Enantioselective synthesis and enantiomeric amplification of amino acids under prebiotic conditions

Article abstract of DOI:10.1021/ol8007099

(Chemical Equation Presented) A plausible origin of biomolecular homochirality is advanced, where α-methyl amino acids found on meteorites transfer their chirality in the synthesis of normal amino acids. This asymmetry can be amplified to nearly homochiral levels, thus providing the necessary prerequisite for life to start on this planet and elsewhere in the universe.

Full text of DOI:10.1021/ol8007099

ORGANIC
LETTERS
2008
Vol. 10, No. 12
2433-2436
Enantioselective Synthesis and
Enantiomeric Amplification of Amino
Acids under Prebiotic Conditions
Mindy Levine, Craig Scott Kenesky, Daniel Mazori, and Ronald Breslow*
Department of Chemistry, Columbia UniVersity, New York, New York 10027
rb33@columbia.edu
Received March 28, 2008
ABSTRACT
A plausible origin of biomolecular homochirality is advanced, where r-methyl amino acids found on meteorites transfer their chirality in the
synthesis of normal amino acids. This asymmetry can be amplified to nearly homochiral levels, thus providing the necessary prerequisite for
life to start on this planet and elsewhere in the universe.
One of the most remarkable features of living organisms is their
homochirality, the fact that they contain amino acids and sugars
of only one enantiomeric form. We have investigated a potential
prebiotic source of homochirality: R-methyl amino acids that
are delivered to earth by carbonaceous chondritic meteorites
(Figure 1). These amino acids, which lack a racemizable
cyanide, and water, all of which have been detected by
microwave spectroscopy in interstellar space.1,5 The nonm-
ethylated amino acids that are also seen in carbonaceous
chondritic meteorites are formed from aldehydes, and they
are racemic. This must reflect their easy racemization by loss
of the R-proton. The R-methyl amino acids, formed from
methyl ketones, cannot racemize.
Many theories have been advanced to account for their
asymmetry,6–8 and one of the most interesting ideas is that
synchrotron radiation from supernova remnants contains
circularly polarized light.9 In a neutron star with circulating
electrons, synchrotron radiation would have one chirality
above the circulation plane and the opposite below it. Thus,
one form of circularly polarized light can be directed into
our part of the universe, with the opposite polarized light
sent in the other direction.
Figure 1. R-Methyl amino acids found on meteorites, and their %
ee of the L-enantiomer.
Circularly polarized light has been shown to preferentially
photolyze one enantiomer of a racemic amino acid, leading
to some enantioenrichment in the remaining amino acid.10
Circularly polarized light that struck the parent body of
R-methine proton, are found in meteorites with small but
measurable L enantiomeric excesses (ee’s).1
Their isotopic distributions confirm that they have extra-
terrestrial origins.2–4 They are presumably formed by Strecker
reactions involving carbonyl compounds, ammonia, hydrogen
(5) Peltzer, E. T.; Bada, J. L. Nature 1978, 272, 443
(6) Bonner, W. A.; Kavasmaneck, P. R.; Martin, F. S.; Flores, J. J.
Science 1974, 186, 143
(7) Bonner, W. A.; Kavasmaneck, P. R. J. Org. Chem. 1976, 41, 2225
(8) Bailey, J.; Chrysostomou, A.; Hough, J. H.; Gledhill, T. M.; McCall,
.
.
(1) Pizzarello, S. Acc. Chem. Res. 2006, 39, 231.
(2) Pizzarello, S.; Feng, X.; Epstein, S.; Cronin, J. R. Geochim.
Cosmochim. Acta 1994, 58, 5579.
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A.; Clark, S.; Me´nard, F.; Tamura, M. Science 1998, 281, 672
(9) Bonner, W. A. Origins Life 1991, 21, 59.
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(3) Pizzarello, S.; Huang, Y.; Fuller, M. Geochim. Cosmochim. Acta
2004, 68, 4963.
(10) Flores, J. J.; Bonner, W. A.; Massey, G. A. J. Am. Chem. Soc.
1977, 99, 3622.
(4) Pizzarello, S.; Huang, Y. Geochim. Cosmochim. Acta 2005, 69, 599.
10.1021/ol8007099 CCC: $40.75
Published on Web 05/21/2008
2008 American Chemical Society

carbonaceous chondritic meteorites could thus have resulted
in the preferential partial photolysis of one enantiomer of
R-methyl amino acids, leading to the observed modest L ee’s.
The R-methylated amino acids can serve as the seeds of
terrestrial homochirality provided two conditions are met:
(i) they must be able to transfer their chirality to biomolecules
such as simple amino acids and sugars, and (ii) there must
be a mechanism for chiral amplification of meteoritic amino
acids or of proteinogenic amino acids up to near homo-
chirality. Here we show that 96% enantiopure L-R-meth-
ylvaline can transaminate phenylpyruvate to L-phenylalanine
in up to 37% ee and pyruvate to L-alanine in up to 20% ee
with copper catalysis. We extend this novel copper-catalyzed
decarboxylative transamination to the synthesis of L-valine
from L-R-methylisoleucine. Furthermore, we demonstrate that
the ee’s of various proteinogenic amino acids can be
amplified above the thermodynamic limits via preferential
kinetic dissolution. Our results demonstrate both conditions
that are necessary for meteoritic R-methyl amino acids to
be a plausible source of biomolecular homochirality, on this
planet and elsewhere in the universe.
Scheme 1. Synthesis of L-R-Methyl Amino Acids
in 32% overall yield and 96% enantiopurity (92% ee). While
L-R-methylisoleucine has been identified in meteorites,1 it
has not been previously synthesized by chemical means. We
synthesized it in 8% overall yield from L-isoleucine. The
relative stereochemistry of compound 2b is assumed by
analogy to known compound 2a (whose stereochemistry is
assigned by comparison of its optical rotation to literature
values).
Our previously reported decarboxylative transamination
(Figure 2) utilized enantiopure D-R-methylvaline to afford
The reaction of 4 equiv of L-R-methylvaline with 1 equiv
of sodium phenylpyruvate and 1 equiv of cupric sulfate
afforded L-phenylalanine. A variety of reaction times (1-120
min) and temperatures (120-160 °C) were screened, and
the best results were obtained at 160 °C for 60 min, wherein
L-phenylalanine was obtained in 37% ee. Shorter reaction
times or lower reaction temperatures led to lower ee’s. This
is a higher transfer of chirality than we had obtained without
the copper salt. Moreover, the desired enantiomer of phe-
nylalanine is now produced: L-R-methylvaline affords L-
phenylalanine. Zinc salts (zinc is less abundant than copper
in meteorites)14,15 were poorer catalysts and did not lead to
the reversal of chiral induction that we saw with copper. The
reaction conditions (solvent-free, 120-160 °C) mimic plau-
sible conditions on prebiotic earth.
Figure 2. Transamination reaction between R-methylvaline and a
ketoacid.
To gain mechanistic insight into this reversal of enanti-
oselectivity in the copper-catalyzed reaction, we performed
DFT/B3LYP calculations using the Maestro interface with
Jaguar version 7.4 using a 6-31TM** basis set on a dual
processor Dell Precision 490 workstation. One potential
reaction intermediate is a square planar copper(II) complex
containing two molecules of imine 3 as bidentate ligands
(Figure 3). Calculations of the van der Waals surface of this
complex suggest that only one face of the R-carbon is
up to 10% L-phenylalanine product under solvent-free
conditions, so the meteoritic L-R-methylvaline would have
produced D-phenylalanine.11 We have now found that the
decarboxylative transaminations that occur when R-meth-
ylvaline is heated neat with R-ketoacids can be catalyzed
by metal ions.
The R-methylated amino acids were synthesized via
stereospecific methylation of the corresponding cis-oxazo-
lidinones (Scheme 1).12 L-R-Methylvaline (2a), which has
been synthesized previously,13 was synthesized in our hands
Scrimin, P. J. Org. Chem. 2004, 69, 3849. Synthesis of 2a: (c) Obrecht,
D.; Bohdal, U.; Broger, C.; Bur, D.; Lehmann, C.; Ruffieux, R.; Scho¨nholzer,
P.; Spiegler, C.; Mu¨ller, K. HelV. Chim. Acta 1995, 78, 563.
(11) Breslow, R.; Levine, M. Tetrahedron Lett. 2006, 47, 1809.
(12) Seebach, D.; Fadel, A. HelV. Chim. Acta 1985, 68, 1243.
(14) Strasdeit, H.; Bu¨sching, I.; Behrends, S.; Saak, W.; Barklage, W.
(13) Synthesis of 12a: (a) Qi, J.; Roush, W. R. Org. Lett. 2006, 8, 2795.
Synthesis of 14a: (b) Formaggio, F.; Barazza, A.; Bertocco, A.; Toniolo,
C.; Broxterman, Q. B.; Kaptein, B.; Brasola, E.; Pengo, P.; Pasquato, L.;
Chem. Eur. J. 2001, 7, 1133
(15) Greenland, L.; Goles, G. G. Geochim. Cosmochim. Acta 1965, 29,
1285
.
.
2434
Org. Lett., Vol. 10, No. 12, 2008

(ii) serine, also found on meteorites, can be dehydrated and
hydrolyzed under prebiotic conditions to yield pyruvic acid.18
Heating 96% enantiopure L-R-methylvaline with sodium
pyruvate and cupric sulfate at 160 °C for 60 min under
solvent-free conditions yields L-alanine in 20% ee. In the
absence of copper, only low ee’s of the undesired D-alanine
were obtained.
Furthermore, both valine and its corresponding 2-hydroxy-
3-methyl butanoic acid have been found on meteorites.16,5
A simple oxidation of the alcohol or amine would yield the
desired valine ketoacid precursor. The oxidation of the
alcohol can occur via the iron oxide found on meteorites.17
Moreover, pyruvic acid, formed from the dehydration and
hydrolysis of serine, can further react with normal amino
acids, such as valine, to produce 3-methyl-2-oxobutanoic acid
and the byproduct alanine.18
Figure 3. Optimized geometry of Cu(II) complexed with two imine
molecules. The green ribbon indicates the face of the R-carbon that
is accessible for protonation; H atoms have been omitted for clarity.
accessible to contact with a second molecule, as the other
face is blocked by the bulky benzyl groups. Protonation from
the accessible face leads to L-phenylalanine.
Valine can be synthesized via our enantioselective tran-
samination. L-R-Methylisoleucine, found on meteorites in
7.0% L ee,19 reacted with 3-methyl-2-oxobutanoate in the
presence of cupric sulfate to produce L-valine in up to 23%
ee. In the absence of copper, no reaction occurred.
This modest ee was then amplified via the preferential
kinetic dissolution of L-valine crystals more quickly than the
racemate crystal. A 500 mg dry mixture of DL-valine and
L-valine was placed in a pipet plugged with glass wool, and
1 mL of room temperature water was passed through. The
solution that passed through was enantioenriched in L-valine.
When we started with 5% ee L-valine and subjected the
mixture to three rounds of preferential dissolution, we found
that the ee of the solution was enriched up to 46% L ee (a
73:27 ratio) (Figure 5). This is identical with the 45% ee
Further examination of this complex indicates the potential
for a competing reaction pathway of a concerted decarboxy-
lative protonation (Figure 4), as the carboxylic acid proton
Figure 4. Formation of D-phenylalanine by concerted decarboxy-
lative proton transfer, both without copper and in the copper
complex.
in the complex is 2.65 Å above the R carbon. Protonation
of the R carbon would lead to D-phenylalanine. A combina-
tion of both reaction pathways (enantioselective protonation
and concerted decarboxylative protonation) would account
for the observed moderate ee’s in the phenylalanine product.
This copper-catalyzed decarboxylative transamination can
be extended to the synthesis of amino acids that are more
prebiotically relevant. In particular, both alanine and its
corresponding R-hydroxy acid, lactic acid, have been found
on meteorites.16 A simple oxidation of either the alcohol or
the amine would yield pyruvic acid as a substrate for our
enantioselective transamination. This oxidation could have
been accomplished in at least two ways: (i) iron oxide found
on meteorites can oxidize lactic acid to pyruvic acid,17 and
Figure 5. Amplification of valine and alanine via preferential kinetic
dissolution.
room temperature thermodynamic limit reported by Black-
mond.20 By switching to 1 mL of ice-cold water and
subjecting the 46% ee L-valine mixture to two more rounds
(16) Cronin, J. R.; Pizzarello, S. Meteorites and the Early Solar System;
Kerridge, J., Matthews, M. S., Eds.; University of Arizona Press: Tucson,
AZ, 1988; pp 819-857..
(18) Bada, J. L.; Glavin, D. P.; McDonald, G. D.; Becker, L. Science
1998, 279, 362.
(19) Cronin, J. R.; Pizzarello, S. Science 1997, 275, 951.
(20) Klussmann, M.; Iwamura, H.; Mathew, S. P.; Wells, D. H.; Pandya,
U.; Armstrong, A.; Blackmond, D. G. Nature 2006, 441, 621.
(17) Kerridge, J. F.; MacKay, A. L.; Boynton, W. V. Science 1979,
205, 395.
Org. Lett., Vol. 10, No. 12, 2008
2435

of preferential kinetic dissolution, the enantiomeric enrich-
ment increased up to 53% ee L-valine.
an average phenylalanine ee of 95%, although the results in
this case are only slightly higher than the margin of error
(Table 2). Preferential kinetic dissolution of tryptophan can
Alanine can also be amplified using preferential kinetic
dissolution. When 1 mL of ice-cold water was passed through
a 500 mg mixture of 5% ee L-alanine over 3 min, the solution
that passed through contained 15% ee L-alanine (Figure 5).
Repeating this procedure four more times led to a 55% ee
L-alanine solution, which is close to the 60% ee room temper-
ature eutectic point of alanine reported by Blackmond.20
Previously, we had amplified aqueous solutions with 1%,
5%, or 10% ee of phenylalanine up to 90% ee, by slow
evaporation that enriched the ee of the solution as the
racemate precipitated.21 With most amino acids, including
phenylalanine, the racemic crystals are less soluble than the
homochiral crystals.22 Blackmond independently used a
related process to achieve enantioenrichment, allowing small
amounts of a partially racemic amino acid to dissolve in
water at equilibrium.20,23 With our evaporation process,
involving selective partial crystallization, we see that tryp-
tophan can be amplified up to 89% ee (Table 1). These limits
Table 2. Enantiomeric Amplification of Amino Acids after
Preferential Kinetic Dissolutiona
component
initial ee, % final ee, %, rt final ee, %, 0 °C
L-phenylalanine
80
85
80
85
90.3 ( 1.0
91.4 ( 3.2
97.5 ( 1.1
97.8 ( 1.3
94.3 (1.4
95.2 ( 0.03
L-tryptophan
a All values are L ee and represent an average of three runs.
proceed even with room temperature water, to yield a final
average ee of 98%, significantly higher than the 89% ee
thermodynamic maximum (Table 2).
We have thus demonstrated both conditions necessary for
the R-methyl amino acids found on meteorites to be a
plausible source of biological homochirality. First, these
enantioenriched amino acids can transfer their chirality to
several proteinogenic amino acids, including some that were
present on prebiotic earth. This transfer takes place under
credible prebiotic conditions. Second, the moderate ee’s
produced in the proteinogenic amino acids can be amplified
via a straightforward, prebiotically relevant procedure, via
either evaporation of water or preferential dissolution by
Blackmond’s equilibration, or by our kinetic process. Am-
plification via evaporation of water could have occurred on
prebiotic earth in a drying lake bed near a site of meteorite
landing. Preferential dissolution may have occurred when
river or rainwater passed over an amino acid mixture,
dissolving the single enantiomer with enriched enantiopurity
and carrying it downstream. After such enrichment, biology
could start and produce dominant organisms that use the
dominant enantiomers, on this planet and elsewhere in the
universe.
Table 1. Enantiomeric Concentration of Tryptophan after Two
Partial Crystallizations from Watera
component
initial ee, %
final ee, %
D
10
5
1
10
5
1
89.0 ( 0.6
86.7 ( 1.6
86.0( 0.2
84.6 ( 4.6
86.9 ( 4.2
81.1 ( 4.3
L
a All values represent an average of two runs.
reflect equilibrium values, dictated by thermodynamics.
Preferential kinetic dissolution is able to overcome these
thermodynamic limits, because the on and off rates for
homochiral crystallizations can involve a smaller kinetic
barrier than those rates for the racemate. Via kinetic
dissolution, a 91% ee of L-phenylalanine was produced from
an initial mixture of 85% L ee, which is similar to the 90%
ee obtained at equilibrium. As in the case of valine and
alanine, using ice-cold water for the kinetic dissolution of
phenylalanine increases the selectivity, allowing us to obtain
Acknowledgment. This work was supported by the
National Institutes of Health and the National Science
Foundation. M.L. thanks Novartis Pharmaceuticals for a
Graduate Research Fellowship.
Supporting Information Available: The syntheses of
R-methyl amino acids, HPLC parameters, and optimization
of the transamination reaction. This material is available free
of charge via the Internet at http://pubs.acs.org.
(21) Breslow, R.; Levine, M. Proc. Natl. Acad. Sci. U.S.A. 2006, 103,
12979.
(22) Huang, J.; Yu, L. J. Am. Chem. Soc. 2006, 128, 1873.
(23) Klussmann, M.; Izumi, T.; White, A. J. P.; Armstrong, A.;
Blackmond, D. G. J. Am. Chem. Soc. 2007, 129, 7657
.
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