Mechanism of pH-dependent photolysis of aliphatic amino acids and enantiomeric enrichment of racemic leucine by circularly polarized light.

Article abstract of DOI:10.1021/ol0155788

It has been proposed that the origin of biological homochirality may be the result of irradiation of a racemic sample of amino acids by circularly polarized light (CPL). To determine the mechanism of enantiomeric enrichment, the irradiation of aliphatic amino acids by CPL was undertaken. An enantiomerically enriched sample (e.g., L isomer enrichment from r-CPL) was found to result from the preferential excitation/decomposition of one enantiomer over another via a Norrish Type II mechanism (leucine, valine, and isoleucine), with the enantiomeric excess dependent on the degree of protonation of the amino/carboxylic acid moiety.

Full text of DOI:10.1021/ol0155788

ORGANIC
LETTERS
2
001
Vol. 3, No. 6
21-924
Mechanism of pH-Dependent Photolysis
of Aliphatic Amino Acids and
9
Enantiomeric Enrichment of Racemic
Leucine by Circularly Polarized Light
†
†
†
‡
Hideo Nishino, Atsuko Kosaka, Guy A. Hembury, Hiroshi Shitomi,
‡
,†
Hideo Onuki, and Yoshihisa Inoue*
Inoue Photochirogenesis Project, ERATO, JST, 4-6-3 Kamishinden,
Toyonaka 560-0085, Japan, and Quantum Radiation DiVision,
Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba 305-8568, Japan
inoue@chem.eng.osaka-u.ac.jp
Received January 17, 2001
ABSTRACT
It has been proposed that the origin of biological homochirality may be the result of irradiation of a racemic sample of amino acids by
circularly polarized light (CPL). To determine the mechanism of enantiomeric enrichment, the irradiation of aliphatic amino acids by CPL was
undertaken. An enantiomerically enriched sample (e.g., L isomer enrichment from r-CPL) was found to result from the preferential excitation/
decomposition of one enantiomer over another via a Norrish Type II mechanism (leucine, valine, and isoleucine), with the enantiomeric excess
dependent on the degree of protonation of the amino/carboxylic acid moiety.
A variety of biotic and prebiotic theories have been pro-
posed to explain the origin of the homochirality in bio-
molecules, and a number of mechanisms have been inves-
pure compound toward left- and right-handed circularly
polarized light (l- and r-CPL), thus giving rise to different
photoreaction rates between enantiomers. The degree of
preferential excitation is determined by the anisotropy
1
tigated. One of the most probable hypotheses is that
amino acid enantiomeric enrichment arose from inter-
stellar circularly polarized light (CPL) irradiation of a racemic
amino acid sample, via an “absolute asymmetric synthesis”
factor (g factor), g ) (ꢀ
+ ꢀ )/2 and 0 e g < 2 (ꢀ is the molar extinction
coefficient).
l
- ꢀ
r
)/ꢀ ) ∆ꢀ/ꢀ, where ꢀ ) (ꢀ
l
r
2
-6
(AAS) process. The principle of AAS is based on the
different molecular absorption coefficients of an optically
(2) Inoue, Y. Chem. ReV. 1992, 92, 741.
3) Everitt, S. R. L.; Inoue, Y. Asymmetric Photochemical Reactions in
(
Solution; Ramamurthy, V., Schanze, K. S., Eds.; Marcel Dekker: New York,
1999; p 71.
(4) Rau, H. Chem. ReV. 1983, 83, 535.
(5) Kuhn, W.; Knopf, E. Z. Phys. Chem., Abt. 1930, B, 291.
†
ERATO, JST.
‡
Quantum Radiation Division, Electrotechnical Laboratory.
(1) Bonner, W. A. Top. Stereochem. 1988, 18, 1.
1
0.1021/ol0155788 CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/01/2001

Scheme 1. Representation of the Major Pathway for the Enantioenrichment of rac-Leua
a
The perferential excitation of D-Leu and its resulting decomposition leads to an L isomer enriched sample. The minor pathway (excitation/
decomposition of L-Leu) is not shown.
The enantiomeric enrichments of some amino acids have
amino acid. On decreasing the pH, the intensity of the g
factor increases at all wavelengths, but particularly in the
215-220 nm region, although the profile at pH 11 shows
some deviation. This is crucial to understanding the possible
mechanism by which an excess of one enantiomer may be
produced in the CPL irradiation process, as it is the
magnitude of the g factor that dictates the efficiency of the
enantiodifferentiating process, and thus the available ee. At
pH 1 the carboxyl group of Leu is in the carboxylic acid
state and the amino group is in the ammonium state, with
the UV and CD bands at 208 nm arising from the n,π*
transition of the carboxyl group. As the pH changes from 1
to 7, the amino/carboxyl moiety changes to the zwitterionic
state. At pH 11 Leu exists as the aminocarboxylate anion.
2,7-9
previously been obtained by CPL irradiation,
e.g., the
8
AAS of leucine (Leu) at pH 1 by laser irradiation. However,
the pH dependence of the % ee generated by CPL irradiation
has not been determined, although it is expected that the
amphoteric character of amino acids will influence the
outcome of such reactions. Indeed, even the detailed pH
dependence of the amino acid g factor has not been reported.
The mechanism by which these AAS reactions occur has
been, as yet, unresolved, key to which has been the poor
identification of the photoproducts.1
0-12
Although various
photochemical mechanisms have been proposed, these
are not able to adequately explain the photoproducts de-
tected.12
Here we first examine the pH dependence of the chiroptical
properties of Leu. Then on irradiation of a racemic Leu
rac-Leu) sample by l- or r-CPL (generated by a polarizing
The UV and CD changes are in agreement with the reported
13
15,16
literature
and also show a dependency on the ionic state
(
(and thus pH) of the amino/carboxyl moiety (pKa1 ) 2.33,
1
7
undulator installed in an electron storage ring) we show how
pH controls the efficiency of the AAS and thus enantiomeric
enrichment. Subsequently, the photodecomposition products
of several aliphatic amino acids are identified, verifying the
pH dependence of the % ee. Then, crucially, it is revealed
that the mechanism of the ee generating photodecomposition
of aliphatic amino acids on CPL irradiation is via the
preferential excitation and subsequent decomposition of one
enantiomer over another (Scheme 1). These results reveal
an insight into the conditions necessary for significant
enantioenrichment of a racemic amino acid sample and,
consequently, a further clue as to the origin of homochirality
in biomolecules.
pKa2 ) 9.74).
Figure 2 shows the final CD spectra of rac-Leu irradiated
at 215 nm by CPL at various pHs. At pHs 7 and 11 the
Figure 1 shows the variation of the g factor for D- and
L-Leu at different pHs. Although the g factor of leucine at
pH 1 has been reported,14 Figure 1 shows the first compre-
hensive analysis of the pH dependence of the g factor of an
(
6) Kuhn, W. Trans. Faraday. Soc 1930, 26, 293.
(7) Inoue, Y.; Tsuneishi, H.; Hakushi, T.; Yagi, K.; Awazu, K.; Onuki,
H. Chem. Commun. 1996, 2627.
8) Flores, J. J.; Bonner, W. A.; Massey, G. A. J. Am. Chem. Soc. 1977,
9, 3622.
(
9
7
(
(
9) Norden, B. Nature 1977, 266, 567.
10) Lapinskaya, E. M.; Khenokh, M. A. Zh. EVol. Biokhim. Fiziol. 1971,
, 14.
(
(
(
11) Kolomiichenko, M. A. Ukr. Biokhim. Zh. 1968, 40, 57.
12) Schaich, K. M. CRC Crit. ReV. Food Sci. Nutr. 1980, 13, 131.
13) See Supporting Information.
Figure 1. g factor of D- and L-Leu at various pHs.
Org. Lett., Vol. 3, No. 6, 2001
9
22

D- and L-Leu) are achiral (Scheme 1), the % op is equivalent
to the % enantiomeric excess (ee). As the solution pH
changes from 1 to 11, the ee of Leu becomes smaller and
almost zero at pHs 7 and 11.
According to calculations using Kagan’s equation, which
describes the theoretical relationship between the ee, the %
2
,3,14
conversion, and the g factor,
the observed ee at pH 1 is
1
3
in good agreement with that predicted.
However, according to Kagan’s theory, the ee should
remain at 0.1% above pH 7, but the observed % ee was
significantly smaller and approaching zero. Even though at
pHs 7 and 11 there is almost no ee, rac-Leu still undergoes
significant achiral photochemical decomposition on CPL and
linearly polarized light (LPL) irradiation (% decomposi-
tions: pH 1 ) 15.4-14.7; pH 2 ) 14.7-12.3; pH 3 ) 9.9-
1
8
9
.8; pH 7 ) 7.5-6.5; pH 11 ) 7.3-5.9). Although the
degree of photodecomposition becomes smaller as the
solution pH changes from pH 1 to 11, Leu is clearly
decomposed over the entire pH range.
To rationalize the generation of a significant ee at low
pH, but the generation of achiral photoproducts at high pH,
product analysis of several aliphatic amino acids irradiated
with LPL at pHs 1 and 7 was undertaken (Table 1). From
Figure 2. CD spectra of Leu irradiated at various pHs with CPL
dose: 60.3 ( 0.3 mA/h).
(
intensities of the CD absorptions are nearly equal to zero,
but as the pH is reduced, corresponding increases in the
intensity of the CD signal are observed, revealing greater
enantiomeric enrichment. When Leu was irradiated at pHs
Table 1. Results of Product Analysis of Aliphatic Amino
Acids Irradiated with LPL at pH 1 and 7. Doses Were pH 1,
6
0.0-60.8 mA/h, and pH 7, 80.1-81.2 mA/h
products (% yield)
amino acid pH concn (mM) % convn Gly other amino acid
1
and 2, the CDmax values were 208 and 205 nm, respectively,
with the CD absorptions at pH 1 and 2 closely reflecting
the corresponding molar CD spectra.13 The signs of the
observed CDs mean that the preferential excitation and
decomposition of D-Leu proceeds by r-CPL irradiation and
that of L-Leu is by l-CPL irradiation.
Gly
Ala
Val
Leu
Ile
Gly
Ala
Val
Leu
Ile
1
1
1
1
1
7
7
7
7
7
12.4
5.7
5.7
5.2
5.4
12.3
5.4
5.6
5.2
5.6
3.9
14.5
23.6
17.4
22.8
4.9
3.8
6.4
8.1
3.3
0
0
12.4
8.0
11.8
0
0
0
0
0
0
0.9
3.7
0
0
0
0
0
0
0
Figure 3 depicts the pH dependence of the % optical purity
(
op) of Leu.13 Each op was estimated using the equation op
)
{θ215/33(∆ꢀ215 of D- or L-Leu)}/[Leu]. However, for this
study, as all the photoproducts are known and (apart from
product analysis, Gly was detected in the photodecomposition
products of rac-Ala, rac-Leu, rac-Val, and rac-Ile. In the
cases of rac-Leu, rac-Val, and rac-Ile at pH 1, Gly was the
main product, with the amount of Gly reaching approxi-
mately 50% of the total amount of photodecomposition
product. Crucially though, in terms of the reaction mecha-
nism, Gly was not detected in the case of Ala, in disagree-
1
1
ment with reported literature. The inference that can be
made from this is that the ee generating mechanism (resulting
(
14) Balavoine, G.; Moradpour, A.; Kagan, H. B. J. Am. Chem. Soc.
974, 96, 5152.
15) H a˚ kansson, R. Chiroptical properties of acid deriVatiVes; Patai, S.,
Ed.; Wiley: Chichester, UK., 1992; Vol. 2, Issue Pt. 1, p 67.
1
(
Figure 3. pH dependence of the % ee of Leu irradiated with CPL
at 215 nm (dose: 60.3 ( 0.3 mA/h): (O) l-CPL, (b) r-CPL, (4)
LPL.
(
(
16) Toniolo, C. J. Phys. Chem. 1970, 74, 1390.
17) Pine, S. H.; Hendrickson, J. B.; Cram, D. J.; Hammond, G. S.
Organic Chemistry, 4th ed.; McGraw-Hill Kogakusha, Ltd.: Tokyo, 1980.
(18) Mittal, L. J.; Mittal, J. P.; Hayon, E. J. Phys. Chem. 1973, 77, 1482.
Org. Lett., Vol. 3, No. 6, 2001
923

in Gly production) requires the presence of a hydrogen in
the γ-position, which is not present in Ala.
These results further indicate that it is the n,π* transition
of the carboxyl group of Leu that governs the pH dependence
of the ee and the decomposition; in light of the pH
dependence of the ionic composition of Leu,13 this idea is
thought to be reasonable. It can thus be clearly seen that all
the different aspects of the reaction such as the ee, %
decomposition, and yield of Gly correspond directly with
each other and to the ionic state of Leu.
Irrespective of the solution pH if a significant g factor
exists at the irradiation wavelength, and the photoreaction
proceeds via the excited state, an ee should be observed.
The reason this is not observed at high pHs is because in
the delocalized carboxylate form the biradical necessary for
γ-hydrogen abstraction cannot form. Therefore, it is specu-
lated that some part of the decomposition at pHs 7 and 11
proceeds via a thermal process and/or an indirect photo-
chemical process.
To summarize, the irradiation of rac-Leu by l- or r-CPL
results in an enantiomerically enriched sample, which occurs
via the differential decomposition of the excited state D and
L isomers via a Norrish Type II mechanism, forming the
biologically important molecule glycine as the main product.
The occurrence of this enantiomeric enriching mechanism
On the basis of the products detected by GC-MS13 and
the production of Gly (nonproduction for Ala), it can be
inferred that the photodecomposition of Leu, Val, and Ile at
pH 1 proceeds initially via preferential excitation to the
biradical of one enantiomer over another (D by r-CPL, and
vice versa). Both excited enantiomers then decompose via
hydrogen abstraction by the excited carboxyl group from the
(and the magnitude of the ee generated) was found to be
entirely dependent on the ionic state (and thus pH) of the
amino/carboxylic acid moiety, as indicated by the variation
of the g factor with pH, as well as the % decomposition.
Even though Leu absorbs light between 190 and 240 nm in
the zwitterionic and carboxylate forms, irradiation by CPL
did not result in the formation of enantiomeric excess, while
an ee of 1.3% was obtained at pH 1 for 55% decomposition,13
clearly revealing the necessity of the amino acid to be in
the carboxylic acid form for successful enantiomeric enrich-
ment by CPL irradiation. These results indicate that for
successful enantioenrichment of a meteorite born amino acid
18
γ-position in a Norrish Type II reaction (Scheme 2). Thus,
Scheme 2. Proposed Norrish Type II Leu Enantioenrichment
Mechanism at low pHs
1
9
sample, the carboxylic acid form is required, which is as
yet an unresolved issue. Further, this mechanism is thought
to be applicable to the other aliphatic amino acids, an issue
that is currently under investigation.
Acknowledgment. We would like to thank Prof. M. H.
Engel (School of Geology and Geophysics, The University
of Oklahoma), Prof. J. R. Cronin (Department of Chemistry
and Biochemistry, Arizona State University), and Dr. Michael
Oelgem o¨ ller and Dr. V. V. Borovkov for their useful
discussion and help with manuscript preparation.
in the case of r-CPL irradiation of rac-Leu, an ee of the L
isomer is obtained. The excited carboxyl group abstracts a
hydrogen from the γ-position, resulting in cleavage of the
C2-C3 bond, giving Gly and isobutylene. The isobutylene
then decomposes to the detected products by radical and/or
ionic processes. This demonstrates that the photodecompo-
sition begins via the excitation of the carboxyl group, which
is why the g factor determines the ee. This enantioselective
activation of the carboxyl group, in principle, should be
applicable to the other aliphatic amino acids which possess
a γ-hydrogen and have similar chiroptical properties (such
as Val and Ile).
Supporting Information Available: The chiroptical
properties of D- and L-Leu at various pHs ranging from 1 to
1
1, the pH dependence of the decomposition of Leu and the
yield of Gly, and the results of product analysis of aliphatic
amino acids irradiated with LPL irradiation at pH 1 and 7.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL0155788
(19) Pizzarello, S.; Cronin, J. R. Geochim. Cosmochim. Acta 2000, 64,
329.
924
Org. Lett., Vol. 3, No. 6, 2001