Chiral Asymmetry in the Photoionization of Gas-Phase Amino-Acid

Letter pubs.acs.org/JPCL

Chiral Asymmetry in the Photoionization of Gas-Phase Amino-Acid Alanine at Lyman‑α Radiation Wavelength Maurice Tia,† Barbara Cunha de Miranda,† Steven Daly,† François Gaie-Levrel,†,§ Gustavo A. Garcia,† Ivan Powis,‡ and Laurent Nahon*,† †

Synchrotron SOLEIL, l’Orme des Merisiers, Saint Aubin BP 48, 91192 Gif sur Yvette Cedex, France School of Chemistry, University of Nottingham, Nottingham NG7 2RD, United Kingdom

‡

ABSTRACT: Gas-phase pure enantiomers of alanine, the simplest proteinaceous chiral amino acid, are investigated by photoelectron circular dichroism, a direct chiroptical, orbital-sensitive effect giving rise to large asymmetries in the photoelectron angular distribution upon photoionization by circularly polarized light. Here we report electron imaging measurements made at the Lyman-α radiation photon energy (10.2 eV) that reveal a strong overall asymmetry for the outermost orbital. Despite the anticipated presence of different conformers, this asymmetry is effectively independent of sample temperature (and hence of conformer population). Furthermore, because of the associated recoiling of the corresponding ion, photoionization by circularly polarized light can generate an asymmetric flux of gas-phase alanine cations, allowing us to deduce an enantiomeric excess, in a given line of sight, of up to 4%. In addition to the implications for the origin of biomolecular asymmetry, these studies pave the way for future chiroptical analytical studies of more complex biomolecules such as peptides. SECTION: Spectroscopy, Photochemistry, and Excited States with the Pj being Legendre polynomials, while θ is the direction of the emitted electron and p is the polarization of the ionizing radiation: p = 0 for linear, p = +1 for left circular polarization (LCP), and p = −1 for right circular polarization (RCP). For CPL, p = ±1, θ is measured from the photon propagation axis. Because b1 is nonzero only for chiral systems photoionized with CPL, it encapsulates the chiral contribution, and because of the odd, cosine form of the first Legendre polynomial term it describes a forward−backward asymmetry in the electron emission direction. Very importantly, the b1{±1} coefficients are antisymmetric to the swapping of either the light helicity or the enantiomer, while the b2 coefficients do not vary with such exchanges. Therefore, for a given enantiomer, the PECD is defined as the difference between the angular distributions obtained with LCP and RCP radiation, such that PECD = 2b1{+1} cos(θ), corresponding to a maximum asymmetry of 2b1{+1} in the forward−backward direction. Analytic potentialities of PECD have recently been enhanced by the demonstration of table-top laser-induced multiphoton PECD,10 in addition to the synchrotron radiation (SR)-based one-photon scheme used so far. Because of the high PECD sensitivity to the molecular potential, measurements of b1, the so-called dichroic parameter, can be used to probe molecular geometry and structures,11 including conformers.12−14 There is therefore a considerable interest in applying such a technique to the study of biomolecules, especially amino acids, which are

W

ithin a bottom/up approach of biomolecular complexity, the gas-phase study of the elementary subunits of key macrobiomolecules allows one to probe, in solvent-free1 or controlled microsolvated2 environments, electronic and structural characteristics that will be involved in the formation of larger molecular structures. This is the case of amino acids, the building blocks of proteins, which are also chiral and may provide a clear dichroic signature when probed with a chiroptical technique such as electronic circular dichroism (CD) in absorption.3 Because CD is an electric-dipoleforbidden effect, giving rise to very small relative signals on the order of 10−4, direct CD data on gas-phase amino acids are scarce and very challenging to obtain.4 In this context, there is a deep interest in examining the chiroptical properties of pure gas-phase enantiomers of amino acids with a more sensitive and electronic orbital-specific chiral probe, namely, photoelectron circular dichroism (PECD), observed as a remarkable forward−backward asymmetry (with respect to the photon axis) of the photoelectron angular distribution produced by the circularly polarized light (CPL)ionization of pure enantiomers of randomly oriented chiral molecules. First evidenced a decade ago,5 the electric-dipole-allowed PECD asymmetry6 may reach several tens of %,7−9 that is, orders of magnitude above classical CD. Quite generally, for randomly oriented molecules the photoelectron angular distribution function is written as Ip(θ ) = 1 +

b1{p}P1(cos

Received: July 9, 2013 Accepted: July 29, 2013 Published: July 29, 2013

{p}

θ ) + b2 P2(cos θ ) © 2013 American Chemical Society

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dx.doi.org/10.1021/jz4014129 | J. Phys. Chem. Lett. 2013, 4, 2698−2704

The Journal of Physical Chemistry Letters

Letter

known to possess several conformers. This we achieve for the first time in this communication. The origin of biomolecular asymmetry, or homochirality of lifethe fact that, for instance, amino acids found in proteins are all of the L-typeremains an unsolved and puzzling mystery,15 which probably traces back to the origin of life itself. This symmetry breaking has been the subject of many postulated scenarios.16 Amino acids were discovered in carbonaceous meteorites with enantiomeric excesses (ee) and isotopic composition, indicating an extraterrestrial origin. They were also synthesized in the laboratory by far-UV irradiation of interstellar/circumstellar medium (ISM/CSM) ice analogs.17 Were there to be an interstellar or circumstellar origin of elementary building blocks of life, one should look for an asymmetric bias applied before this organic matter was delivered on our planet, inducing a significant ee. Such a bias could be induced by CPL, for which astronomical sources have been reported.18 Along this line, several asymmetric photochemical processes were proposed and simulated in condensed matter19 by using UV CPL, leading to significant ee, in the few % range. In this context, one could wonder if PECD could be an alternative photophysical asymmetric process, acting on gas-phase amino acids, which could lead to noticeable ee. One of the challenges in studying gas-phase biomolecules is that these species are, in general, thermolabile, that is, very difficult to generate intact in the gas phase. We used two different, complementary methods of vaporization: resistive heating in an oven, followed by a supersonic molecular beam expansion, and aerosol thermodesorption (TD). As compared with the oven-resistive heating approach, the interest in the TD method is that the vaporization is “soft”, leading to intact neutral parent molecules without any spurious decomposition products. It has the drawback of producing a lower target density, at least in the case of amino acids, and it leads to a rather hot internal temperature of the produced molecules. Also, the achieved electron energy and ion mass resolutions are lowered in this mode of operation because of the presence of the TD hot tip in the interaction region. We used here the aerosol-TD method mainly as a benchmark for the mass spectrum analysis of the oven-produced alanine, allowing for identification of any spurious thermal decomposition peaks, but also to study the PECD dependence on varying Boltzmann conformer populations associated with the different characteristic temperatures of the two sources. With both methods, the gaseous neutral alanine molecules are photoionized by vacuum ultraviolet (VUV) CPL from a variable polarization SR beamline, and the corresponding electron and ion images, detected in coincidence, are obtained via a double-imaging photoelectron−photoion coincidence (PEPICO) spectrometer, as represented schematically in Figure 1a. For a given enantiomer, photoelectron images are recorded for any chosen peak in the mass spectrum by alternating RCP and LCP light. The (LCP-RCP) difference image yields the b1 parameters, while the (LCP+RCP) sum image provides the corresponding photoelectron spectrum (PES). The adiabatic ionization energy of alanine lies at 8.82 eV,20 and the cross section for the production of the parent molecules (m/z 89) rises from this onset to reach a plateau around 18 eV21 and then decreases. Lyman-α radiation is dominant in the VUV spectrum of ISM/CSM environments, and so we restrict consideration in this work to ionization at this wavelength. The corresponding photon energy (10.2 eV) is

Figure 1. (a) Overall experimental scheme and time-of-flight spectra. GF stands for gas filter and PSD stands for position sensitive detectors. (b) Time-of-flight (TOF) spectra of alanine photoionized at 10.2 eV, with neutral species produced by aerosol thermodesorption (black line) or resistive heating associated with an adiabatic expansion (red line). Inserts correspond to ion images showing the kinetic energy release for a few ion masses.

only sufficient to eject photoelectrons from the highest occupied molecular orbital (HOMO) of neutral alanine.22 Figure 1b shows the time-of-flight (TOF) spectra of alanine recorded at 10.2 eV. With the cooled molecular beam inlet (red curve), the parent ion (m/z 89) abundance amounts to 0.9 ± 0.2% of the total ion yield. (See Table 1.) In the aerosol-TD spectrum, the parent peak at m/z 89, barely visible because of the poor mass resolution, amounts to 0.7 ± 0.4% of the total alanine signal. This is slightly below the abundance of ∼1% derived at 10 eV from the Pan et al. data20 using a laserdesorption method. This is probably due to the fact that in our aerosol-TD method the internal temperature of the molecule in the released plume can reach a maximum of 373 K, probably higher than that obtained by laser-desorption, leading to an enhanced unimolecular fragmentation of the parent ion in a hot ground state and to a lowering of the fragmentation threshold, as already observed on tryptophan.23,24 From these data, we estimate that in typical ISM/CSM environment with temperatures ranging from 10 to 200 K in hot cores19 the abundance of alanine molecules surviving ionization at 10.2 eV as parent ion is in the 0.5 to 2% range. Both TOF spectra are dominated by the fragment m/z 44, corresponding to the loss of COOH, the main fragmentation channel, as already reported.20 Note that in the TOF obtained when alanine is produced by aerosol-TD (black curve) the peak at m/z 28 is a contribution of N2, used as the nebulizing gas here, ionized by residual, unfiltered second-order SR light, while the peak at m/z 30 corresponds to the first-order light ionization of NO, present as an impurity in the N2 gas cylinder. The TOF spectrum obtained by resistive heating (red curve in Figure 1b) in the molecular beam sample reservoir, shows additional peaks (m/z 17, 59, 73). These ions are as cold as the parent molecule, as inferred from their TOF peak width and 2699



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Table 1. Global Analysis on the 10.2 eV CPL Ionization/Fragmentation of Oven-Produced Alanine ion (m/z) 89

description parent

abundance (%)a

translational temperature (K)

0.9

85

dichroic parameter b1 −0.019 ± 0.003 +0.021 ± 0.005 L-alanine: −0.0288 ± 0.0006 D-alanine: +0.0290 ± 0.0020 0.001 ± 0.001 0.00 ± 0.01 L-alanine:

D-alanine:

a

44

fragment

73 17

spurious spurious

96.1b

256 99 100

Of total alanine-related signal, as measured from the time-of-flight spectrum. bRemaining 3% corresponds to the alanine fragment m/z 45.

89), as selected via the PECD−PICO scheme. A clear PECDinduced forward−backward asymmetry with respect to the light axis is visible, with a sign reversing upon switching the enantiomer. The analysis of these images leads to the PES and b1 distribution over the HOMO electronic band, as visible in Figure 2b. The PES is in perfect agreement with previously reported spectra, with a vertical IE around 9.7 eV.22,25 The b1 curves of the two enantiomers show an excellent mirroring, as theoretically expected. The magnitude of the b1 parameters increases slowly across the PES band profile. This variation with ionization energy may be associated with (unresolved) changes in the vibrational state of the cation26 but is most probably simply attributable to the changing photoionization dynamics as the momentum or energy of the emitted electron reduces to higher ionization energy. The PES-weighted average (over the whole HOMO band) is found to have an absolute b1 value of 0.029. Note that this value is the same as the one obtained by filtering only on the m/z 44 ion (Table 1), which is not surprising considering that this species by far dominates the TOF spectrum. A similar measurement performed with the aerosol-TD method, producing neutral molecules with higher internal energy (300−373 K) shows again a clear forward−backward asymmetry visible on the raw difference electron image (see Figure 3a), corresponding to an average b1 magnitude of also ∼0.03, as can be seen in Figure 3b. At the chosen photon energy of 10.2 eV, the PECD of alanine therefore appears to be insensitive to the temperatures in the range considered here, that is, to the alanine conformer population. In Figure 4, we present similar data to that of Figure 2b but filtered exclusively on the parent mass (m/z 89). The corresponding PES now spans only the electron energy range 8.7 to 9.5 eV, showing that the alanine parent ion is stable only in the low binding energy part of the HOMO−1 photoionization channel. The dichroic parameter b1 again exhibits the expected mirroring between the two enantiomers but now, due to sampling only the lower ionization energy (nonfragmenting) regions of the HOMO band, the absolute value averaged over the corresponding PES band is 0.020 (Table 1). We note that in other, colder environments, a reduced internal energy content of the alanine neutral may render its cation stable against fragmentation to higher ionization energies. The mean magnitude of the b1 parameter could then approach the overall HOMO band average of 0.03. Even so, the value found here for the parent-only dichroic parameter, b1 = 0.02, corresponds to a remarkable 4% chiral asymmetry, which is orders of magnitude more intense than any classical CD signal. Alanine possesses several conformers, with the three lowest energy conformers lying within ∼20 meV of each other.27,28 Therefore, even at the 85 K temperature of our molecular beam a Boltzmann population should include contributions from all three, while the two higher energy conformers should be even

the coincident ion images; translational temperatures extracted from the ion images are on the order of 85−100 K (see Table 1), due to the adiabatic cooling. This indicates that these peaks correspond to spurious contributions due to thermal decomposition of alanine in the oven prior to the supersonic expansion and do not come from a dissociative ionization process of the nascent neutral alanine, which would otherwise give rise to hotter translational distributions, as, for instance, in the case of the m/z 44 fragment. In addition, the analysis of electron images filtered on these different spurious fragment ions shows their corresponding b1 parameters to be vanishing, which is not surprising because the thermally induced neutral fragments from which they derive are themselves nonchiral species. All of the contributions that are not related to the ionization of neutral alanine have been filtered out in the PECD/PES data treatment presented below by means of the electron/ion coincidences in the so-called PECD−PICO scheme. In Figure 2a we show for the oven vaporization the raw difference images obtained at Lyman-α radiation energy for electrons correlated to all alanine-related ions (m/z 44, 45, and

Figure 2. PECD and PES measurement recorded at 10.2 eV on ovenproduced D- and L-alanine. (a) Raw difference images, filtered on all alanine-related ions, showing a PECD-induced forward−backward asymmetry with respect to the light-propagation axis (depicted by the arrow). (b) Corresponding PES and b1 dichroic parameter distribution over the HOMO band. The blue (respectively, red) data correspond to the D- (respectively, L-) enantiomer. 2700



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more prominent in the TD experiments. Previous work12,14,29,30 has shown that different molecular conformers may have very different dichroic parameters, especially for slow electrons31 as here, perhaps even leading to some mutual cancellation. Nevertheless, despite the expected presence of several conformers potentially having different b1 values, the measured PECD is relatively intense in the present experiments, and the lack of any obvious temperature dependence (see Figures 2 and 3) seems to imply that the varying conformer populations do not display a self-canceling PECD. This apparent lack of a distinct temperature/conformer population dependence is then somewhat surprising, given the marked conformer dependence found in previous PECD investigations.12,14,29,30 It is likely due here either simply to a chance compensation between the different conformer contributions to the PECD signal at this single photon energy under the two temperature regimes sampled or to the fact that at this energy the three conformers exhibit quite similar PECD in terms of magnitude and most of all sign, leading to a weak or even vanishing temperature dependence. This last reason seems to be the dominant one as it can be inferred from previously published conformer-resolved PECD theoretical analysis.28 Indeed, for the kinetic energy corresponding to 10.2 eV, the calculated HOMO b1 values are −0.04, 0.00, and −0.03 for, respectively, conformers 1 (of lowest energy), 2, and 3. Therefore, in the perfect cooling limit, where the conformer population relaxes completely to the lowest energy structure, the effective b1 calculated value would be −0.04, while in the hot temperature limit, assuming a 1:1:1 population, the conformer-average b1 values would reach −0.024. A full conformer analysis study should be carried out in the future via systematic orbital-dependent PECD measurements recorded over a large photon energy range and a comparison with refined theoretical calculations, which is beyond the scope of the present paper. Nevertheless, at 10.2 eV photon energy, the average measured HOMO b1 value of −0.03 for the Lenantiomer happens to be in excellent agreement in magnitude and most of all in sign with already published theoretical predictions for alanine PECD,28 showing that through

Figure 3. PECD and PES measurement recorded at 10.2 eV on TDproduced D-alanine. (a) Raw difference image, filtered for all alaninerelated ions, showing a PECD-induced forward−backward asymmetry with respect to the light propagation axis (depicted by the arrow). (b) Corresponding PES and b1 dichroic parameter distribution over the HOMO band.

Figure 4. Parent-filtered (m/z 89) PECD and PES measurement recorded at 10.2 eV on oven-produced D-(blue curve) and L-alanine (red curve). The average b1 value of ∼0.02 corresponds to a 4% asymmetry. 2701



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depend on the temperature, as discussed above, an important point considering the large range of temperatures encountered in the ISM/CSM. The above scenario could be strengthened in the future by systematic studies on other amino acids to check the magnitude and sign of the dichroic parameter at the Lyman-α wavelength. Nevertheless, on the basis of the present data, we can infer that PECD-induced asymmetries lead to the creation of a nonracemic flux of amino acids. These could have been seeded on Earth via comets and meteorites and might therefore, in combination with other phenomena, have assisted the emergence of life while also triggering the appearance of biomolecular asymmetry.

comparison with calculations, the absolute configuration of gasphase biomolecules can be ascertained. In a broader analytical context, these findings suggest that even “floppy” biological systems may have a clear PECD signature, making this chiroptical probe a potentially sensitive tool for their study in the gas phase. We have also demonstrated here that the PECD−PICO scheme can provide a multiplexed chiroptical characterization of a mixture of compounds (some of which may be nonchiral). Indeed, owing to the recording of electrons coincident with mass-selected ions, one can selectively determine (by appropriate data filtering) the PECD of a given species in the mass spectrum and therefore its chiroptical properties such as the absolute configuration. As an illustration of this capability, it is clear that we could have discarded m/z 73 as not being a fragment resulting from dissociative ionization of the neutral alanine, not only on the basis of its cold translational temperature but also by the fact that its dichroic parameter is zero. (See Table 1.) Furthermore, this study paves the way to extended chiroptical studies on other amino acids and peptides, either bare or microsolvated. In particular, the formation of a peptidic bond should possess a PECD signature because PECD has been shown to be sensitive to clustering.32 Regarding a possible implication of PECD in the origin of biomolecular asymmetry, let us now consider that alanine is present as a racemate in the ISM/CSM. Gas-phase molecules are likely produced by evaporation in hot cores,33 by photon- or energetic beam impact-induced desorption of molecules synthesized at icy grain surfaces, or by ion-neutral reactions in the surroundings of grain molecular mantles.34 Note that so far the only claimed amino-acid detection in the ISM is that of glycine,34 the simplest and nonchiral proteic amino acid, and this observation remains controversial. Nevertheless, larger gasphase amino acids are probably also present but may be more difficult to detect. Let us further consider that this racemic mixture of gas-phase alanine is embedded into a partially CPL field with a given and constant helicity over a large region of space, as in the Orion Nebulae, similar to the one from which the Sun was formed, as discussed in ref 35. Then, the fact that b1 is antisymmetric with the swapping of enantiomers means that the PECD-induced electron asymmetry at Ly-α radiation will be opposite for the two enantiomers. Because of momentum conservation, the corresponding recoiling ions will also have opposite asymmetries. This process therefore leads to an asymmetry in the flux of nonfragmented alanine ions in a given line of sight, which may reach a maximum value of 2b1, that is, 4% in the limit of pure CPL (and possibly more for a vibrationless population of neutral parent). In other words, along the photon axis direction, the distribution of recoiling ions present an asymmetry of 4%, which corresponds to a net ee of the same value. This enantioenriched gas-phase alanine ion cloud of a given handedness, separating from its counterpart, may then be captured, neutralized, and embedded into comets and meteorites seeding Earth with an exogenous organic matter presenting an initial ee. Because of the low internal energy content on the produced parent ion, racemization or enantiomer interconversion processes are very unlikely. The ion recoil speed distribution is quite peaked in absolute value because the Lyman-α ionization leading to the parent ion production gives rise to a rather narrow electron distribution (see PES of Figure 4) with a kinetic energy Ek ≈ 1.1 ± 0.2 eV. We also note that at the Lyman-α wavelength the induced enantiomeric excess does not

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EXPERIMENTAL METHODS For both vaporization methods, commercially available samples of D- and L-alanine (Aldrich, 99% purity) were used. For the oven-based resistive heating experiments, the sample was placed in the oven reservoir, nested into layers of fiberglass to limit thermal decomposition. Heating at ∼450 K produced a vapor that was mixed with 0.5 bar of He before being expanded through a 50 μm nozzle. The supersonic expansion was then skimmed to form a molecular beam that crossed the photon beam at a right angle, provided by the variable polarization undulator-based VUV beamline DESIRS36 at Synchrotron SOLEIL (St. Aubin, France). (See also http://www. synchrotron-soleil.fr/Recherche/LignesLumiere/DESIRS.) For the aerosol-TD method, a fragmentation-free density of neutral alanine molecules in the photoionization source region was produced by TD of the corresponding homochiral aerosol, according to a method that was recently developed on DESIRS.24 In brief, an atomizer was used to produce nanoparticles by nebulizing a 0.5 g/L solution of D-alanine in water. Downstream, the resulting aerosol was dried and then transmitted in the “jet chamber” through an aerodynamic lens system. The aerosols then crossed the 0.7 mm diameter skimmer and impinged onto the thermodesorber a 3 mm diameter hot tip made of porous tungsten heated at a temperature of 373 K. The resulted plume of intact gas-phase alanine was ionized by the VUV SR, as in the molecular beam configuration, that is, in the interaction region of the doubleimaging PEPICO spectrometer Delicious III.37,38 This spectrometer accelerates the produced ions and electrons in opposite directions, perpendicular to the molecular and photon beams. It couples a modified velocity map imaging (VMI) electron analyzer to an ion-imaging/TOF mass spectrometer and can be operated in electron-ion coincidence mode, used here to select different ion masses corresponding to the ionization of the nascent neutral alanine and allowing the rejection of any background or spurious electron signal due to impurities or decomposition products. The apparatus is capable of 5% (10% with the thermodesorber tip inserted) electron energy resolving power on the edge of the detector. The ionimaging capabilities allowed the ion kinetic energy release (KER) to be measured, from which the corresponding Boltzmann temperature of mass-selected ions is subsequently extracted. With both vaporization methods, PECD at hν = 10.2 eV was measured by recording several mass-filtered photoelectron images for alternate light helicities (PECD−PICO) and then subtracting them to obtain a difference image, which was later treated using the pBasex inversion algorithm39 to recreate the original angular distribution of the difference. The full 2702



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(11) Nahon, L.; Powis, I. In Chiral Recognition in the Gas Phase; Zehnacker, A., Ed.; CRC Press - Taylor & Francis: Boca Raton, FL, 2010; pp 1−26. (12) Garcia, G.; Nahon, L.; Harding, C. J.; Powis, I. Chiral Signature in Angle-Resolved Valence Photoelectron Spectroscopy of Pure Glycidol Enantiomers. Phys. Chem. Chem. Phys. 2008, 10, 1628−1639. (13) Harding, C. J.; Mikajlo, E. A.; Powis, I.; Barth, S.; Joshi, S.; Ulrich, V.; Hergenhahn, U. Circular Dichroism in the Angle-Resolved C 1s Photoemission Spectroscopy of Gas-Phase Carvone Enantiomers. J. Chem. Phys. 2005, 123, 234310. (14) Turchini, S.; Catone, D.; Contini, G.; Zema, N.; Irrera, S.; Stener, M.; Di Tommaso, D.; Decleva, P.; Prosperi, T. Conformational Effects in Photoelectron Circular Dichroism of Alaninol. ChemPhysChem 2009, 10, 1839−1846. (15) Meierhenrich, U. Amino Acids and the Asymmetry of Life; Springer: Berlin, 2008. (16) Evans, A. C.; Meinert, C.; Giri, C.; Goesmann, F.; Meierhenrich, U. J. Chirality, Photochemistry and the Detection of Amino Acids in Interstellar Ice Analogues and Comets. Chem. Soc. Rev. 2012, 41, 5447−5458. (17) Munoz Caro, G. M.; Meierhenrich, U. J.; Schutte, W. A.; Barbier, B.; Arcones Segovia, A.; Rosenbauer, H.; Thiemann, W. H. P.; Brack, A.; Greenberg, J. M. Amino Acids from Ultraviolet Irradiation of Interstellar Ice Analogues. Nature 2002, 416, 403−406. (18) Bailey, J.; Chrysostomou, A.; Hough, J. H.; Gledhill, T. M.; McCall, A.; Clark, S.; Menard, F.; Tamura, M. Circular Polarization in Star-Formation Regions: Implications for Biomolecular Homochirality. Science 1998, 281, 672−674. (19) Meinert, C.; de Marcellus, P.; d’Hendecourt, L. L.; Nahon, L.; Jones, N. C.; Hoffmann, S. V.; Bredehoft, J. H.; Meierhenrich, U. J. Photochirogenesis: Photochemical Models on the Absolute Asymmetric Formation of Amino Acids in Interstellar Space. Phys. Life Rev. 2011, 8, 307−330. (20) Pan, Y.; Zhang, L. D.; Zhang, T. C.; Guo, H. J.; Hong, X.; Sheng, L. S.; Qi, F. Intramolecular Hydrogen Transfer in the Ionization Process of α-Alanine. Phys. Chem. Chem. Phys. 2009, 11, 1189−1195. (21) Jochims, H. W.; Schwell, M.; Chotin, J. L.; Clemino, M.; Dulieu, F.; Baumgartel, H.; Leach, S. Photoion Mass Spectrometry of Five Amino Acids in the 6−22 eV Photon Energy Range. Chem. Phys. 2004, 298, 279−297. (22) Farrokhpour, H.; Fathi, F.; De Brito, A. N. Theoretical and Experimental Study of Valence Photoelectron Spectrum of D,LAlanine Amino Acid. J. Phys. Chem. A 2012, 116, 7004−7015. (23) Wilson, K. R.; Jimenez-Cruz, M.; Nicolas, C.; Belau, L.; Leone, S. R.; Ahmed, M. Thermal Vaporization of Biological Nanoparticles: Fragment-Free Vacuum Ultraviolet Photoionization Mass Spectra of Tryptophan, Phenylalanine-Glycine-Glycine, and β-Carotene. J. Phys. Chem. A 2006, 110, 2106−2113. (24) Gaie-Levrel, F.; Garcia, G.; Schwell, M.; Nahon, L. VUV StateSelected Photoionization of Thermally-Desorbed Biomolecules by Coupling an Aerosol Source to an Imaging Photoelectron/Photoion Coincidence Spectrometer: Case of the Amino-Acids Tryptophan and Phenylalanine. Phys. Chem. Chem. Phys. 2011, 13, 7024−7036. (25) Powis, I.; Rennie, E. E.; Hergenhahn, U.; Kugeler, O.; BussySocrate, R. Investigation of the Gas-Phase Amino Acid Alanine by Synchrotron Radiation Photoelectron Spectroscopy. J. Phys. Chem. A 2003, 107, 25−34. (26) Garcia, G.; Nahon, L.; Daly, S.; Powis, I. Vibrationally Induced Inversion of Photoelectron Forward-Backward Asymmetry in Chiral Molecule Photoionization by Circularly Polarized Light. Nat. Commun. 2013, 4, 2132. (27) Jaeger, H. M.; Schaefer, H. F.; Demaison, J.; Csaszar, A. G.; Allen, W. D. Lowest-Lying Conformers of Alanine: Pushing Theory to Ascertain Precise Energetics and Semiexperimental R-e Structures. J. Chem. Theory Comput. 2010, 6, 3066−3078. (28) Powis, I. Photoelectron Spectroscopy and Circular Dichroism in Chiral Biomolecules: L-Alanine. J. Phys. Chem. A 2000, 104, 878−882.

procedure, helicity and b1 sign convention as well as precautions to minimize purely instrumental effects have been previously described.40 The DESIRS beamline delivered left- and right-handed circular polarization states with corresponding absolute circular polarization rates above 97%, as accurately measured by a dedicated VUV polarimeter.41 A gas filter was filled with 0.25 mbar of argon to eliminate higher order radiation (undulator harmonics) that could be transmitted by the grating. We selected a low-dispersion (200 grooves/mm), high-flux grating with monochromator slits settings corresponding to a typical photon bandwidth of 12 meV at 10.2 eV.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address §

François Gaie-Levrel: Chemistry and Biology Division, Laboratoire National d’Essai, France. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS F.G.-L. acknowledges the support from RTRA-Triangle de la Physique. We are grateful to J.-F. Gil for technical assistance and to the SOLEIL staff for running the facility and providing beamtime under project 2011705.

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REFERENCES

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Chiral Asymmetry in the Photoionization of Gas-Phase Amino-Acid

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