FULL PAPER
DOI: 10.1002/chem.201301831

Photoreaction of rac-Leucine in Ice by Circularly Polarized Synchrotron
Radiation: Temperature-Induced Mechanism Switching from Norrish Type II
to Deamination
Hideo Nishino,[b] Masahito Hosaka,[c] Masahiro Katoh,[d] and Yoshihisa Inoue*[a]Abstract: The delivery of extraterrestrial organics to primitive Earth is considered to have triggered the origin
and subsequent evolution of life.
Indeed, enantiomericallyenriched
amino acids of nonterrestrial origin
have been found in carbonaceous
meteoACHTUNGRErites, and enantioselective photodecomposition by circularly polarized
light (CPL) in outer space has been
proposed to have played some role in
the initial enantiomeric bias. To experimentally examine this possibility andelucidate the photoreaction mechanisms, we have studied the photolysisof racemic leucine (rac-Leu) in acidic
and neutral ice/water media at 21–
298 K with left- and right-CPL in an attempt to detect enantiomerically enriched d- and l-Leu, respectively. Comprehensive product analyses revealed
Keywords: amino acids · chirality ·photolysis · reaction mechanism ·temperature effects

Introduction
The origin of homochirality in the biosphere is one of themost crucial issues in chemical evolution, for which a varietyof biotic and prebiotic theories and chemical and physicalmechanisms have hitherto been proposed.[1–10] Recent meteorite studies have revealed that carbonaceous meteorites
carry biologically important organics, including aliphaticamino acids.[2, 3] Crucially, both the natural and unnaturalamino acids found in meteorites are not racemic but enriched enantiomerically as well as isotopically in 13C and 15N,
which indicates a nonterrestrial origin of the enantiomer imbalance.[3][a] Prof. Dr. Y. Inoue
Department of Applied Chemistry, Osaka University
2-1 Yamada-oka, Suita 565-0871 (Japan)
Fax: (+ 81) 6-6879-7923
E-mail: inoue@chem.eng.osaka-u.ac.jp[b] Dr. H. Nishino
Entropy Control Project (Japan) Science and Technology Agency
Current address: Osaka Municipal Technical Research Institute
1-6-50 Morinomiya, Joto-ku, Osaka 536-8553 (Japan)[c] Prof. Dr. M. Hosaka
Synchrotron Radiation Research Center, Nagoya University
Furo-cho, Chikusa-ku, Nagoya 464-8603 (Japan)[d] Prof. Dr. M. Katoh
Institute for Molecular Science
National Institutes of Natural Sciences
Myodaiji, Okazaki 444-8585 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301831.

Chem. Eur. J. 2013, 19, 13929 – 13936

that the CPL-induced deracemizationof Leu proceeds in both acidic andneutral ice matrices even at 21 K, andthat the main mechanism switchesfrom Norrish-type II g-hydrogen abstraction to SNi deamination on lowering the temperature. The potential roleof the CPL-induced photodecomposition of amino acids as a source of theenantiomer imbalance in meteorites isdiscussed.

In the 1980s, Rubenstein, Bonner et al. proposed a cosmic
scenario in which circularly polarized synchrotron radiationfrom electrons orbiting neutron stars enantioselectively photodecomposed racemic amino acids in an icy comet to leave
enantiomerically enriched antipodes that were subsequently
delivered to the primitive Earth and triggered biomolecularhomochirality.[4] However, circularly polarized light (CPL)
has never been detected in the vicinity of a neutron star andthus the emission of CPL from neutron stars was questioned.[5] However, the star-forming region of the Orion molecular cloud was found to emit CPL in the IR region,[6] presumably provoked by Mie scattering on aligned interstellar
dust grains.[6, 7] The proposal and discovery of such interstellar CPL sources stimulated the investigation to elucidate theplausibility of the cosmic scenario,[4, 6, 8, 9] including the photochemical enantiomer enrichment of amino acids by CPL inouter space.[1a, b, 9] Indeed, a variety of (racemic) aliphaticamino acids are known to be produced upon vacuum ultraviolet (VUV) irradiation of a matrix of H2O, CH3OH, CO2,
CO, and NH3 at 12 K under high vacuum.[10] Nevertheless,the subsequent process, that is, the enantioselective photodecomposition of racemic amino acids by CPL, has not been
experimentally proven or disproven under interstellar conditions.
The principle of enantiomer enrichment upon CPL photolysis of a racemic substrate[11] rests on the relative difference in molar extinction coefficients (e) of an enantiomer
pair under left- or right-CPL (l- or r-CPL), which leads tounequal rates of excitation and subsequent reaction. The
anisotropy factor (g), a measure of preferential excitation, isdefined as the normalized difference in e of an enantiopure 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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compound towards l- and r-CPL at a given wavelength: g =(eler)/e = De/e, in which e = (el+er)/2 and hence 0  g <2.[11, 12]
It is crucial that the Kuhn–Condon zero-sum rule[13] does
not apply to the CPL photolysis of aliphatic amino acids inwater/ice media simply because the media absorb the VUVCPL (< 200 nm) and only the UV CPL of 200–250 nm is absorbed by amino acids.[12]In 1977, Bonner and co-workers reported that CPL photolysis of rac-Leu in 0.1 m HCl at 212.8 nm led to 2–2.5 %enantiomeric excess (ee) in the remaining Leu at 59–75 %conversion, although no photodegradation mechanism was
proposed.[14]In our recent study,[12] rac-Leu and rac-alanine (Ala) were
photolyzed with CPL at 215 nm in acidic and neutral aqueous solutions at room temperature to give enantiomericallyenriched Leu and Ala, irrespective of the solution pH.
However, the mechanism was significantly pH-dependent,
switching from Norrish-type II g-hydrogen abstraction inacidic solutions to the less efficient deamination to an a-lactone by intramolecular nucleophilic substitution (SNi) in neutral solutions,[12, 15] as a result of the change in the excitedchemical species from carboxylic acid to carboxylate; forthe detailed mechanisms in acidic and neutral solutions, see
Scheme 1.
Meierhenrich et al. performed the CPL photolysis of athin film of pure rac-Leu at 182 nm to obtain enantiomerically enriched l- and d-Leu in 0.88  0.28 and 2.60  0.16 %ee, respectively, upon l- and r-CPL irradiation, presumably
at ambient temperatures.[16a] More recently, the same groupreported the results of the CPL photolysis of a 2:1:1 mixtureof H2O, CH3OH, and NH3.[16b] The mixture was first irradiated at 80 K by CPL at 187 nm to produce radical species
trapped in the low-temperature matrix, which was allowed
to thaw and react to afford a mixture of stable products containing amino acids. The resulting mixture was subjected toa second CPL irradiation at room temperature to enantioselectively photodecompose the racemic amino acids producedin the first irradiation; l- and d-enriched Ala were obtained
in 0.71  0.30 and 1.34  0.40 % ee upon l- and r-CPL irradiation at 187 nm, respectively.[16b] Takano et al. reported thatan aqueous extract of high-molecular-weight complex organics (of up to 3000 Da), prepared by irradiating a gaseous
mixture of CO, NH3, and H2O with 3 MeV protons, gave land d-Ala in 0.65  0.23 and 0.44  0.31 % ee upon l- and rCPL irradiation at > 200 nm and subsequent hydrolysis byacid.[17]However, none of our experiments,[12] nor those of Meierhenrich[16] or Takano,[17] has directly proven the occurrenceof the enantioselective decomposition of racemic aminoacids or identified the products derived therefrom underconditions similar to interstellar ice (10–20 K).
In this study to elucidate the feasibility and the detailed
mechanism of the photoreaction of amino acids in ice matrices and hopefully to obtain some insight into the cosmic scenario, we chose rac-Leu as a representative aliphatic aminoacid substrate for the following reasons: (1) Enantiomerical-

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ly enriched Leu has been found in carbonaceous meteorites,[3a] 2) VUV irradiation of a mixture of simple starting materials (H2O, CH3OH, CO2, CO, and NH3) produces racLeu,[10] and 3) Leu photodecomposes efficiently, at least atambient temperatures.[12] The photolysis of rac-Leu was performed in neutral and acidic water and ice media at temperatures ranging from 298 to 21 K to elucidate the effects oftemperature and pH on the photoreaction mechanism ofamino acids by using unpolarized UV light (214 nm) from a
Y-line lamp and circularly polarized synchrotron radiation(215 nm) from a polarizing undulator[18a] installed in theelectron storage ring UVSOR-II.[18b]Results and Discussion
Temperature- and pH-dependent anisotropy factor: In the
CPL photolysis experiment, the anisotropy factor (g = De/e)of the chiral substrate plays a decisive role in determining
the efficiency of photochemical enantiomer enrichment.
Hence we first examined the chiroptical properties of l-Leu
in acidic and neutral aqueous solutions at 268–318 K.
Aliphatic l-amino acids are known to show weak n,p*transitions (e = 45–70 m1 cm1) with a positive Cotton effect(De = 1–2 m1 cm1) at around 210 nm and an extremely
weak negative Cotton effect (j De j  0.001 m1 cm1) at 250–
255 nm in aqueous solutions at pH 1.[12, 19] This is exactly thecase with l-Leu. As shown in Figure 1, the anisotropy spectrum (Figure 1c) is broader than the UV and CD spectra(Figure 1a,b), affording a relatively large positive g factor ofaround 0.03 at 210–220 nm and a much smaller negative gfactor of around 0.002 at 255 nm.
CD spectral analysis and product study: The Leu samples
photolyzed by circularly polarized and unpolarized light inacidic and neutral media at various temperatures were subjected to CD spectral analysis and then to qualitative andquantitative product analyses.[16] The CD spectral changes
caused by CPL irradiation at 21 and 81 K are shown in
Figure 2 and Figure S3 (in the Supporting Information), respectively, and the nonvolatile and volatile products (seeFigures S4–S16) and their yields obtained upon photolysis in
the temperature range 21–298 K are summarized in Tables 1
and 2, respectively.
Because the g factor is dimensionless and hence independent of the volume change caused by varying the solution temperature, it is convenient to examine precisely theeffects of temperature on the chiroptical properties. On
cooling the acidic Leu solution, the g factor was appreciably
enhanced at 210–220 nm at the expense of that at 255 nm(Figure 1c), which has been attributed to the shift in theequilibrium of amino acid conformers and hence is reversible in nature.19b Upon further cooling, the aqueous Leu solution solidified to give an ice matrix, which was glassy but
not completely transparent and hence slightly scattered incident light. Although the frozen Leu samples were not suitable for spectral analysis, it is likely that the conformer com- 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 2013, 19, 13929 – 13936

Photoreaction of rac-Leucine

FULL PAPER

Figure 1. UV (top), CD (middle), and anisotropy spectra (bottom) of
acidic (left) and neutral (right) aqueous solutions of l-Leu (10.6 mm forthe acidic solution and 106 mm for the neutral solution) at 318–268 K.
The UV and CD spectra at 318 and 293 K are corrected for the volume
changes caused by temperature variation, whereas the anisotropy factor
is inherently free from the errors arising from volume change as g = De/e.

position, and therefore the g factor, near the freezing point(268 K) are preserved or at least not significantly altered in
the ice matrix.
In a neutral solution at pH 7, zwitterionic l-Leu shows
only a structureless absorption tail at 200–250 nm in boththe UV and CD spectra (Figure 1 d, e), whereas the anisotropy spectrum (Figure 1f) shows a broad positive peak at 210–
230 nm that is greatly enhanced up to 0.02 on cooling thesolution to near the freezing temperature, that is, 273.5 K.
From a technical point of view, it was convenient that theanisotropy spectra are relatively flat and the maximal valuesof 0.02–0.03 are reached in the same wavelength region forboth the acidic and neutral solutions. This allowed us to exploit the optimal conditions for CPL photolysis withoutfine-tuning the irradiation wavelength (which is achieved
only by altering the energy of electron acceleration in thesynchrotron).
Figure 2a illustrates the CD spectra of the neutral Leu
samples irradiated at 21 K with l- and r-CPL; the spectra are

Chem. Eur. J. 2013, 19, 13929 – 13936

Figure 2. CD spectra of rac-Leu solutions recorded at 296 K before andafter CPL irradiation at 215 nm in an ice matrix at 21 K. (a) Neutral samples ([rac-Leu] = 7.0 mm, pH298 K = 7), in which the conversions upon irradiation of l- and r-CPL (dose: 860 mA h) were 21 and 17 %, respectively.
(b) Acidic samples ([rac-Leu] = 30.5 mm, pH298 K = 1), in which the conversions upon irradiation of l- and r-CPL (dose: 400 mA h) were 6 and 7 %,respectively.

consistent in shape with those of d- and l-Leu, respectively,
which indicates preferential photodecomposition of the antipode (l- and d-Leu), as anticipated by the CD sign at
215 nm (Figure 1d). Naturally, no appreciable CD signals developed upon irradiation with linearly polarized light (LPL).
The optical purity (which is normally equivalent to the ee
but determined from the optical rotation) of the enantiomerically enriched Leu was determined from the induced ellipticity to be 0.3 and + 0.3 % (at 17 and 21 % conversion)
for l- and r-CPL, respectively. The acidic rac-Leu samples irradiated at 21 K with l- and r-CPL also exhibit mirror-image
CD spectra with extrema at 200–210 nm (Figure 2b). Fromthe ellipticity induced, the optical purity was determined tobe 0.1 and + 0.1 % (at 6 and 7 % conversion) for l- and rCPL irradiation, respectively.
Upon irradiation of Leu in an acidic aqueous solution at
298 K, Gly was produced as the major product in 47 % yield(based on the consumed Leu), together with a small amountof ammonia (13 % yield). In contrast, the photolysis of aneutral solution of Leu afforded a-hydroxyisocaproic acid
(38 % yield) and ammonia (73 % yield) as the major prod- 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Y. Inoue et al.

Table 1. Nonvolatile products detected in the rac-Leu samples irradiated at 214 nm with a Y-line lamp (YL) or at 215 nm with left-circularly polarizedsynchrotron radiation (l-CPL) in acidic and neutral aqueous solutions and ice matrices at various temperatures.[a]
T
[K]

Light
sourcepH298 K

298

YL

268
238

YL
YL

183

YL

123

YL

81

l-CPL

77

YL

1
7
1
1
7
1
7
1
7
1
7
1

21

l-CPL

Irradiation
Time
Dose
[h]
ACHTUNGRE[mA h][b]Leu[mm][c]Absorbance[d]

1.5
5
1.5
4
5
4
5
4
5

28.1
8.12
28.1
29.4
6.75
29.4
6.75
29.4
6.75
30.5
7.08
29.4
29.4
6.75
30.5
6.95

0.24[e]0.05[e]
0.24
0.28
0.05
0.28
0.05
0.28
0.05
0.29
0.05
0.28
0.28
0.05
0.29
0.05

200
860
4
12
10

7
1
7

400
860

Product yield [mm (%)][e]NH3iBuCH2NH2
iBuCH(OH)CO2H

Conversion
ACHTUNGRE[mm (%)]

Gly

11.8 (41.9)
1.24 (14.8)
10.6 (37.8)
7.48 (25.5)
0.84 (12.4)
4.54 (15.5)
0.78 (11.6)
2.76 (9.4)
0.77 (11.4)
1.75 (5.7)
1.41 (19.9)
0.72 (2.5)
1.54 (5.2)
0.89 (13.2)
1.75 (5.7)
1.30 (18.7)

5.48 (46.5)
03.86 (36.3)
1.11 (14.8)
00.82 (18.1)
0
0.19 (6.9)
00.14 (8.0)
0
0
0
0
0
0

1.5 (13)0.9 (73)
0.9 (8)1.8 (24)0.4 (47)
0.9 (20)
0.3 (38)
0.5 (18)
0.3 (39)0.6 (34)
0.6 (43)
0.2 (28)
0.5 (32)
0.2 (22)0.6 (34)
0.5 (38)

0.1 (1)0.1 (8)0.1 (1)
< 0.1
< 0.1
< 0.1
0.1 (13)< 0.1
0.1 (13)0.1 (6)0.1 (7)
< 0.1
< 0.1
< 0.1
0.1 (6)0.1 (8)< 0.1
0.47 (38)
0
0
0
0
0
0
0
0
0
0
0
0
0
0

[a] Irradiated at 298–123 K in a Unisoku USP-203 cryostat, at 81 K in a continuous-flow liquid N2 cryostat, at 77 K in a Suprasil Dewar vessel filled withliquid N2, and at 21 K in a continuous-flow liquid helium cryostat. [b] Light intensity defined as the accumulated electron beam current of the electron
storage ring UVSOR-II. [c] Lower concentrations were employed for neutral samples, due to the lower solubility of zwitterionic Leu. [d] Absorbance calculated from the concentration and the effective pathlength (1.8 mm), unless noted otherwise. [e] Product yield based on the consumed Leu. In all irradiated samples, trace amounts of CH3CO2H and HCO2H were detected. Effective pathlength: 1.5 mm.

Table 2. Gaseous and ether-extracted products detected upon gas chromatographic analyses of the rac-Leu samples photolyzed at 215 nm with left-circularly polarized synchrotron radiation (l-CPL) in acidic and neutral ice matrices at 81 and 21 K.[a]T[K]

pH298 K

81

1
7
1
7

21

Dose
ACHTUNGRE[mA h][b]

Conc.
[mm]

AbsorACHTUNGRE
bance[c]Conversion
ACHTUNGRE[mm (%)]

CO

CO2

200
860
400
860

30.47
6.95
30.47
6.95

0.28
0.05
0.28
0.05

1.75 (6.4)
1.23 (17.7)
2.05 (6.8)
1.38 (19.9)

–[e]
–[e]
> 0.04 (> 2)> 0.04 (> 3)

–[e]
–[e]
> 0.07 (> 3)> 0.12 (> 9)

Product yield [mm (%)][d]iBuCHOCH3CH=CH2
0.07 (4)0.09 (7)0.12 (6)0.16 (12)

–[e]
–[e]
> 0.01 (> 0.1)
> 0.01 (> 0.1)

ACHTUNGRE(CH3)2C=CH2
–[e]
–[e]
< 0.01< 0.01[a] Irradiated in a continuous-flow liquid N2 (81 K) or He (21 K) cryostat. [b] Light intensity defined as the accumulated electron beam current of
UVSOR-II. [c] Absorbance calculated from the concentration and the effective pathlength (1.8 mm). [d] Product yield based on the consumed Leu.
[e] Yield not determined.

ucts. The rate of photodecomposition, or conversion, in general, was reduced by lowering the irradiation temperature,
but the effect was more significant for the acidic samples(Table 1). As shown in Figure 3a, a dramatic change was observed for the Gly yield, which suddenly dropped at thefreezing point (indicated by the vertical broken line in Figure 3a) and gradually faded out upon further cooling, finallydisappearing at 77 K. The sudden drop in Gly yield at thefreezing point may be ascribed to the reduction of conformational freedom upon freezing, which hinders the access ofthe excited carbonyl oxygen to the g-hydrogen and subsequent abstraction (Scheme 1). It is interesting, however, thatsome g-hydrogen abstraction still occurs even in ice matrices, at least at temperatures higher than 77 K, for which two
independent mechanisms (one static and one dynamic) may
be responsible. It is reasonable to assume that (1) part of
the Leu molecules in the ice matrix are frozen in conformation(s) that are suitable for g-hydrogen abstraction and(2) there still remains some partial mobility available for theexcited carbonyl moiety to approach and abstract the g-hy-13932

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drogen atom. It is likely that both of these mechanisms may
operate in ice matrices at higher temperatures but become
structurally and/or energetically unfeasible at temperatures
below 77 K.
In contrast, the ammonia yield is much less sensitive totemperature and in fact increases at lower temperatures atthe expense of Gly (Figure 3a). This indicates that the SNi
attack of the excited carboxylate moiety on the a-carbon
atom, leading to deamination, is conformationally more tolerant and energetically more feasible.
Leu in neutral ice matrices shows very different photobehavior. As shown in Figure 3b, the conversion and the ammonia yield only moderately decrease on lowering the temperature from 298 (solution) to 238 K (ice), whereas the formation of a-hydroxyisocaproic acid, the main product of thesolution-phase irradiation, is completely suppressed in ice,even at 238 K. This result is not unexpected because theattack of water on the intermediate a-lactone, leading to ahydroxyisocaproic acid, is not feasible in ice matrices, as discussed below (Scheme 1).[12, 19a, 20a]

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Photoreaction of rac-Leucine

FULL PAPERin 4–6 % yield for the acidic sample (Table 2). Crucially, theformation of isovaleraldehyde was not proportional to theperiod of irradiation, but showed an induction period, whichis in sharp contrast to the linear dependence of the ammonia yield (Figure 4). This indicates that isovaleraldehyde is a

Figure 4. Yields of ammonia (*) and isovaleraldehyde (iBuCHO, ~) as afunction of the photodecomposition of neutral Leu (6.63–6.95 mm) upon
irradiation by l-CPL of varying doses (100–860 mA h) at 21 K.
Figure 3. Chemical yields of ammonia (*) and Gly (*) obtained upon irradiation of (a) acidic and (b) neutral rac-Leu samples in aqueous solutions at 268–298 K and in ice matrices at 21–238 K. The vertical broken
line indicates the freezing point of each solution.

Gas chromatographic (GC) analysis revealed that the gaseous products collected from the irradiated neutral andacidic Leu samples contained CO in > 2–3 % yield and CO2
in > 3–9 % yield, whereas the diethyl ether extract contained
isovaleraldehyde in 7–12 % yield for the neutral sample andproduct derived from a secondary photoreaction of the initial product. We propose isobutyl-a-lactone as the mostplausible intermediate for the formation of isovaleraldehyde
upon irradiation in ice; this lactone, which is derived fromthe photochemical SNi deamination of Leu, is highly reactive
and immediately attacked by water to give a-hydroxyisocaproic acid in aqueous solutions.[12, 20] In low-temperature ice
matrices, the a-lactone produced is likely to be trapped
intact and suffer secondary photodecomposition to isovaler-

Scheme 1. Photochemical reactions of rac-Leu in acidic and neutral aqueous solutions and ice matrices.

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Y. Inoue et al.

aldehyde and CO (Scheme 1, bottom row). Experimentally,
the detection of propene, a photodecomposition productfrom isovaleraldehyde, in the photolyzed solution (Table 2)
may support this mechanism (Scheme 1, middle right).

Conclusion
In this study, we have identified the major volatile and nonvolatile products obtained upon direct irradiation of Leu inaqueous solutions and also in ice matrices. Based on the results of the product study, we have discussed the photochemical behavior of Leu (as a representative aliphaticamino acid) in acidic and neutral media at temperatures
ranging from 298 to 21 K to reveal that the photoreaction
mechanism and the major products derived therefrom are
critical functions of the phase, temperature, and pH of themedia.
In acidic media, the overall photoreaction is greatly decelerated by lowering the irradiation temperature, particularly
below the freezing point, and the main photoreaction mechanism switches from the Norrish-type II g-hydrogen abstraction, which affords Gly and isobutene, to SNi deamination to
give isobutyl-a-lactone, which is trapped in situ and subsequently photodecarbonylated to isovaleraldehyde in ice matrices.
In neutral media, the photoreaction proceeds consistently
by the SNi deamination mechanism to give the a-lactone,
which is followed by the nucleophilic attack of water leading
to a-hydroxyisocaproic acid in aqueous solutions at ambient
temperatures or alternatively by photodecarbonylation toisovaleraldehyde in ice matrices at cryogenic temperatures.
The overall efficiency of the photoreaction is surprisingly insensitive to the irradiation temperature (and medium),
maintaining the same level of conversion throughout thetemperature range examined (21–298 K). This suggests that
the SNi deamination survives as the major photoreaction
process under interstellar conditions (10–20 K).
Intriguingly, the photoreaction mechanism converges tothe deamination process in both acidic and neutral media attemperatures below 77 K. This indicates that the amino acidexists in its zwitterionic form in ice matrices at lower temperatures, irrespective of the original pH at 298 K, because
the deamination occurs only upon excitation of the carboxylate moiety of a zwitterionic amino acid.[12, 19a, 20a] In this connection, it is crucial to point out that the dissociation of HCladsorbed on an ice surface is discouraged at low temperatures[21a,b] and HCl on amorphous ice at 70 K is totally molecular[21a.c] whereas around 85 % of HCl is molecular in ice
nanocrystals at 60 K.[21d] This means that HCl becomes more
molecular, or less acidic, at lower temperatures and that it isdifficult to protonate zwitterionic amino acid in ice matrices.
This shift in equilibrium rationalizes the switching of the
major photoreaction mechanism from the g-hydrogen abstraction by an excited carboxylic acid to the SNi deamination by an excited carboxylate (Scheme 1).

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On the other hand, we are less certain about the potential
roles of different ice phases in controlling the photobehavior
as we cooled the sample solutions at a constant rate to thedesignated temperatures and no literature information isavailable on the phases of ice matrices containing 7–30 mm
rac-Leu (and HCl). However, the experimental results obtained in this study (i.e., no clear jump is observed in conversion or product yield at the phase transition temperatures) suggest that the photobehavior is not significantly affected by the phase transitions from metastable phase I
cubic (Ic) to phase I hexagonal (Ih) and then to phase XI observed upon cooling pure ice,[22] or more likely the ice matrices containing Leu and/or HCl are amorphous in nature andhence experience no clear phase transition.
From the point of view of the chemical evolution, it is crucial that not only the CPL photoreaction of rac-amino acids
in ice has been experimentally proven to proceed under interstellar conditions, but also that the major photoreaction
mechanisms and products have been elucidated. These results substantiate the cosmic scenario in which interstellar
ice analogues containing simple starting materials irradiatedby VUV light produce a mixture of racemic amino acids[10]that subsequently photodecompose enantioselectively by
CPL radiated from the star-formation region en route toprimitive Earth.
However, the enantiomeric excesses obtained in the CPL
photolysis of aliphatic amino acids are generally low (0.1–
2.6 % ee),[12, 16a, b, 17] including the present case. It has beenshown that the enantioselectivity obtained by CPL photodecomposition of a racemic mixture is highly conversion-dependent and can be enhanced to 90 % ee and even to 99 %ee at 88 and 96 % conversion, respectively, if the g factor is
1.0.[11] In reality, the g factors of aliphatic amino acids, including Leu (Figure 1), are typically 0.02 or slightly higher
at the irradiation wavelength (215 nm).[23] This means thatan ee of 4.6 % will be achieved at 99 % conversion and furthermore that the last single molecule will have statistically
48 % ee on average even when one mole of an amino acidwith g = 0.02 is subjected to CPL photolysis.[11] Hence, theunexpectedly high ee values of up to 50–60 % reported recently by Pizzarello and co-workers[3] for nonterrestrial isoleucine and allo-isoleucine extracted from the Murchisonand other meteorites, and also by Glavin et al.[3m] for nonterrestrial proteinogenic aspartic acid and glutamic acid fromtwo out of the three examined fragments of the Tagish Lake
meteorite, are evidently unexplainable by the CPL photodecomposition mechanism alone and indicate the operation ofother mechanisms such as preferential crystallization to conglomerates[24] during the aqueous alteration in a meteorite
parent body.[3m] Nevertheless, it is also worth noting that essentially all of the proteinogenic amino acids found in the
Tagish Lake meteorite are l-enriched,[3m] which suggests a
certain seeding or biasing mechanism to trigger the enantiomeric imbalance in which the CPL photodecomposition
would have played some role.

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Photoreaction of rac-Leucine

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Experimental Section
Materials: rac-Leu ( 99 % pure), d-Leu (99 % pure, 97 % ee), and l-Leu( 98 % pure, 99 % ee) were purchased from Wako and used without further purification.
Sample preparation: Acidic Leu solutions (28.1–30.5 mm) were preparedby dissolving rac-, d-, or l-Leu in an aqueous 0.1 m HCl solution(pH298 K 1.03) for photoirradiation or in an aqueous 0.5 m HCl solution for
UV and CD spectral measurements, whereas neutral Leu solutions (6.8–
8.1 mm) at pH298 K 6.78 were prepared by dissolving rac-, d- and l-Leu indistilled water. The acidic and neutral Leu solutions, with a thickness of
1.5–1.8 mm, were relatively transparent at 215 nm, showing absorbancesof 0.24–0.29 and approximately 0.05, respectively. Immediately after their
preparation, each Leu solution of 0.7 mL volume was placed in a regular(10  10  40 mm) or thin (2  10  40 mm) quartz cell, degassed by threefreeze–pump–thaw cycles, sealed under high vacuum (< 105 Pa), andthen subjected to spectral examination and photoirradiation at ambientand lower temperatures.
Spectroscopy: UV and CD spectra of the acidic and neutral l-Leu solutions prepared above were recorded at 268–318 K on a JASCO V-550
spectrophotometer fitted with an HMC-358 temperature controller andon a J-720WI spectropolarimeter fitted with a PMH-354WI temperature
controller. Spectral analysis at temperatures lower than the freezing
point was not feasible or the result was not reproducible due to surface
roughness and scattering of the frozen samples.
Photoirradiation: The Leu samples prepared above were irradiated at
214 nm by using unpolarized light from an Ushio Y-line lamp in solution
at 268–298 K and also in ice at 77–238 K.
The frozen samples for unpolarized light irradiation were prepared as follows. A quartz cell containing 0.7 mL of the Leu solution was graduallycooled to 238 K by horizontally placing the cell in a cooling bath. Oncefrozen, the icy sample was quickly transferred to a Unisoku USP-203
cryostat (with three optical windows for spectral examination and irradiation) and further cooled to the desired temperature of  123 K at a rateof 10 K min1 or directly to 77 K by immersing the sample cell in liquid
N2, and then irradiated by a Y-line lamp.
For CPL irradiation, a quartz cell containing the Leu solution was fixed
on a brass sample holder, which was placed horizontally in a vacuum
chamber connected to the polarizing undulator beam-line and cooled to
21 or 81 K first at a rate of 2–5 K min1 (298–220 K) and then at 10–
20 K min1 (< 220 K) in a continuous-flow helium or nitrogen cryostat
with temperature monitored by a thermocouple attached to the cell surface. The quartz cell containing a frozen Leu sample of around 1.5–
1.8 mm thickness was rotated to the upright position and irradiated at
21  3 or 81  2 K with CPL at 215 nm generated by the polarizing undulator installed in UVSOR-II at the Institute for Molecular Science (seeFigures S1 and S2 in the Supporting Information).[18]Product analyses: The irradiated samples were warmed to ambient temperature in a plastic bag with a septum cap. The volatile products were
sampled by using a gas-tight syringe, and an aliquot of the aqueous solution was extracted with diethyl ether, both of which were analyzed by gas
chromatography (GC). The rest of the solution was subjected to qualitative and quantitative analyses as described below.[12]The gaseous inorganic and organic products collected in a gas-tight syringe were analyzed by GC on a Shimadzu GC-14A instrument (with a
TCD detector) fitted with a column (2.1 m  3 mm i.d.) of molecularsieves (5 , 60/80 mesh). The ether extract was subjected to GC-MS analysis on an HP-6890GC/5973MSD instrument fitted with a CP-PoraBOND Q column (25 m  0.25 mm i.d.) to characterize and quantify theorganic products. The yields obtained were corrected for the distribution
coefficients of the analytes between the ether (2 mL) and Leu solution(0.2 mL), which were determined on a Shimadzu GC-14A with a J&W
DB wax column (30 m  0.25 mm i.d.). Nonvolatile products in the photolyzed Leu samples were characterized and quantified by using the aminoacid analysis, ion chromatography, and capillary electrophoresis, as reported previously.[12]Chem. Eur. J. 2013, 19, 13929 – 13936

CD spectral analysis of the CPL-irradiated samples was carried out to determine their optical purity (op) at 296 K in a cylindrical cell (13 mm
i.d.  2 mm thickness) on a JASCO J-725 spectropolarimeter with a modified optical system for higher light intensity and sensitivity. The opticalpurities were calculated by using the equation op = q215/ACHTUNGRE(33De215ACHTUNGRE[Leu]), in
which q215 is the ellipticity at 215 nm of a CPL-irradiated sample, De215 is
the molar CD of enantiopure Leu at the same wavelength, and [Leu] theconcentration of Leu remaining in the photolyzed solution.[12]Acknowledgements
This work was supported by the Joint Studies Program of the Institutefor Molecular Science and the Japan Society for the Promotion of Science through a Grant-in-Aid for Scientific Research (A, grant no.
21245011) both of which are gratefully acknowledged. We thank themembers of the UVSOR facility, Institute for Molecular Science, in particular Mr. Eiken Nakamura and Mr. Takayuki Murakami, for the operation of the linear accelerator and the continuous flow liquid He/N2 cryostat. We also thank Ushio Inc. for the use of a Y-line lamp.

[1] a) W. A. Bonner, Top. Stereochem. 1988, 18, 1; b) W. A. Bonner,
Origins Life Evol. Biosphere 1991, 21, 59; c) J. Alain, C. Corinne,
Origins Life Evol. Biosphere 2002, 32, 129; d) U. J.; Meierhenrich,
G. M. MuÇos Caro, W. A. Schutte, B. Barbier, A. A. Segovia, H.
Rosenbauer, W. H. P. Thiemann, A. Brack, European Space Agency
Special Publication 2002, SP-518, 25; e) C. Meinert, J. H. Bredeohoft, J.-J. Filippi, Y. Baraud, L. Nahon, F. Wien, N. C. Jones, S. V.
Hoffmann, U. J. Meierhenrich, Angew. Chem. 2012, 124, 4562;
Angew. Chem. Int. Ed. 2012, 51, 4484.
[2] a) J. Podlech, Angew. Chem. 1999, 111, 501; Angew. Chem. Int. Ed.
1999, 38, 477; b) S. Pizzarello, Acc. Chem. Res. 2006, 39, 231; c) U. J.
Meierhenrich, J.-J. Filippi, C. Meinert, P. Vierling, J. P. Dworkin,
Angew. Chem. 2010, 122, 3826; Angew. Chem. Int. Ed. 2010, 49,3738; d) J. E. Hein, D. G. Blackmond, Acc. Chem. Res. 2012, 45,2045.
[3] a) M. H. Engel, B. Nagy, Nature 1982, 296, 837 (see also ref. [3f] forfurther discussion); b) M. H. Engel, S. A. Macko, J. A. Silfer, Nature
1990, 348, 47; c) S. Pizzarello, J. R. Cronin, R. V. Krishnamurthy, S.
Epstein, Geochim. Cosmochim. Acta 1991, 55, 905; d) J. R. Cronin,
S. Pizzarello, Science 1997, 275, 951; e) M. H. Engel, S. A. Macko,
Nature 1997, 389, 265; f) S. Pizzarello, J. R. Cronin, Geochim. Cosmochim. Acta 2000, 64, 329; g) M. A. Sephton, Nat. Prod. Rep. 2002,
19, 292; h) S. Pizzarello, M. Zolensky, K. A. Turk, Geochim. Cosmochim. Acta 2003, 67, 1589; i) S. Pizzarello, Y, Huang, M. R. Alexandrer, Proc. Natl. Acad. Sci. USA 2008, 105, 3700; j) D. P. Glavin,
J. P. Dworkin, Proc. Natl. Acad. Sci. USA 2009, 106, 5487; k) D. P.
Glavin, M. P. Callahan, J. P. Dworkin, J. E. Elsila, Meteorit. Planet.
Sci. 2010, 45, 1948; l) S. Pizzarello, D. L. Schrader, A. A. Monroe,
D. S. Lauretta, Proc. Natl. Acad. Sci. USA 2012, 109, 11949; m) D. P.
Glavin, J. E. Elsila, A. S. Burton, M. P. Callahan, J. P. Dworkin,
R. W. Hilts, C. D. K. Herd, Meteorit. Planet. Sci. 2012, 47, 1347.
[4] a) E. Rubenstein, W. A. Bonner, H. P. Noyes, G. S. Brown, Nature
1983, 306, 118; b) W. A. Bonner, E. Rubenstein, Biosystems 1987,
20, 99.
[5] J. A. Roberts, Nature 1984, 308, 318.
[6] a) J. Bailey, A. Chrysostomou, J. H. Hough, T. M. Gledhill, A.
MaCall, S. Clark, F. Mnard, Science 1998, 281, 672; b) M. Buschermçhle, D. C. B. Whittet, A. Chrysostomou, J. H. P. Hough, W. Lucas,
A. J. Adamson, B. A. Whitney, M. J. Wolff, Astrophys. J. 2005, 624,821.
[7] a) K. Kobayashi, T. Kasamatsu, T. Kaneko, J. Koike, T. Oshima, T.
Saito, T. Yamamoto, H. Yanagawa, Adv. Space Res. 1995, 16, 21;b) B. T. Draine, Astrophys. J. 2003, 598, 1017.
[8] J. Or, Nature 1961, 190, 389.

 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

13935

Y. Inoue et al.

[9] U. Meierhenrich, B. Barbier, R. Jacquet, A. Chabin, C. Alcaraz, L.
Nahon, A. Brack, European Space Agency Special Publication 2001,
SP-496, 167.
[10] a) G. M. MuÇoz Caro, U. J. Meierhenrich, W. A. Schutte, B. Barbler,
A. A. Segovia, H. Rosenbauer, W. H. P. Thiemann, A. Brack, J. M.
Greenberg, Nature 2002, 416, 403; b) M. P. Bernstein, J. P. Dworkin,
S. A. Sandford, G. W. Cooper, L. J. Allamandola, Nature 2002, 416,
401; c) C. Meinert, J.-J. Filippi, P. Marcellus, L. L. S. Hendecourt,
U. J. Meierhenrich, ChemPlusChem 2012, 77, 186.
[11] a) W. Kuhn, E. Knopf, Z. Phys. Chem., Abt. B 1930, 7, 292; b) G.
Balavoine, A. Moradpour, H. B. Kagan, J. Am. Chem. Soc. 1974, 96,5152; c) H. Rau, Chem. Rev. 1983, 83, 535; d) H. Nishino, A. Nakamura, Y. Inoue, J. Chem. Soc., Perkin Trans. 2 2001, 1693.
[12] H. Nishino, A. Kosaka, G. A. Hembury, F. Aoki, K. Miyauchi, H.
Shitomi, H. Onuki, Y. Inoue, J. Am. Chem. Soc. 2002, 124, 11618.
[13] S. F. Mason, Nature 1997, 389, 804.
[14] J. J. Flores, W. A. Bonner, G. A. Massey, J. Am. Chem. Soc. 1977, 99,3622.
[15] A. D. Mclaren, D. Shugar, Photochemistry of Proteins and Nucleic
Acids, Pergamon Press, Oxford, 1964.
[16] a) U. J. Meierhenrich, L. Nahon, C. Alcaraz, J. H. Bredehçft, S. V.
Hoffmann, B. Barbier, A. Brack, Angew. Chem. 2005, 117, 5774;
Angew. Chem. Int. Ed. 2005, 44, 5630; b) P. de Marcellus, C. Meinert, M. Nuevo, J.-J. Filippi, G. Danger, D. Deboffle, L. Nahon, L.
Le Sergeamt d’Hendecourt, U. J. Meierhenrich, Astrophys. J. 2011,
727, L27.
[17] Y. Takano, J. Takahashi, T. Kaneko, K. Marumo, K. Kobayashi,
Earth Planet. Sci. Lett. 2007, 254, 106.

13936

www.chemeurj.org[18] a) S. Kimura, M. Kimada, H. Hama, X. Marchal, T. Tanaka, H. Kitamura, J. Electron Spectrosc. Relat. Phenom. 1996, 80, 437; b) M.
Katoh, K. Hayashi, T. Honda, Y. Hori, M. Hosaka, T. Kinoshita, S.
Kouda, Y. Takashima, J. Yamazaki, Nucl. Instrum. Methods Phys.
Res. Sect. A 2001, 467/468, 68.
[19] a) I. Mandl, A. D. McLaren, J. Am. Chem. Soc. 1951, 73, 1826; b) R.
Hkansson in Chemistry of Acid Derivatives, Vol. 2 (Ed. S. Patai),Wiley, Chichester, 1992, Pt. 1.
[20] a) O. L. Chapman, P. W. Wojtkowski, W. Adam, O. Rodriguez, R.
Ruckt schel, J. Am. Chem. Soc. 1972, 94, 1365; b) O. L. Chapman,
Pure Appl. Chem. 1974, 40, 511.
[21] a) L. Delzeit, B. Rowland, J. P. Devlin, J. Phys. Chem. 1993, 97,10312; b) N. Uras, M. Rahman, J. P. Devlin, J. Phys. Chem. B 1998,
102, 9375; c) H. Kang, T. H. Shin, S. C. Park, I. K. Kim, S. J. Han, J.
Am. Chem. Soc. 2000, 122, 9842; d) J. P. Devlin, N. Uras, J. Sadlej,
V. Buch, Nature 2002, 417, 269.
[22] A. Leadbetter, R. C. Ward, J. W. Clark, P. A. Tucker, T. Matsuo, H.
Suga, J. Chem. Phys. 1985, 82, 424.
[23] H. Nishino, A. Kosaka, G. A. Hembury, K. Matsushima, Y. Inoue, J.
Chem. Soc., Perkin Trans. 2 2002, 582.
[24] a) C. Viedma, Origins Life Evol. Biosphere 2001, 31, 501; b) A. Alvarez Rodrigo, H. Lorenz, A. Seidel-Morgenstern, Chirality 2004,
16, 499; c) C. Viedma, J. E. Ortiz, T. de Torres, T. Izumi, D. Blackmond, J. Am. Chem. Soc. 2008, 130, 15274.

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Received: May 13, 2013
Revised: July 7, 2013
Published online: August 28, 2013

Chem. Eur. J. 2013, 19, 13929 – 13936