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Review
Molecular Asymmetry in Prebiotic Chemistry: An
Account from Meteorites
Sandra Pizzarello
School of Molecular Sciences, Arizona State University, Tempe, AZ 85287, USA; [email protected];
Tel.: +1-480-965-3370
Academic Editors: Pier Luigi Luisi and David Deamer
Received: 8 February 2016; Accepted: 7 April 2016; Published: 13 April 2016
Abstract: Carbonaceous Chondrite (CC) meteorites are fragments of asteroids, solar planetesimals
that never became large enough to separate matter by their density, like terrestrial planets. CC contains
various amounts of organic carbon and carry a record of chemical evolution as it came to be in the
Solar System, at the time the Earth was formed and before the origins of life. We review this record as
it pertains to the chiral asymmetry determined for several organic compounds in CC, which reaches
a broad molecular distribution and enantiomeric excesses of up to 50%–60%. Because homochirality
is an indispensable attribute of extant polymers and these meteoritic enantiomeric excesses are still,
to date, the only case of chiral asymmetry in organic molecules measured outside the biosphere, the
possibility of an exogenous delivery of primed prebiotic compounds to early Earth from meteorites is
often proposed. Whether this exogenous delivery held a chiral advantage in molecular evolution
remains an open question, as many others regarding the origins of life are.
Keywords: meteorites; asteroids; abiotic; prebiotic; chemical evolution; enantiomeric excess
1. Carbon-Containing Meteorites and Cosmochemical Evolution
One of the paradigms of chemical evolution proposes that the abiotic formation of increasingly
complex molecules throughout cosmochemical and Earthly processes could eventually lead to life’s
precursor molecules. It finds its rationale in the observation of phenomena, both leading to, and
proceeding from, the origin of life, such as the formation of complex organic molecules in interstellar
media (e.g., [1]), and a terrestrial phylogeny that evolved from much simpler organisms (e.g., [2]).
To date, such chemical evolution has found direct experimental scrutiny, solely through the
analyses of Carbonaceous Chondrites (CC) meteorites, stony fragments of asteroids containing
abundant organic carbon, ranging in complexity from kerogen-like macromolecular materials to
simple soluble compounds, several of which are identical to terrestrial biomolecules. Because CC
Asteroidal parent bodies are planetesimals found between Mars and Jupiter that never accreted
large enough to undergo the extensive differentiation involved in planet formation, their meteoritic
fragments uniquely offer a pristine record of abiotic organic chemistry in the early Solar System, as
found in a planetary setting, at a unique juncture in the long history of the biogenic element, and at its
closest to the onset of life. Their studies, therefore, have offered an understanding, not only of how
compounds identical to life’s molecules were formed abiotically, but also about the organic inventory
accreted by early Earth, e.g., through its heavy bombardment from comets and meteorites [3,4].
Comprehensive molecular and isotopic analyses for over forty-five years, and continuing today,
have revealed that CCs may contain a complex organic suite of compounds, which are seen as the
products of the synthetic capabilities of both solar and pre-solar environments [5]. Their complex
range varies from polar compounds, such as amino acids, to non-polar hydrocarbons (e.g., [5–7])
and, when detected simply by their elemental compositions, may include thousands of molecular
species [8]. From the perspective of a possible continuity between cosmochemical and prebiotic
Life 2016, 6, 18; doi:10.3390/life6020018
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Life 2016, 6, 18
2 of 9
evolution, this diverse suite would not appear to indicate any selective synthetic trend and, in fact,
indicate a fundamental distinction between abiotic and biological chemical processes in their gaining
of molecular complexity. To say it with the words of Shakespeare, therefore, a meteoritic organic input
of thousands H, C, N, O containing molecules would have offered “too much of a good thing” for an
evolutionary transition towards the origins of life.
Nevertheless, within this evident heterogeneity, several meteoritic compounds have been found to
contain L-enantiomeric excesses (ee), i.e., to be more abundant in the L-chiral form like the amino acids
of terrestrial proteins [9], offering the first unequivocal indication that a purely chemical evolutionary
process, as the one believed to be responsible for the formation of meteorite organics, may also have
been at work in chiral selection, a property known since the time of Pasteur to be intimately associated
with life.
The following review reports on ee distributions found within meteorite organics so far, the
possible venues that might have lead to their formation, and the effect that the input of such selected
molecules might have had in prebiotic chemistry.
2. Enantiomeric Excesses in Meteoritic Compounds
The search for chiral asymmetry in meteorites was first undertaken by Engel and Nagy [10]
and, although carried out carefully, was controversial [11] because of the pervasive distribution
of homochirality in the biosphere and the possibility of terrestrial contamination in carbonaceous
meteorites resulting from bacteria, molds, or their remnants. However, ee were ultimately detected in
the Murchison meteorite for some amino acids not common in terrestrial proteins and abundant in
meteorites, the 2-methyl, 2-aminoacids (2maa), and to have the same configuration (L) as protein amino
acids [9]. To date, L-ee have been detected for a number of amino acids, as well as other compounds,
and their inventory is summarized in Table 1 [12]. As shown, the larger number comprises the amino
acids of several meteorites and involves molecular species that lack an α-H, with the exception of the
diastereomers of isoleucine, as further detailed below, and the one-time finding for aspartic acid [13].
Lactic acid is the only hydroxy acid to display ee [14], while two recent communications have reported
their detection in several sugar acids derivatives [15] and two amines, sec-butyl- and 1-methylbutyl
amines [16]. These latter studies, appear to point at the possibility of diverse asymmetric effects in
cosmochemistry. These meteoritic enantiomeric excesses are still, to date, the only case of molecular
asymmetry measured outside the biosphere.
The overall extent of ee in 2maa varies significantly, between 0% and 18%, and even within short
distances in the same meteorite stone [17,18]. For Isovaline (2-amino, 2-methylbutanoic acid), the
most abundant of the acids, the absence of any contribution from terrestrial contaminants within
this chiral heterogeneity was validated by Murchison, using 13 C-, and D isotopic analyses [17–19],
where the two enantiomers were found to have comparable enrichments. Isovaline δD values were
particularly high, +3600‰ [19], and near to those measured spectroscopically for organic molecules in
the interstellar medium.
It should be noted that racemization, the conversion of one enantiomer into another, is possible
for 2H-amino acids in water through a repeat statistical abstraction and reacquisition of the α-H and
because of its vicinity to the electron-withdrawing carboxyl group. Racemization, with time and
proper conditions, will lead to racemic solutions (equal amounts of the two enantiomers). The fact
that all amino acids found non-racemic in meteorites have a 2-methyl in place of a 2-H and cannot
racemize, easily raises the question of whether the twelve racemizable meteoritic amino acids detected
having terrestrial protein counterparts might have initially carried ee but racemized during the water
phases known to have affected CC parent bodies. The understanding of the processes involved in
meteoritic amino acids’ syntheses and the apparently odd ee findings for diastereomer amino acids in
several meteorites may be able to address the question.
compounds,
and
their
inventory
summarized
in
[12].same
shown,
theaslarger
number
acids [9].
date,
L-ee
haveis been
detected
forTable
number
ofAs
amino
acids,
well
other
in
meteorites,
theTo
2-methyl,
2-aminoacids
(2maa),
and
toa have
the
configuration
(L)and
asas
protein
withamino
the
exception
of
the
diastereomers
of
isoleucine,
as 1further
detailed
below,
the
one-time
comprises
the
amino
acids
of
several
meteorites
and
involves
molecular
species
that
lack
another
α-H,
compounds,
and
their
inventory
is
summarized
in
Table
1
[12].
As
shown,
the
larger
number
amino
[9]. To
date,
haveacid
beenisdetected
a number
of to
amino
acids,eeas
wellwhile
as
finding
foracids
aspartic
acid
[13].L-ee
Lactic
the onlyforhydroxy
acid
display
[14],
two recent
with the exception
of reported
the
diastereomers
of isoleucine,
as further
detailed
below,
and
the
comprises
theand
amino
acids
of several
meteorites
and
involves
molecular
species
that
lackone-time
an
α-H,
compounds,
their
inventory
is summarized
in
1 [12].
As acids
shown,
the
larger
number
communications
have
their
detection
in Table
several
sugar
derivatives
[15]
and two
finding
aspartic
acid
acid
isofthe
onlyand
hydroxy
acidmolecular
to
displayspecies
ee [14],and
while
two
with
thefor
exception
of
the[13].
diastereomers
isoleucine,
as further
detailed
below,
the
one-time
comprises
the
amino
acids
ofLactic
several
meteorites
involves
that
lack
anrecent
α-H,
amines, sec-butyl- and 1-methylbutyl amines [16]. These latter studies, appear to point at the
communications
have
reported
their
inhydroxy
several
sugar
acids
derivatives
[15]
and
two
finding
aspartic
acid
[13].
Lactic
aciddetection
isofthe
only
acid
to
display
ee [14],and
while
recent
with
thefor
exception
of
the
diastereomers
isoleucine,
as further
detailed
below,
thetwo
one-time
possibility
ofsec-butyldiverse
asymmetric
effectsamines
in cosmochemistry.
These meteoritic
enantiomeric
excesses
Life 2016,
6, 18 have
amines,
and
1-methylbutyl
[16].
These acid
latter
appear
to[15]
point
the
communications
their
inhydroxy
several
sugartostudies,
acids
derivatives
andat
two
finding
for aspartic
acidreported
[13]. Lactic
aciddetection
is the only
display
ee
[14], while
two
recent
are still,
to
date,
the
only
case
of
molecular
asymmetry
measured
outside
the
biosphere.
possibility
of diverse
asymmetric
effectsdetection
in cosmochemistry.
These
meteoritic
enantiomeric
excesses
amines,
sec-butyland
1-methylbutyl
amines
[16].
These latter
appear
to [15]
point
at two
the
communications
have
reported
their
in several
sugar studies,
acids derivatives
and
are still, to
date,
theand
only
case of molecular
measured
outside
theappear
biosphere.
possibility
of
diverse
asymmetric
effectsamines
in asymmetry
cosmochemistry.
These
meteoritic
enantiomeric
amines,
sec-butyl1-methylbutyl
[16]. These
latter
studies,
to pointexcesses
at the
Table
1.
Amines
and
amino-,
hydroxy,
and
sugar-acids
displaying
enantiomeric
in
Table
Amines
and
amino-,
hydroxy,
and sugar-acids
displaying
excesses
in
are
still, 1.
to
date,
the only
case
of molecular
measured
outside
theenantiomeric
biosphere.
possibility
of
diverse
asymmetric
effects
in asymmetry
cosmochemistry.
These
meteoritic
enantiomeric
excessesexcesses
Table
1. Amines
and
amino-,
hydroxy,asymmetry
and sugar-acids
displaying
enantiomeric
excesses in
carbonaceous
chondrites.
carbonaceous
chondrites.
are
still,
to
date,
the only
case
of molecular
measured
outside
the biosphere.
carbonaceous
chondrites.
Table
1. Amines
and amino-, hydroxy, and sugar-acids displaying enantiomeric excesses in
Compound
Formula
carbonaceous
chondrites.
Table
1. Amines
and amino-,
Compound
Formula
Compound Formula
carbonaceous chondrites.
Compound Formula
Compound Formula
NH
22
NH
NH2
NH2
OO
Life 2016, 6, 18
Life 2016, 6, 18
Life 2016, 6, 18
O OH
OH
NH
NH2O 2OH
NHO
O 2OH
NHO2 OH
OH
NH
O
2OH
NHNH
2 O2
OH
O
O2 OH
NH
NH
2OH
O
OH
NHNH
2 2OH
NH2
O
O
Compound
Distribution
1
hydroxy,
and sugar-acids displaying ee%
enantiomeric
excesses2 inDistribution 2
Compound
1 ee%
1
Compound
Compound
Compound
Sec-butylamine
Sec-butylamine
Sec-butylamine
Sec-butylamine
Sec-butylamine
2
ee%
Distribution
GRA 95229 3,*
ee% 1
Distribution
GRA 95229 3,* 2 GRA 95229 3, *
LAP 02342LAP 02342
ee% 1 (S)(S)
Distribution
LAP95229
023423,* 2
GRA
(S)
MIL3,*07525MIL 07525
MIL95229
07525
LAP
02342
0-18GRA
0-18
(S) 0-18
PCA 91082PCA 91082
PCA
91082
MIL 07525
LAP
02342
0-18
(S)
EET 92049
PCA
91082
MIL
07525
EET 92049EET 92049
0-18
EET 92049
PCA
91082
L L MN, MY
*, Or
2amino2methylbutanoic a. 4 a. 4
MN,
MY5 *, Or 5
2amino2methylbutanoic
EET
92049
5
6,* 5
4 4
(isovaline)
2.5-19.6
LAP02342
L
MN,
MY
*,
Or
2amino2methylbutanoic
a.
L
MN,
2amino2methylbutanoic
a.
6,* MY *, Or
(isovaline)
2.5-19.6
LAP02342
6,
(isovaline)
(isovaline)
2amino2methylbutanoic
a. 4
2amino2methylpentanoic
(isovaline)
2amino2methylpentanoic
(2methylnorvaline)
2amino2methylpentanoic
(2methylnorvaline)
(2methylnorvaline)
2amino2methylpentanoic
2amino2methylpentanoic
2amino2methylhexanoic
(2methylnorvaline)
(2methylnorvaline)
2amino2methylhexanoic
(2methylnorleucine)
2amino2methylhexanoic
(2methylnorleucine)
2amino2methylhexanoic
(2methylnorleucine)
(2methylnorleucine)
2amino2methylhexanoic
(2methylnorleucine)
6,* 5
LAP02342 *
2.5-19.6
LAP02342
L 2.5-19.6
MN,
MY *, Or
L
6,*
2.5-19.6
LAP02342
MN, MY
L
1.4-2.8
L
MN, MY
MN,
MY
1.4-2.8
L 1.4-2.8
L
MN, MY
L 1.4-2.8 MN, MY
1.4-2.8
MN, MY
1.8
L L
MN, MY
MN, MY
1.8
L 1.8
MN, MY
1.8
L
3 of
9
MN,
MY
3 of 9
1.8
3 of 9
2amino2methylheptanoic
L
MN
3 of 9
OH
2amino2methylheptanoic
L
MN
O OH
NH
2
Life 2016, 6, 18
3 of 9 MN
2amino2methylheptanoic
L
NH
2amino2methylheptanoic
L
MN
O O 2OH
MN,
MY
Life 2016, 6, 18
3 of 9
O
MN, MY
2amino3methylpentanoic
L
NH
2
O
2amino2methylheptanoic
MN
OHOH
2amino3methylpentanoic
L
Life 2016, 6, 18
3 of 9
GRA
95229
OH
MN, MY
GRA
95229
(isoleucine)
4-50
MN,
MY
NH2ONH
O 2OH
2amino2methylheptanoic
L
MN
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3 of 9
2amino3methylpentanoic
L LAP
(isoleucine)
4-50
02342
2amino3methylpentanoic
L
NH2
GRA
95229
LAP
02342
OH
ONH
95229
O 2OH
2amino2methylheptanoic
L
MN
O
MN,
MY
(isoleucine)
4-50 GRA
MN,
MY
Life 2016, 6, 18
3 LAP
of 9 02342
(isoleucine)
4-50
O
2amino3methylpentanoic
L
NH2O
MN,
MY
2amino3methylpentanoic
D
NH
O
LAP
02342
MN,
MY
2amino2methylheptanoic
L
MN
OH
GRA
95229
OH2OH
2amino3methylpentanoic
D
GRA
95229
2amino3methylpentanoic
L
(isoleucine)
4-50
OH2
NH2ONH
GRA
95229
(alloisoleucine)
2-60
2amino2methylheptanoic
L
MN
OH
MN,
MY
GRA
95229
H2N
O OH
LAP
02342
(alloisoleucine)
2-60
LAP 02342
2amino3methylpentanoic
L
D
H2N
(isoleucine)
4-50
NH2O
NH
LAP
02342
MN,
MY
MN, MY
GRA
95229
LAP
02342
MN,
MY
OO OH2OH
2amino2methylheptanoic
L
MN
2amino3methylpentanoic
D
2amino3methylpentanoic
L
(isoleucine)
4-50
(alloisoleucine)
2-60
O OH
2amino3methylpentanoic
D
NHO
H2N
2amino2,3methylbutanoic
L
2O
MN,02342
MY
GRA 95229
GRA
95229
NH
LAP
OH
MN,
MY
2
GRA
95229
2amino2,3methylbutanoic
L
OH
(alloisoleucine)
2-60 MN, MY
2amino3methylpentanoic
L
(isoleucine)
4-50
2amino3methylpentanoic
D
(alloisoleucine)
2-60
(2methylvaline)
1.0
2 OOH
OH
LAP 02342
O
N NH
H2NH
GRA
95229
LAP
GRA
95229
MN,02342
MY
2 OH
LAP
02342
(2methylvaline)
1.0
2amino2,3methylbutanoic
L
(isoleucine)
4-50
NH
D
NH
2amino3methylpentanoic
L
(alloisoleucine)
2-60
2O
H2N O
LAP
02342
MN,
MY
O2OH
OH
GRA
95229
LAP 02342
(2methylvaline)
1.0
O
2amino3methylpentanoic
D
2amino2,3methylbutanoic
L
(alloisoleucine)
2-60
(isoleucine)
4-50
O
N NH
H2NH
O
MN,02342
MY
2 2 OH
L
2amino2,3methylpentanoic
GRA
95229
LAP
OH
OH
2amino3methylpentanoic
D
2amino2,3methylpentanoic
MN, MY
2amino2,3methylbutanoic
L
1.4-5.2
(alloisoleucine)
2-60
(2methylvaline)
1.0
2amino2,3methylbutanoic
L GRA
O OH
OH
H2N NH
NH
O
O2OH
95229
2
1.4-5.2
MN,
MY
LAP 02342
MN, MY
L
MY
NH2
2amino2,3methylbutanoic
L
(alloisoleucine)
2-60
(2methylvaline)
1.0
(2methylvaline)
1.0 MN,
2amino2,3methylpentanoic
MN,02342
MY
2amino3methylpentanoic
D
H2N NH
OH
OO2OH
MN,
MY
LAP
OH
1.4-5.2
GRA
95229
O
allo
L
2amino2,3methylbutanoic
(2methylvaline)
1.0
NH2
(alloisoleucine)
2-60
allo
L
OO2OH
2amino2,3methylpentanoic
H2N NH
MN,
MY
OH
LAP
L
MN,02342
MY
2amino2,3methylpentanoic
2.2-10.4
1.4-5.2
2amino2,3methylbutanoic
O OH
(2methylvaline)
1.0
O
NH
NH
2amino2,3methylpentanoic
MN,
MY
2.2-10.4
2OH
2OH
allo
L
O
NH2
L
1.4-5.2
L
(2methylvaline)
1.0
MN, MY
MY
O
2amino2,3methylbutanoic
L
NH
OH
O2 2O
NH
2amino2,3methylpentanoic
MN,
2amino2,3methylpentanoic
MN, MY
OH
2amino2,3methylpentanoic
2.2-10.4
O
O
MY
L 1.4-5.2 MN,
allo
1.4-5.2
NH
2
OOH
NH
L
(2methylvaline)
1.0
2methylglutamic
MN
2amino2,3methylpentanoic
MN,
MY
2
HO
OH
OH
NH2
allo
L
2methylglutamic
MN
3-2
2amino2,3methylpentanoic
2.2-10.4
O OH
1.4-5.2
HO O
ONH
NH
MN, MY
2
3-2
2amino2,3methylpentanoic
2OH
L
ONH
allo
L
2
2amino2,3methylpentanoic
2.2-10.4
1.4-5.2
2methylglutamic
MN
7,*,
L
HO O ONH
OH
O 2O
MN
MN,
MY
OH
3-2
O NH
7,*,
2amino2,3methylpentanoic
OH
allo
L
MN MY
2amino2,3methylpentanoic
2.2-10.4
allo
L MN,
2
O
O
1.4-5.2
O
NH
2methylglutamic
MN
MN,
MY
2
L
GRA95229 *
Lactic
OH
MN, MY
HO
OH
NH
2OH
L 2.2-10.4
allo
3-2
O OH
2amino2,3methylpentanoic
2.2-10.4
7,*, *
2amino2,3methylpentanoic
GRA95229
Lactic
3.0-12-3
MN
NH
2methylglutamic
MN
MN,
MY
2O 2OH
HO O OH NH
3.0-12-3
LAP
02342
*
L
OH O OH
3-2
2amino2,3methylpentanoic
2.2-10.4
LAP
02342
allo
L
7,*, *
GRA95229
*
Lactic
OH
O 2OH
2methylglutamic
MN
MN
2
ONH
HO O OHONH
MN,
MY
3.0-12-3
OH
L
3-2
OH
OHO ONH
7,
2amino2,3methylpentanoic
2.2-10.4
O
O
LAP
02342
MN
2methylglutamic
MN 8*, **
2OH
GRA95229
Lactic
OH
HO
Threonic
D
MN
NH
2OH
HO
8
L
3-2
L
3.0-12-3
Threonic
D
MN
O
7,*,
OH
O
OH
NH
OH
2methylglutamic
HO
MN
2methylglutamic
MN
GRA95229
*
Lactic
HO O
LAPMN
02342
OHOHO 2OH
3-2
L
3-2
3.0-12-3
OH
L
8
7,*,
OHO
Threonic
MN
2
1 Values HO
2 Meteorites where ee D
MN
OH
GRA95229
* are
Lactic
ONH
OH
OH
LAP
02342
from
[12]
unless
otherwise
indicated;
of
the
given
compounds
2methylglutamic
MN
HO
OH
1 Values from [12] unless otherwise indicated; 2 Meteorites where3.0-12-3
Lof the given compounds
ee3-2
O
7,8*,
OH
OH
MN
OH
ONH
GRA95229
* are
Lactic
OH
LAP
02342
D
MN
found. MN,
Murchison;
MY, Murray; Threonic
Or,
Orgueil; * isotopic data
acquired
for
individual
2
OH
HO Murchison;
L acquired
3.0-12-3
found.
MY,
Murray;
Or,
Orgueil;
*
isotopic
data
for
individual
1 ValuesMN,
2
OH
from
[12]
unless
Meteorites
where
ee D
of the
given
compounds
are
OH
MN 7, *,
O OH
GRA95229
Lactic
3O
4 acid; 5otherwise
7 [14];
8 [15],
7,8*, *sugar
OH
Threonic
MN
LAP
02342
OH
[18]; 6 [39];indicated;
this last study
detected
other
ee-containing
enantiomers;
MN
HO 3 [16];
L ee-containing
4 acid; 5 [18]; 6 [39]; 7 [14]; 8 [15], this last study detected
3.0-12-3
[16];
other
sugar
enantiomers;
L
OH
8
MY, Murray;
Or,
Orgueil;
* isotopic
acquired
for
individual
1found.
2 Meteorites
Lactic
OHMurchison;
O OH
LAP
02342
OH
Threonic
D
MN
ValuesMN,
from
[12]
unless otherwise
indicated;
where eedata
of the
given
compounds
acids.
GRA95229
** are GRA95229 *
Lactic
OH
HO
3.0-12-3
3 [16]; 4 acid; 5 [18]; 6 [39]; 7 [14]; 8 [15],
3.0-12-3
8
1acids.
2 Meteorites
OH
this last
study
detected
other
ee-containing
enantiomers;
O OH
OH
Threonic
D
MN
ValuesMN,
from
[12]
unless
otherwise
indicated;
where
ee
of
the
given
compounds
are LAP 02342 *
OH
found.
Murchison;
MY,
Murray;
Or,
Orgueil;
*
isotopic
data
acquired
for
individual
LAP 02342 *sugar
HO
8
1
2
acids.
3
4
5
6
7
8
Threonic
D
MN
OH
Values
from
[12]
unless
Meteorites
where
ee
of the
given
compounds
are
found.
MN,
Murchison;
MY,
Murray;
Or,significantly,
Orgueil;
* study
isotopic
data
acquired
for
individual
[16];
acid;
[18];
[39];indicated;
[14];
[15],
this last
detected
other
ee-containing
sugar
enantiomers;
The
overall
of ee otherwise
in
2maa
varies
between
0%
and
18%,
and
even
within
OHextent
O OH
HO
The overall
extent
of ee in 2maa
varies
significantly,
between 0% and 18%, and even within
2 Meteorites
3 Murchison;
4 acid; 5otherwise
7 [14];
8 [15],
OH
ValuesMN,
from
[12]
unless
indicated;
where
eedata
of the
given
compounds
areacid),
found.
MY,
Or,
Orgueil;
* study
isotopic
acquired
for individual
8
[16];
[18]; 6Murray;
[39];stone
this
last
detected
other
ee-containing
sugar
acids.
short1enantiomers;
distances
in
the
same
meteorite
[17,18].
For
Isovaline
(2-amino,
2-methylbutanoic
Threonic
D
MN
OH
HO inextent
shortfound.
distances
the same
stone
[17,18].
For Isovaline
(2-amino,
2-methylbutanoic
acid),
1The
2
overall
of eemeteorite
in
2maa
varies
between
0%
and
18%,
and
even
within
4 acid;
5otherwise
6Murray;
7 [14];
8significantly,
Values
from3Murchison;
[12]
where
ee
of
the
given
compounds
are
MN,
MY,
Or,
Orgueil;
* isotopic
data
acquired
for
individual
[16];
[18];
[39];indicated;
[15],
this
study
detected
other
ee-containing
sugar
enantiomers;
acids.
the most
abundant
ofunless
the
acids,
the
absence
ofMeteorites
anylast
contribution
from
terrestrial
contaminants
OH
Threonic
D
MN 8
the
most
abundant
of
the
acids,
the
absence
of
any
contribution
from
terrestrial
contaminants
3extent
4 acid;
5 MY,
6Murray;
7 [14];
8significantly,
short
distances
in
the
same
meteorite
stone
[17,18].
For
Isovaline
(2-amino,
2-methylbutanoic
acid),
found.
MN,
Murchison;
Or,
Orgueil;
*
isotopic
data
acquired
for
individual
13
The
overall
of
ee
in
2maa
varies
between
0%
and
18%,
and
even
within
acids.
[16];
[18];
[39];
[15],
this
last
study
detected
other
ee-containing
sugar
enantiomers;
1
2
withinValues
this1 chiral
heterogeneity
was
validated
byMeteorites
Murchison,
using
C-,
and
Dcompounds
isotopic
analyses
from [12]
unless
otherwise
indicated;
where
ee of13ee
the
are are found. MN,
2 Meteorites
Values
from
[12]
unless
otherwise
indicated;
where
ofgiven
the
given
compounds
within
this
chiral
heterogeneity
was
validated
by
Murchison,
using
C-,
D isotopic
analyses
4 acid;
5 in
6 the
7 [14]; 8significantly,
the
most
abundant
ofsame
the
acids,
absence
of
any
contribution
from
terrestrial
contaminants
The
overall
of eemeteorite
2maa
varies
between
0%
andand
18%,
and
even
[16];
[18];
[39];
[15],
this
last
detected
other
ee-containing
sugar
enantiomers;
acids.
short
distances
in3extent
the
stone
[17,18].
For
Isovaline
(2-amino,
2-methylbutanoic
acid),
3within
4
5
found.
MN,the
Murchison;
MY,
Murray;
Or,
Orgueil;
* study
isotopic
data
acquired
for
individual
[17–19],
where
two
enantiomers
were
found
have
comparable
enrichments.
Isovaline
δD
values
were
Murchison;
MY,
Murray;
Or,
Orgueil;
* to
isotopic
data
acquired
for individual
enantiomers;
[16];
13C-,
[17–19],
where
the
two
enantiomers
were
found
to by
have
comparable
enrichments.
Isovaline
δD
values
were acid; [18];
within
this
heterogeneity
was
validated
Murchison,
using
and
D
isotopic
analyses
6 chiral
7
8
The
overall
extent
of
ee
in
2maa
varies
significantly,
between
0%
and
18%,
and
even
within
acids.
short
distances
in
the
same
meteorite
stone
[17,18].
For
Isovaline
(2-amino,
2-methylbutanoic
acid),
3
4
5
6
7
8
the
most
abundant
of
the
acids,
the
absence
of
any
contribution
from
terrestrial
contaminants
[39];+3600‰
[14]; acid;
[15],
last
study
detected
other
ee-containing
sugarother
acids.
[16];
[18];
[39];
this last
study detected
sugar in
enantiomers;
particularly
high,
[19],this
and
near
to[14];
those[15],
measured
spectroscopically
foree-containing
organic molecules
particularly
high,
+3600‰
[19],
and2maa
near
to
those
spectroscopically
for
organic
molecules
in
13C-,
The
overall
extent
of
eemeteorite
in
varies
significantly,
between
0%
andand
18%,
andδD
even
within
[17–19],
where
the
two
enantiomers
were
found
tomeasured
have
comparable
enrichments.
Isovaline
values
were
short
distances
in
the
stone
[17,18].
For
Isovaline
(2-amino,
2-methylbutanoic
acid),
the
most
abundant
ofsame
the
acids,
the
absence
of
any
contribution
from
terrestrial
contaminants
within
this
chiral
heterogeneity
was
validated
by
Murchison,
using
D isotopic
analyses
acids.
the interstellar
medium.
the
interstellar
medium.
13
The
overall
extent
of
ee
in
2maa
varies
significantly,
between
0%
and
18%,
and
even
within
short
in
the
meteorite
stone
[17,18].
For
Isovaline
(2-amino,
2-methylbutanoic
acid),
particularly
high,
+3600‰
[19],
andwas
near
to
those
spectroscopically
for
organic
molecules
in
the
most
abundant
ofsame
the
acids,
the
absence
of
any
contribution
from
terrestrial
contaminants
within
this
chiral
heterogeneity
validated
by
Murchison,
C-, and
Danother,
isotopic
[17–19],
where
the
two
enantiomers
were
found
tomeasured
have
comparable
enrichments.
Isovaline
δD
values
were
Itdistances
should
be
noted
that
racemization,
the
conversion
of
one using
enantiomer
into
is analyses
possible
Itdistances
should
be
noted
that
racemization,
the[17,18].
conversion
of
one using
enantiomer
into
another,
is analyses
possible
13C-, and
short
in
the
same
meteorite
stone
For
Isovaline
(2-amino,
2-methylbutanoic
acid),
the
most
abundant
of
the
acids,
the
absence
of
any
contribution
from
terrestrial
contaminants
interstellar
medium.
within
this
chiral
heterogeneity
was
validated
by
Murchison,
D
isotopic
[17–19],
where
the
two
enantiomers
were
found
to
have
comparable
enrichments.
Isovaline
δD
values
were
The
overall
extent
of
ee
in
2maa
varies
significantly,
between
0%
and
18%,
and
even
within
particularly
high,
+3600‰
[19],
and
near
to
those
measured
spectroscopically
for
organic
molecules
in
for 2H-amino acids in water through a repeat statistical abstraction and reacquisition of the α-H and
13C-,
for
2H-amino
acids
inofsame
water
through
avalidated
repeat
statistical
abstraction
and
reacquisition
of
the
α-H
and
the
most
abundant
the
acids,
the
absence
of
any
contribution
from
terrestrial
contaminants
within
this
chiral
heterogeneity
was
by
Murchison,
using
and
Danother,
isotopic
analyses
Itdistances
should
be
noted
that
racemization,
the
conversion
of
onegroup.
enantiomer
into
istime
possible
[17–19],
where
the
two
enantiomers
were
found
tomeasured
have
comparable
enrichments.
Isovaline
δD
values
were
particularly
+3600‰
[19],
and
near
to
those
spectroscopically
for
organic
molecules
in
short
in
the
meteorite
stone
[17,18].
For
Isovaline
(2-amino,
2-methylbutanoic
acid),
the
interstellar
medium.
because
of high,
its
vicinity
to
the
electron-withdrawing
carboxyl
Racemization,
with
and
13C-, and D isotopic
because
of
its
vicinity
to
the
electron-withdrawing
carboxyl
group.
Racemization,
with
time
and
within
this
chiral
heterogeneity
was
validated
by
Murchison,
using
analyses
[17–19],
where
the
two
enantiomers
were
found
to
have
comparable
enrichments.
Isovaline
δD
values
were
for
2H-amino
acids
in
water
through
a
repeat
statistical
abstraction
and
reacquisition
of
the
α-H
and
particularly
high,
+3600‰
[19],
and
near
to
those
measured
spectroscopically
for
organic
molecules
in
interstellar
medium.
the
most
abundant
of
the
acids,
the
absence
of
any
contribution
from
terrestrial
contaminants
It
should
be
noted
that
racemization,
the
conversion
of
one
enantiomer
into
another,
is
possible
proper conditions, will lead to racemic solutions (equal amounts of the two enantiomers). The fact
proper
conditions,
will
lead
to
racemic
solutions
(equal
amounts
ofand
the
two
enantiomers).
Thewere
fact
[17–19],
where
the
two
enantiomers
were
found
tomeasured
have
comparable
enrichments.
Isovaline
δD
values
13C-,
particularly
high,
+3600‰
[19],
and
near
to
those
spectroscopically
for
organic
molecules
in
because
of
its
vicinity
to
the
electron-withdrawing
carboxyl
group.
Racemization,
with
time
and
the
interstellar
medium.
It
should
be
noted
that
racemization,
the
conversion
of
one
enantiomer
into
another,
is
possible
within
this
chiral
heterogeneity
was
validated
by
Murchison,
using
and
D
isotopic
analyses
for
2H-amino
acids
in
water
through
a
repeat
statistical
abstraction
reacquisition
of
the
α-H
and
that all amino acids found non-racemic in meteorites have a 2-methyl in place of a 2-H and cannot
Life 2016, 6, 18
3 of 9
Life 2016, 6, 18
4 of 9
A possible pathway proposed for the formation of 2-amino-, and 2-hydroxy acids in meteorites
is the addition of ammonia and HCN to ketones and aldehydes in the presence of water (Figure 1a,
Reference [12] and the references therein). This is a reasonable hypothesis, for at least some of the
amino acids because all the needed reagents are abundant components of interstellar ices and likely
Life 2016, 6, 18
4 of 9
constituents
of primitive asteroids [20,21]. Although producing an asymmetric carbon and, therefore,
a chiral molecule in most cases, this type of synthesis is non-stereospecific because the HCN addition
therefore, a chiral molecule in most cases, this type of synthesis is non-stereospecific because the
would
be random
give
amounts
D-, amounts
and L-enantiomers.
HCN
addition and
would
be equal
random
and giveof
equal
of D-, and L-enantiomers.
Figure 1. (a) The Strecker synthesis for the possible formation of amino acids in meteorites ([12] and
Figure 1. (a) The Strecker synthesis for the possible formation of amino acids in meteorites ([12]
references therein); (b) Structural relation of the alloisoleucine and isoleucine diastereomers; (c)
and references therein); (b) Structural relation of the alloisoleucine and isoleucine diastereomers;
Enantiomeric excesses detected for the isoleucine diastereomers in the GRA 95229 meteorites using
(c) Enantiomeric
excesses detected
for the isoleucine
in the GRA 95229 meteorites using
Gas Chromatography-Mass
Spectrometry,
single iondiastereomers
traces [22].
Gas Chromatography-Mass Spectrometry, single ion traces [22].
However, reaction products become more complex for longer aldehydes that already contain
an asymmetric carbon, such as in the synthesis of the 6-C isoleucine (ile) and alloisolucine (allo)
diastereomers (Figure 1b) from DL 2-methylbutanal [12]. In this case, were an ee already present in
Life 2016, 6, 18
5 of 9
Life 2016, 6, 18
5 of 9
However, reaction products become more complex for longer aldehydes that already contain
theasymmetric
precursor aldehyde,
e.g., of
those
product
amino acids(allo)
that
an
carbon, such
asthe
in (S)
theconfiguration,
synthesis of the
6-Cdiastereomeric
isoleucine (ile)
and alloisolucine
carried
the
(S)
portion
of
the
precursor
through
their
synthesis
will
be
more
abundant
than
their
diastereomers (Figure 1b) from DL 2-methylbutanal [12]. In this case, were an ee already present in the
respectivealdehyde,
enantiomers
any
ee would
be revealed
in different
compounds.
precursor
e.g., and
of the
(S)original
configuration,
those
diastereomeric
product
amino acids that carried
Inportion
the above
example,
thisthrough
would be
thesynthesis
(RS) allo and
ile compounds
or, intheir
the respective
formalism
the (S)
of the
precursor
their
will (SS)
be more
abundant than
used
for
amino
acids,
D
-allo
and
L
-ile.
As
can
be
seen,
these
two
sets
of
diastereomers
are
also each
enantiomers and any original ee would be revealed in different compounds.
other’s
racemization
product
(calledbeepimerization
in (SS)
this ile
case,)
and, because
theformalism
3C is too
far
In the
above example,
this would
the (RS) allo and
compounds
or, in the
used
removed
from
the
carboxyl
and
will
racemize
much
more
slowly,
if
at
all,
original
ee
may
be
for amino acids, D-allo and L-ile. As can be seen, these two sets of diastereomers are also each other’s
preserved
much
longer.
racemization product (called epimerization in this case,) and, because the 3C is too far removed from
Such was
distribution
foundmore
in the
extracts
group of
meteoritesmuch
collected
in
the carboxyl
andthe
will
racemize much
slowly,
if at of
all,a original
ee pristine
may be preserved
longer.
Antarctica
(Renazzo-type
or
CR)
(Figure
1c,
[22,23])
and,
based
on
the
above
formative
premise,
as
Such was the distribution found in the extracts of a group of pristine meteorites collected in
well as upon
confirmingor
the
indigeneity
all four
individual
by 13C isotopic
Antarctica
(Renazzo-type
CR)
(Figure 1c,of[22,23])
and,
based onenantiomers
the above formative
premise,analyses
as well
[22],
theconfirming
findings were
interpretedoftoallsignify
that theirenantiomers
precursor aldehyde
carriedanalyses
an original
as
upon
the indigeneity
four individual
by 13 C isotopic
[22],
S-asymmetry
to
the
meteorite’s
parent
body.
CR
meteorites’
organic
composition,
therefore,
offers
the findings were interpreted to signify that their precursor aldehyde carried an original S-asymmetrya
new
account
of body.
cosmochemical
evolution’s
capabilities
toward
the selective
of
to
theexciting
meteorite’s
parent
CR meteorites’
organic
composition,
therefore,
offers aproduction
new exciting
very
large
abiotic
ee,
of
up
to
60%
in
the
case
of
the
isoleucine
diastereomers,
in
some
meteorites
account of cosmochemical evolution’s capabilities toward the selective production of very large abiotic
(Table
ee,
of up1).to 60% in the case of the isoleucine diastereomers, in some meteorites (Table 1).
3.
The Likely
Likely Locals
Locals for
for Asymmetric
Asymmetric Syntheses
Syntheses
3. The
The
in the
amino
acidsacids
of meteorites
has been
debated
since their
initial
detection,
The origin
originofofee ee
in the
amino
of meteorites
haslong
been
long debated
since
their
initial
and
is still and
not is
known
(e.g.,
[24]).(e.g.,
The [24]).
first hypotheses
to be put to
forward
their possible
detection,
still not
known
The first hypotheses
be put proposed
forward proposed
their
asymmetric
decomposition
upon UV upon
photolysis
[25] and, following
hint of the
amino
high
possible asymmetric
decomposition
UV photolysis
[25] and, the
following
hintacids’
of amino
isotopic
enrichments
deuterium, the
theirlikelihood
syntheses in
acids’ high
isotopic inenrichments
in likelihood
deuterium,of the
of cold,
theirdeuterium-enriching
syntheses in cold,
cosmic
environments [7].
The two
enantiomers[7].
of aThe
chiral
molecule
have identical
chemical
properties,
deuterium-enriching
cosmic
environments
two
enantiomers
of a chiral
molecule
have
but
differ in
some physico-chemical
features,
[26], and,
as a result,
react/interact
identical
chemical
properties, but differ
in e.g.,
somepolarizability
physico-chemical
features,
e.g., may
polarizability
[26],
differently
with other
molecules
or entitieswith
(a common
illustration
couldorbeentities
that of (a
thecommon
distinct
and, as a result,
maychiral
react/interact
differently
other chiral
molecules
illustration
couldcan
be that
the distinct
waysaims).
two people can join hands for different aims).
ways
two people
join of
hands
for different
The effects
effects of
of UV
UV circularly
circularly polarized
polarized light
light (CPL)
(CPL) irradiation,
irradiation, which
which can
can have
have right-handed
right-handed or
or
The
left-handed helical
helicalpaths
pathsofofpropagation
propagation(Figure
(Figure2),2),
were
discussed
and
studied
in detail
left-handed
were
discussed
and
studied
in detail
[27][27]
andand
for
for amino
in particular.
experiments
showirradiation
UVCPL generating
irradiation adsorption
generating
amino
acids acids
in particular.
Several Several
experiments
did showdid
UVCPL
adsorption
bands
by the compounds
with the
sign
as the irradiating
CPL
[28],
andover
evenaover
bands
by the
compounds
with the same
signsame
as the
irradiating
CPL [28],
and
even
largea
large wavelength
[29], however,
the maximum
ee achieved
found
to at
be,most,
at most,
(at
wavelength
range range
[29], however,
the maximum
ee achieved
werewere
found
to be,
9% 9%
(at an
an unlikely
1 [30]),
far lower
found
in meteorites.
unlikely
pH pH
1 [30]),
i.e., i.e.,
far lower
thanthan
found
in meteorites.
Figure 2.
2. Graphic
Graphic representation
representation of
of Left
Left and
and Right
Right Circularly
Circularly Polarized
Polarized Light.
Light.
Figure
These experimental data leave open the possibility that ee were first formed in precursor
molecules, such as the aldehydes mentioned above, which would also justify the findings for ile
and allo. Aldehydes have not been studied in this regard, e.g., their circular dichroism is not known,
Life 2016, 6, 18
6 of 9
These experimental data leave open the possibility that ee were first formed in precursor molecules,
such as the aldehydes mentioned above, which would also justify the findings for ile and allo.
Aldehydes have not been studied in this regard, e.g., their circular dichroism is not known, nor
is their behavior upon CPL irradiation, and the field is, in fact, yet open to inquiry. However, the
proposal of their participation as precursors to the syntheses of amino acids in meteorites would
draw the important inference that, because chiral aldehydes racemize quickly in water, in fact, much
more rapidly than amino acids, the synthetic window for amino acids in meteorites from non-racemic
aldehydes during the asteroidal water processes should have been short and icy [22]. Early parent
body aggregates or protoplanetary disc environments would fit the scenario, with ammonia abundance
possibly providing a lower freezing point of water.
Although the scenario also implies that meteoritic ee would be rare and could be detected for
2-amino acids only in the special cases, such as for the diastereomer compounds described above,
the hypothesis does not explain the ee for 2maa, of which precursors would be ketones that cannot
racemize. Additionally, the large heterogeneity of ee values demonstrated throughout these studies of
meteoritic chiral compounds is very puzzling and has brought other proposals that the formation of
ee could be related to asteroidal parent body effects during aqueous stages [12,31] and, in particular,
catalysis from their mineral phases.
This was first addressed to justify the large variability of isovaline ee between contiguous
fragments of the Murchison meteorite and, by their X-ray diffraction analyses, showed a possible
correlation between isovaline ee and hydrous silicates abundances [16]. In the case of CR meteorites
mentioned above, that their aqueous processes are found to be variable within the meteorite matrix [31]
would also satisfy the observation of varying ee detected between meteorites’ samples.
The recent findings of two non-racemic amines in several CR2 meteorites with ee as high
as 60% [15] were able to further this discussion. Because, as for amino acids, amines’ circular
dichroism [15] and anisotropy spectra [32] do not encourage suggestions of possible asymmetric
effects by UV CPL, it has been proposed that the ketones precursor to these amines, while not chiral,
could have benefited by adsorption upon mineral phases possessing asymmetry. Such minerals, e.g.,
magnetite seen to have in meteorites helical structures [33], could then provide chiral induction during
syntheses. For 2-methylamines, it could have been a reductive amination process made possible by
the large abundance of ammonia known to be present in this type of meteorites, e.g., [34]. If further
confirmed experimentally, processes like these would point to possible important roles for Asteroidal
bodies, their aqueous history and the minerals ensuing from warm hydrous stages in the development
of chiral asymmetry in prebiotic molecules.
4. Prebiotic Chiral Asymmetry and the Origins of Life
The origins of life are utterly unknown, have been debated for close to a century, and the only
achieved consensus is one of life likely emerging from inanimate molecules, about 3.5 billion years
ago and by diverse possible scenarios, such as syntheses in the early atmosphere, the deep Earth or by
input of organics materials from comets and meteorites (e.g., [5,35,36]). As mentioned already, and as
Cronin and Chang [37] remarked pointedly, the known meteoritic inventory discussed above “ . . .
is clearly not the recipe for constructing the progenote . . . and a basic challenge resides in finding
. . . where diversity that characterizes chemical evolution . . . gives way to the selectivity of life”. The
question therefore is: Has the molecular asymmetry of meteoritic compounds changed the odds of
an exobiology?
One distinguishing characteristic of Earth life is that the structure and functions of biopolymers
rely on the exclusive one-handedness of their monomeric components, i.e., most protein amino acids
have an L-configuration while sugars in RNA and DNA have a D-configuration and substitutions
along the polymers with enantiomers of opposite handedness usually result in loss of function. That
the trait of molecular asymmetry could be, or become by evolution, so pervasive as to define life
Life 2016, 6, 18
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processes and that several meteoritic compounds may have ee of the same L-configuration as terrestrial
proteins do seem to provide a new dimension to this debate.
However, the overall data also pose a caveat. As we have seen, the asteroidal aqueous
environments, as recorded in the mineralogy of all meteorites containing organic compounds of
prebiotic interest, tend to racemize either the α-H amino acids or their aldehyde precursors, making ee
for these amino acids scarce in the mature parent bodies. The α-substituted species that do not racemize
would be more abundant but possibly less reactive. On the other hand, their ketone precursors do
not racemize at all and could, in fact, have been very desirable prebiotic reactants. Moreover, some of
the CC meteorites, such as the Renazzo-type, contain, not only abundant amino acids, but also large
abundance of ammonia [38–40], an indispensable reactant for their syntheses. The combination of
mineral catalysts with exogenously delivered key ingredients and chiral compounds, therefore, may
still represent a very good evolutionary chance in prebiotic chemistry.
Acknowledgments: The author is grateful for the near forty years of funding from the NASA Exobiology and
Astrobiology Division and the current support from the National Aeronautics and Space Administration under
Agreement No. NNX15AD94G for the program “Earths in Other Solar Systems”.
Conflicts of Interest: The authors declare no conflict of interest.
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