Photolysis of rac-leucine with circularly polarized synchrotron radiation

Article abstract of DOI:10.1002/cbdv.200900311

Amino acids that pass the RNA machinery in living organisms occur in L-configuration. The question on the evolutionary origin of this biomolecular asymmetry remains unanswered to this day. Amino acids were detected in artificially produced interstellar ices, and L-enantiomer-enriched amino acids were identified in CM-type meteorites. This hints at a possible interstellar/circumstellar origin of the amino acids themselves as well as their stereochemical asymmetry. Based upon the current knowledge about the occurrence of circularly-polarized electromagnetic radiation in interstellar environments, we subjected rac-leucine to far-UV circularly-polarized synchrotron radiation. Asymmetric photolysis was followed by an analysis in an enantioselective GC/MS system. Here, we report on an advanced photolysis rate of more than 99% for leucine. The results indicate that high photolysis rates can occur under the chosen conditions, favoring enantioselective photolysis. In 2014, the obtained results will be reexamined by cometary mission Rosetta.

Full text of DOI:10.1002/cbdv.200900311

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Photolysis of rac-Leucine with Circularly Polarized Synchrotron Radiation
by Uwe J. Meierhenrich*a), Jean-Jacques Filippia), Cornelia Meinerta), Søren V. Hoffmannb),
Jan Hendrik Bredehçftc), and Laurent Nahond)
a) University of Nice-Sophia Antipolis, LCMBA UMR 6001 CNRS, avenue Valrose, F-06108 Nice
(phone: þ33-492-076177; fax: þ33-492-076151; e-mail: Uwe.Meierhenrich@unice.fr)
b) University of Aarhus, Institute for Storage Ring Facilities (ISA), Ny Munkegade 120,
DK-8000 Aarhus C
c) University of Bremen, Institute for Applied and Physical Chemistry, Leobener Strasse,
D-28359 Bremen
d) Synchrotron SOLEIL, lꢀOrme des Merisiers, Saint Aubin BP 48, F-91192 Gif-sur-Yvette
´
Dedicated to Andre Brack
Amino acids that pass the RNA machinery in living organisms occur in l-configuration. The
question on the evolutionary origin of this biomolecular asymmetry remains unanswered to this day.
Amino acids were detected in artificially produced interstellar ices, and l-enantiomer-enriched amino
acids were identified in CM-type meteorites. This hints at a possible interstellar/circumstellar origin of
the amino acids themselves as well as their stereochemical asymmetry. Based upon the current
knowledge about the occurrence of circularly-polarized electromagnetic radiation in interstellar
environments, we subjected rac-leucine to far-UV circularly-polarized synchrotron radiation. Asym-
metric photolysis was followed by an analysis in an enantioselective GC/MS system. Here, we report on
an advanced photolysis rate of more than 99% for leucine. The results indicate that high photolysis rates
can occur under the chosen conditions, favoring enantioselective photolysis. In 2014, the obtained results
will be reexamined by cometary mission Rosetta.
Introduction. – The origin of cellular life [1], including the evolution of genes and
enzymes, is based on biopolymers that are strongly selective towards the chirality of
their monomer subunits [2]. Proteins are composed of amino acid monomers of
l-configuration. The evolutionary origin of lifeꢀs molecular asymmetry remains yet
unanswered; one approach among others focuses on asymmetric interstellar phenom-
ena inducing an enantiomeric excess (ee) into organic molecules by enantioselective
photolysis or asymmetric synthesis. This hypothesis is supported by the fact that more
than 16 different amino acids have been identified in simulated interstellar ices [2–4],
and achiral glycine has been detected in a cometary dust sample returned by Stardust
[5]. Besides, enantiomerically enriched amino acids [6–8] and rac-diamino acids [9]
were found in CM type meteorites – a comprehensive summary of these amino acids
can be found in [10] – and among them no chiral amino acid shows an excess of the
d-enantiomer. Amino acidsꢀ asymmetry might have been induced by circularly-
polarized (CP) electromagnetic radiation that has been detected in interstellar
environments and star-forming regions [11][12]. These data and indications point
towards the fact that biomolecular asymmetry might have been induced photochemi-
ꢁ 2010 Verlag Helvetica Chimica Acta AG, Zꢂrich

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CHEMISTRY & BIODIVERSITY – Vol. 7 (2010)
cally in interstellar amino acids long before these amino acids were delivered to the
early Earth and triggered the origin of life on our planet [13][14]. Such a scenario will
be probed (and challenged) in the near future since the Cometary mission Rosetta is
designed to land on a comet nucleus in 2014 and should analyze cometary ice in situ
[15]. The COSAC (Cometary Sampling and Composition) instrument aboard Rosettaꢀs
Lander will focus on the enantioselective analysis of chiral organic molecules including
amino acids by the application of a Chirasil-l-Val, a Cyclodextrin G-TA, and a Chirasil-
Dex CB stationary phase [2]. The COSAC experiment is a combined GC/MS aboard
the Philae cometary surface probe of the Rosetta Lander.
To better understand a possible photochemical origin of biomolecular asymmetry,
we studied a) the asymmetric formation of amino acids [16][17] and biological
cofactors [18] under interstellar conditions using CP light, and b) the asymmetric
photolysis of amino acids subjected to CP electromagnetic radiation at the synchrotron
SOLEIL. Here, we report on recent experiments subjecting the amino acid rac-leucine
to CP synchrotron radiation [19] in the far-UV. It is well-known that due to the chiral
interaction with a ꢃchiralꢀ CP photon, the enantiomers have different absorption cross
sections at certain wavelengths resulting in a non-zero circular dichroism (CD) De¼
eL-leucine ꢀeD-leucine, whose intensity varies with the photon energy. The chosen photon
energy was higher than the leucine dissociation energy, and, therefore, an absorbing
leucine enantiomer will undergo photodissociation, mainly by decarboxylation. An
increasing photolysis rate will induce an increase in ee in the remaining non-photolyzed
leucine sample [20] following a hyperbolic tangent function given in [21]. The optical
yield is moreover dependent on the optical anisotropy factor g¼De /e. The aim of the
present study was to induce photolysis rates of 99% and above in leucine at a
wavelength that corresponds to a large CD value. Irradiation experiments were
performed on solid-state samples at irradiation wavelengths below 200 nm. CD
Measurements show remarkably high values in this region [22], much higher in fact
than above 200 nm in aqueous solution. We assume moreover that our amorphous
leucine samples better represent the occurrence of amino acids under interstellar
conditions than the previously studied aqueous solutions [23][24] or a microcrystalline
phase [22].
Results and Discussion. – By using an ultra high vacuum (UHV) sublimation
chamber, an amorphous l-leucine sample of 150-nm film thickness was deposited onto
a MgF2 window by heating the leucine reservoir to 1328 for 16 min at 10ꢀ8 mbar. VUV/
CD spectra of this film were recorded between 130 and 330 nm at the Institute for
Storage Ring Facilities, University of Aarhus, Denmark, using the synchrotron
beamline UV1 at the ASTRID storage ring. The obtained CD spectrum is given in
Fig. 1. As expected, it depicts an interesting CD signal below 200 nm: a positive band at
184 nm and a negative band at 163 nm noticeably different from the solid state CD
spectrum recorded from microcrystalline d-leucine reported in [22].
A first preliminary test of enantioselective photolysis at beamline DESIRS of the
synchrotron SOLEIL was performed to determine the dimensions of the rac-leucine
film, the obtained photon transmission, the optical density, the alignment of the setup
etc. To this end, an amorphous film of rac-leucine with 2.0 mmꢁ5.4 mm lateral
dimensions and a thickness of 987 nm was condensed onto a MgF2 window using the

CHEMISTRY & BIODIVERSITY – Vol. 7 (2010)
1653
Fig. 1. VUV CD spectra of an amorphous l-leucine film of 150-nm thickness recorded at the synchrotron
facility ISA, University of Aarhus, Denmark. To avoid contributions to the signal from VUV
birefringence and linear dichroism, the sample was rotated around the light axis in steps of 908 leading
to the four visible traces. CD spectra of d-leucine show signals of opposite sign. Signal is given in
millidegrees of ellipticity q, which can be converted to circular dichroism by the equation q [mdeg]¼
3298.2 De.
UHV sublimation chamber in which the amino acid reservoir was heated to 1458 for
50 min. Under inert conditions, this sample ꢃAꢀ was taken out of the sublimation
chamber and immediately installed in the UHV irradiation chamber. It was subjected
to right CP synchrotron radiation of 181 nm (6.85 eV). The footprint of the synchrotron
radiation was 2.5 mmꢁ6.75 mm and thus larger than the deposited leucine film. During
the first hours of irradiation, the transmission monitored by a photodiode downstream
of the irradiation chamber increased as expected due to photolysis of the amino acid.
Then, as assumed, the transmission normalized to the beam current reached a quasi-
constant value. After 17 h, irradiation was stopped; the remaining leucine was
extracted with ultra-pure H2O and derivatized to form a volatile (ethoxycarbonyl)leu-
cine ethyl ester (Leu-ECEE) derivative [25]. The d,l-Leu-ECEE enantiomers were
separated on a Chirasil-l-Val stationary phase and individually quantified in the single-
ion monitoring mode on mass trace m/z 158 using a GC/MS system Agilent 6900/5973
setup at the synchrotron facilities. With the help of an additional non-irradiated 981 nm
rac-leucine film that had been sublimated for 52 min at 1458, extracted, derivatized, and
analyzed identically, a total photolysis rate of 84.6ꢂ0.5% was determined for sample
A. Sample A and the non-irradiated sample were alternately injected eight times to
minimize the error in photolysis rate and ee determination. An ee value of 0.0ꢂ0.5%
was found (see also Table).
For sample B, a 301-nm film of rac-leucine was prepared by 13-min sublimation at
1458. At the synchrotron, sample B was irradiated with right-handed CP synchrotron
radiation at 187 nm (6.63 eV) for 55 h. The circular polarization rate of the synchrotron
radiation was ca. 99%. The wavelength was slightly increased compared to sample A in
order to better match the maximum of the CD spectrum (Fig. 1), and to avoid

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CHEMISTRY & BIODIVERSITY – Vol. 7 (2010)
Table. Photolysis Rates and ee Values of Leucine Samples Obtained by Enantioselective Photolysis
Applying CP Synchrotron Radiation
Sample Film thickness [nm] Irradiation time [h] Circular polarization Photolysis rate [%] ee l-leucine
A
B
C
987
301
300
17
55
56
right
right
left
84.6
99.23
99.875
0.0
5.2
–a)
a) The amount of remaining amino acid was too low to precisely determine the ee.
irradiation at a wavelength close to a zero CD value. After comparison with a non-
irradiated rac-leucine sample of 300 nm that was deposited for 22 min at 1458, a total
photolysis rate of 99.23% was obtained for sample B as depicted in Fig. 2. An ee value
of 5.2ꢂ0.5% of the d-leucine enantiomer was recorded in the remaining leucine film
(Fig. 3). This is to date the highest reported ee value introduced in an amino acid by
means of photochemistry. It is in accordance with theoretical predictions of Kagan et al.
who calculated an ee of 4.6% for a photolysis rate of 99.0% on the basis of an
anisotropy factor of g¼0.02, typical for amino acids under the chosen conditions [21].
The measured ee value of 5.2%, however, needed to be confirmed by irradiation of a
rac-leucine film with opposite-handed synchrotron radiation.
Fig. 2. Enantioselective gas chromatogram of sample B (blue line). The upper black line represents the
chromatogram of the non-irradiated rac-leucine sample illustrating the photolysis rate of 99.23%. In
sample B (blue line) the ee value was determined. Leucine enantiomers were extracted with H2O and
derivatized to form (ethoxycarbonyl)leucine ethyl ester (ECEE) prior to GC analyses.
Sample C was sublimated for 18 min at 1458 giving a 300-nm film of rac-leucine.
Sample C was irradiated for 56 h with the opposite-handed light, i.e., left-handed CP
synchrotron radiation at 187 nm, also CP at 99%. Our strategy was again to obtain a
high total photolysis rate with the opposite ee in the remaining amino acid residue. In

CHEMISTRY & BIODIVERSITY – Vol. 7 (2010)
1655
Fig. 3. Overlaid enantioselective gas chromatograms of baseline-separated leucine enantiomers in sample
B (black line) and in the diluted non-irradiated racemic sample (red line). For each chromatogram, four
samples that were analyzed alternately were averaged. Sample B shows an ee value of 5.2% of the
d-enantiomer compared to the non-irradiated sample. Leucine enantiomers were extracted with H2O
and derivatized to form (ethoxycarbonyl)leucine ethyl ester (ECEE) prior to GC analyses.
sample C, we recorded a photolysis rate of 99.875 (þ0.125–0.5)%. The ee value
however, proved difficult to determine, since the accuracy and precision of the applied
enantioselective GC/MS method is dependent upon the amount of sample reaching the
detector, considering that, after irradiation, the remaining amino acid film is even
thinner than a monolayer. As expected, the standard deviation increases with
decreasing sample quantity. Even after tweaking various GC/MS instrument param-
eters such as threshold values, solvent trapping, and others in order to determine the ee
value of sample C with high precision, the standard deviation of 5% in the ee
determination of this sample remained higher than the obtained ee. In summary and
after a multitude of enantioselective analyses, we cannot establish that an opposite
significant ee was induced into sample C. For future analyses, an enantioselective
multidimensional GCꢁGC system will be applied providing higher sensitivity and
improved detection limits. Here, the combination of a Chirasil-l-Val phase in the first
dimension with a Carbowax phase in the second dimension will allow both the high-
resolution chromatographic separation of enantiomers in the first dimension and the
separation of matrix, such as solvent and column bleeding, from the amino acid
enantiomers in the second dimension. For this application, the GCꢁGC technique
provides higher signal-to-noise ratios and thus more precise ee measurements.

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CHEMISTRY & BIODIVERSITY – Vol. 7 (2010)
Promising results obtained for the amino acid leucine in terms of high photolysis
rates allow us to assume that asymmetric interstellar photochemical processes might
have initiated the origin of molecular asymmetry manifested in living organisms. The
obtained results let us envisage future experiments that will include the amino acid
isovaline which is dialkylated at C(a). Isovaline has been identified in various
meteorites with high ee values of the l-enantiomers [6], but also in many fungal
peptides [26–28]. Isovaline is predicted to be more inert against potential photo-
racemisation reactions. For upcoming experiments, the amino acid filmsꢀ irradiation
chamber will be installed closer to the focal point of the DESIRS beamline so that a
higher photon/molecule ratio is reached in order to shorten irradiation times (ideally
10–15 h per sample).
Cometary mission Rosetta has been designed to deposit the Lander Philae on comet
67P/Churyumov-Gerasimenko in 2014 aiming to enantioselectively analyze organic
molecules including amino acids in cometary matter in situ [2][15][29–34]. A comet
nucleus sample return mission was also proposed recently [35]. Expected results on the
occurrence of amino acids including an eventual ee value will further elicit whether or
not interstellar photochemistry might have induced an asymmetry in biomolecules
which was possibly required for the origin of life on Earth.
We thank Dr. Stephanie Blanchandin for providing the chemistry laboratory facility at synchrotron
SOLEIL, and Jean-FranÅois Gil for the construction of the UHV irradiation chamber used at
synchrotron SOLEIL. This work was supported by the Agence Nationale de la Recherche ANR-07-
BLAN-0293.
Experimental Part
The amino acid sublimation chamber is equipped with an amino acid sublimation source, a quartz
microbalance and a holder for the MgF2 substrates onto which the amino acids are re-condensed (Fig. 4).
Two types of MgF2 substrates can be fitted. Either a disc with a diameter of 22 mm, which can be used for
synchrotron radiation CD measurements, or a 5ꢁ8 mm rectangular substrate, which can be used for
photolysis experiments. The rectangular substrate is fitted with a Teflon and a Cu mask allowing for the
deposition of a 2.0ꢁ5.4-mm film. In the sublimation source, the amino acids are filled into a cylindrical
19-mm3 quartz reservoir which fits inside a metal cylinder. The metal cylinder is resistively heated with a
ThermoCoax heater (ThermoCoax S.A.S., France) and the temp. is measured with a K-type
thermocouple. The sublimation source is mounted on a 1’’ retractable bellows, allowing the source-
substrate distance to be controlled. An integrated shutter allows opening and interrupting the amino acid
flow precisely. The rate of amino acid re-condensation (as an area density change rate) can be monitored
by a 5-MHz quartz crystal microbalance (QCM) using an Inficon-Maxtek 5 silver-coated quartz crystal,
mounted on a 2’’ retractable bellows. The QCM can be inserted between the source and the MgF2
substrate before and after sublimation to measure the mass flow rate. Typical deposition rates of the
amorphous amino acid films are 10–20 nm minꢀ1, and the error in the leucine film-thickness
determination is 5 nm. The UHV vacuum chamber is pumped by a Varian Turbo-V 81M turbo pump,
backed by a BOC Edwards XDS 10 scroll pump. The pressure is monitored by a Balzer Compact Full
Range gauge.
VUV CP Synchrotron radiation was produced by the electromagnetic polarizing undulator
OPHELIE2 (HU640 type) [36] installed on the French storage ring SOLEIL, feeding the DESIRS
beamline [37], whose monochromator was set to zeroth order in order to transmit the full spectrum of the
fundamental radiation on the undulator. Such a spectrum has a 7% relative bandwidth around a central
photon energy which can be tuned over the whole VUV range (5–40 eV). Absolute circular polarization
rates, carefully measured and calibrated with a dedicated VUV polarimeter [38], vary from 90 to 100%

CHEMISTRY & BIODIVERSITY – Vol. 7 (2010)
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Fig. 4. Temperature- and pressure-controlled ultra high vacuum (UHV) system for sublimating and re-
condensing amino acids designed and constructed at the synchrotron facility ISA, University of Aarhus,
Denmark. 1: UHV chamber, 2: MgF2 crystal holder, 3: QCM, 4: manipulator, 5: temperature-controlled
reservoir for amino acids, 6: UHV pump, 7: QCM support, and 8: reservoir support.
over the 6 to 35 eV range. Note that high harmonics of the undulator were cut-off by a gas filter [39] filled
with Xe. Detailed enantioselective GC/MS conditions were reported in [4][22][25].
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