carbon, nitrogen, and sulfur
geochemistry of Archean and Proterozoic shales from the
Kaapvaal Craton, South Africa. Geochim. Cosmochim.
Acta 61, 3441–3459.
Wickramasinghe, N.C., Hoyle, F., Wallis, D.H., 1997. Spectroscopic
evidence for panspermia. Proc. SPIE 3111, 282–
295.