VUV Photodynamics and Chiral Asymmetry in the Photoionization of

Article pubs.acs.org/JPCA

VUV Photodynamics and Chiral Asymmetry in the Photoionization of Gas Phase Alanine Enantiomers Maurice Tia,† Barbara Cunha de Miranda,† Steven Daly,† François Gaie-Levrel,†,§ Gustavo A. Garcia,† Laurent Nahon,*,† and Ivan Powis‡ †

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, U.K.

‡

ABSTRACT: The valence shell photoionization of the simplest proteinaceous chiral amino acid, alanine, is investigated over the vacuum ultraviolet region from its ionization threshold up to 18 eV. Tunable and variable polarization synchrotron radiation was coupled to a double imaging photoelectron/photoion coincidence (i2PEPICO) spectrometer to produce mass-selected threshold photoelectron spectra and derive the state-selected fragmentation channels. The photoelectron circular dichroism (PECD), an orbital-sensitive, conformer-dependent chiroptical effect, was also recorded at various photon energies and compared to continuum multiple scattering calculations. Two complementary vaporization methodsaerosol thermodesorption and a resistively heated sample oven coupled to an adiabatic expansionwere applied to promote pure enantiomers of alanine into the gas phase, yielding neutral alanine with different internal energy distributions. A comparison of the photoelectron spectroscopy, fragmentation, and dichroism measured for each of the vaporization methods was rationalized in terms of internal energy and conformer populations and supported by theoretical calculations. The analytical potential of the so-called PECD-PICO detection techniquewhere the electron spectroscopy and circular dichroism can be obtained as a function of mass and ion translational energyis underlined and applied to characterize the origin of the various species found in the experimental mass spectra. Finally, the PECD findings are discussed within an astrochemical context, and possible implications regarding the origin of biomolecular asymmetry are identified.

1. INTRODUCTION The gas phase offers a solvent-free and substrate-free environment in which molecules can be studied in detail with an optimized interplay between experiment and theory. In particular, because intermolecular and solvent interactions can be neglected, only intramolecular interactions such as noncovalent bonds, which are responsible for conformations, have to be taken into account. In addition, dilute matter may be probed by photons over a wide “transparent” spectral range including the vacuum ultra-violet (VUV), in a regime where a single molecule interacts with a single photon. In the case of biomolecules, the study of elementary building blocks of life such as amino acids or DNA basisin the gas phase takes place in the so-called bottom/up approach of the study of large biopolymers. In the past decade, this approach has provided invaluable information in terms of electronic and molecular structures as well as photodynamics.1,2 Along this line of investigation, the electronic structure of gas phase alaninethe simplest chiral proteic amino acid and the focus of this articlehas been the subject of a large number of both experimental and theoretical works,3−5 stressing the importance of taking into account several low-lying structural conformers6,7 of this floppy molecule, even in jet-cooled conditions.8−10 In parallel, alanine UV/VUV gas phase ion photochemistry has been also studied11−14 and discussed within a prebiotic context.11,15 The photostability of this amino acid © 2014 American Chemical Society

versus fragmentation is an important issue in terms of survival during the journey from the interstellar medium, where amino acids have probably been formed,16 toward earth. Alanine UV/VUV electronic circular dichroism (CD) absorption spectra have been recorded in solution17,18 and as thin films,19−22 and they are discussed within the context of protein structural analysis23 and asymmetric photochemistry.24,25 Chiroptical properties such as these26 have been much less studied in the gas phase than in the condensed matter, because the weakness of the relative signal (in the 10−3 to 10−5 range) makes measurement very challenging in a dilute medium.27 Hence, no gas phase alanine CD spectra are available, to our knowledge, except for a single published study providing simply an upper limit determination of 0.1% over the VUV range.28 Nevertheless, gas phase data on chiral properties of base constituents of biopolymers, for example, may be crucial for the understanding of supra-molecular chirality in biopolymer secondary structures such as the protein α−helix.29 In practice, the chirality of gas phase molecules has sometimes been investigated instead by chiral recognition with other chiral species.30,31 Received: February 14, 2014 Revised: March 18, 2014 Published: March 24, 2014 2765

dx.doi.org/10.1021/jp5016142 | J. Phys. Chem. A 2014, 118, 2765−2779

The Journal of Physical Chemistry A

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Norman procedure52 adjusted by empirical reduction factors of 0.82 and 0.83 (depending on the conformers) was used for determining the overlapping atomic sphere radii. Spherical harmonic basis functions, truncated in each region at a value lmax (see Table 1), are then used in the description of electron

During the past decade, a new direct chiroptical effect, photoelectron circular dichroism (PECD), has been widely studied both theoretically and experimentally on randomly oriented gas phase chiral systems. This electric-dipole-allowed, orbital-specific effect32 is observed by a differential measurement as an intense forward/backward asymmetry (with respect to the photon propagation axis) in the photoelectron angular distribution emitted upon photoionization of pure enantiomers with circularly polarized light (CPL). Such an intense effect, with corresponding asymmetries of up to a few tens of percent,33 strongly depends on the photon energy34,35 and appears to be very sensitive to dynamical36 and static molecular structures,37 including conformers38−43 or clusters.33,44,45 Quantitatively, the PECD asymmetry equals 2b1, where b1, the so-called dichroic parameter, is the coefficient of the odd first Legendre polynomial P1 term in the normalized angular distribution function, IP (θ) = 1 + b1{p}P1(cosθ) + b2{p}P2(cosθ), with θ being the angle between the direction of the emitted electron and the photon propagation axis and p 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)). As symmetry considerations dictate that b1{±1} are nonzero only for chiral systems photoionized with CPL, they encapsulate the chiral contribution. In addition, the b1{±1} parameters are antisymmetric with either the swapping of the enantiomers or the flipping of the light helicity, making PECD a direct chiroptical probe. So far, the majority of the published PECD investigations have not targeted prebiotic molecules, although the modified biomolecule alaninol has been studied.39 Nevertheless, the very first computational PECD study examined alanine,46and we very recently published a first experimental paper on the PECD of alanine measured at the single photon energy of 10.2 eV,47 corresponding to the astrochemically relevant Lyman α radiation. One of the purposes of this last paper was to show how PECD may account, probably with other processes, for the origin of life’s homochirality and the fact that, for instance, only L-amino acids are found in the biosphere. Here, we present a broad study over a large VUV range (9− 18 eV) of the photodynamics and chiral asymmetries in the photoionization of gas phase alanine. After the description of the computational and experimental methods, which include two complementary vaporization techniques, we will show the first threshold photoelectron photoion (TPEPICO) spectrum of alanine, connecting the fragmentation pattern to the electronic structure. Then a complete set of computed and measured PECD data will be presented and discussed in terms of structural analysis (i.e., conformer distribution) as well as within an astrochemical context.

Table 1. Spherical Harmonic Basis Set for CMS Xα Calculations in the Initial State and in the Final State lmax values (initial state) atomic region

outer sphere

conformer 1 conformer 2 conformer 3

7 6 6 lmax

C, N, O 3 4 4 values (final state)

H 1 2 2

atomic region

outer sphere

C, N, O

H

conformer 1 conformer 2 conformer 3

20 13 13

12 9 9

4 6 6

wave functions for each sphere, and a self-consistent potential was found after a number of iterations using the standard bound-state MS-Xα technique.53 The obtained potential was then adapted to take the correct asymptotic Coulombic form for ion plus electron by a Latter tail procedure.54 The continuum electron wave functions were calculated using these corrected potentials but with a larger spherical harmonic basis set (see Table 1). Finally, the dipole matrix elements are calculated in a frozen core approximation, neglecting relaxation effects. 2.2. Experimental Methods. Alanine, like other amino acids, is a thermolabile molecule, and therefore, its vaporization is a challenging task; a simple oven-based resistive heating (RH) method may lead to a severe thermal decomposition and polymerization. In this work a second, complementary method to produce intact neutral alanine in the gas phase, aerosol thermodersorption (TD), has been adopted in order to unambiguously characterize its VUV photoionization and photochemistry. Commercially available samples of D- and Lalanine (Aldrich, 99% purity) were used for both methods. Resistive Heating (RH) Method. Alanine powder was placed into a metal reservoir between two layers of glass wool in order to avoid a contact between the metal and alanine which could increase thermal decomposition. The reservoir was then placed inside a high temperature oven, which is composed of eight heating elements (eight at the bottom and eight at the top), heated at 463 K at the upper part near the nozzle and 448 K at the lower part in order to have a constant gradient of temperature to prevent clogging of the nozzle. The resulting vapor was mixed with 0.5 bar He before being expanded supersonically through a 50 μm nozzle and subsequently skimmed to form a molecular beam. The molecular beam then enters the interaction region of the double imaging photoelectron/photoion coincidence (i2PEPICO) spectrometer DELICIOUS 3,55,56where it crosses the photon beam at a right angle. The photon beam is provided by the variable polarization undulator-based VUV beamline DESIRS57 at Synchrotron SOLEIL (St Aubin, France). Aerosol Thermodesorption (TD) Method. Intact gas phase neutral alanine pure enantiomers were obtained at the photoionization region by thermodesorption of the corresponding homochiral aerosol according to a method that was

2. METHODS 2.1. Calculations. Although the expression of the dichroic parameter has been known since 1976,48,49 the first quantitative calculations were only performed a little more than a decade ago.46,50 Since then, many theoretical works have followed, usually accompanying the experimental results. Here we apply the continuum multiple scattering (CMS-Xα)51 method, which will be briefly described. First, a ground state model potential is generated by partitioning the molecule into overlapping spherical regions on atomic centers, interstitial regions and an extramolecular outer sphere region. To generate the model potential for the alanine conformers, the atomic coordinates already obtained by Jaeger et al.7 and by Powis46 were used. A 2766



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Figure 1. Time-of-flight spectra of alanine photoionized at 9.4 (top, blue), 12 (middle, red), and 14 eV(bottom, green), respectively, with neutral species produced by (a) TD method and by (b) RH method. The square of radius of the ion images shown in (b) is proportional to the translational energy projection into the detector plane. Note that the asymmetric tail in the TOF peaks toward higher m/z arises as a consequence of the inhomogeneous extraction field from ions produced off axis.

recently developed on DESIRS.58 An atomizer (TSI, model 3076) was used to produce alanine nanoparticles by nebulizing a 0.5 g/L solution of enantiopure alanine in water using N2 as nebulizer gas. The resulting aerosol was first dried by diffusion through two silica-gel columns (TSI, model 3062) and then transmitted through an aerodynamic lens system in order to form a narrow nanoparticles beam ( 30 meV) with the cold contribution subtracted. The black solid line corresponds to the PES without the cold contribution subtraction. (b) Photoelectron spectra (black curve) and dichroism (b1), red circles, as a function of the binding energy selected in coincidence with the m/z 86 fragment at 12 eV photon energy.

Figure 9. Calculated b1 values relative to HOMO (top), HOMO-1 (center), and HOMO-2 (bottom) of L-alanine orbitals obtained by a continuum multiple scattering treatment (CMS-Xα) for the three lowest-lying conformer of alanine.

extraction of an experimental b1 relative to only one conformer by estimating the relative Boltzmann population abundances. Such a method was successfully applied for the 3methylcyclopentanone by Turchini et al.40 with two conformers and two temperatures. Concerning theoretical b2 values, some differences are also observed especially for the HOMO depending on the considered conformer, but the trend is always the same for a given orbital, demonstrating one more time the specific sensitivity of the b1 parameter as a conformer probe as compared to b2 (≈β). Although experimental b2 curves can also be obtained from the total photoelectron image, systematic errors such as detector gain inhomogeneities would preclude the precise determination of b2 in the present challenging case of alanine, so that we are not presenting here any experimental b2 values. Previous theoretical PECD calculations have been published by Powis,46 nevertheless a updated calculation has been performed in this work. Specifically, a much larger angular basis for the final state has been adopted, as was subsequently shown to be desirable.41,74 The construction of the model potential has also been refined. Figure 11a presents theoretical CMS-Xα and experimental b1 values associated with the HOMO orbital of L-alanine as a function of the photon energy. Because of the water limit in the aerosol experiment discussed in section 3.1, it was impossible to record experimental dichroic points with enough statistics with the TD vaporization method above 13 eV. Several calculated curves are displayed, according to different conformer populations. These include a Boltzmann conformer distribution at four different temperatures, 373, 350,

310, and 85 K, corresponding to the thermodesorber temperature in the TD method, the alanine internal temperature in the TD method, the alanine internal temperature in the RH method, and the measured translational temperature in the RH method, respectively. At 85 K, conformer 1 is the predominant species, although as mentioned previously, the internal energy and conformer populations will not be in thermal equilibrium due to the fast cooling. We should therefore expect a higher internal energy in the oven experiments, as hinted by the small differences seen between the experimental PECD points recorded with both methods. Furthermore, the agreement with the calculated curves is better when the temperature is increased, adding the contributions from the conformers 2 and 3. Some differences are observed between theory and experiments in Figure 11a, in particular in the threshold region where the theory predicts a negative sign, but the experiment gives a positive sign. This is probably due to the limitations of the CMS-Xα potential model because slow electrons will be expected to display the greatest sensitivity to the molecular potential. The theory also predicts a higher b1 value at 12 eV where a shape resonance is predicted (Figure 5) by the calculations. Shape resonant features are known to be overemphasized by such fixed geometry calculations as conducted here, but the predicted trend seems to be qualitatively followed in the experimental data. However, one can note the shift of the b1 values, especially at 11, 12, and 13 2774



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Figure 11. (a) Calculated b1 values for L-alanine obtained by applying a Boltzmann conformer distribution as a function of the photon energy considering four temperatures for the first band (HOMO orbital): T = 85K (blue curve), T = 310 K (green curve), T = 350 K (purple curve), and T = 373 K (red curve). Experimental points related to the TD experiment are in red and those related to the RH experiment are in blue. (b) Same for the second band (HOMO-1 and HOMO-2 orbitals): T = 85 K (blue curve), T = 310 K (green curve), T = 350 K (purple curve), and T = 373 K (red curve). Experimental points related to the TD experiment are in red and those related to the RH experiment are in blue.

Figure 10. Calculated b2 values relative to HOMO (top), HOMO-1 (center), and HOMO-2 (bottom) orbitals obtained by a continuum multiple scattering treatment (CMS-Xα) for the three lowest-lying conformer of alanine.

temperature T. Again, four temperatures were considered, 373, 350, 310, and 85 K. There is a particularly good agreement between theory and experiment concerning that band which supports our choice of conformer populations. As in the case of the HOMO orbital, the best agreement is found for the 373K curve, pointing to an internal energy for the RH method close to that of the TD method, despite the adiabatic cooling. 3.5. Implication in Astrobiology. The origin of life’s homochirality, a central and still puzzling question in astrobiology,75 traces back probably to the origin of life itself and has been the subject of many scenarios including deterministic ones based upon the consequences of chemical or physical laws.16 In particular, since the discovery of a partially circularly polarized light in massive star forming region76,77 together with the fact that L-enriched amino acids have been found in carbonaceous meteorites,78 a CPL-induced process has appeared as a possible asymmetric bias to which amino acids formed in the interstellar medium would have been exposed during their journey toward earth, leading to significant enantiomeric excesses (ee) that would be later amplified on earth toward homochirality. So far, most of the efforts have been targeted toward asymmetric photochemical processes simulated in the condensed matter25 by using UV CPL from SR, leading to significant ee in the few percent range, such as the asymmetric photolysis on thin films of the racemic amino acid leucine,79 and alanine80 induced by nonvanishing

eV, between the two vaporization experiments leading to different target temperatures, which is well-reproduced by the theory. On the whole, the CMS-Xα curves reproduce the correct trend showing that absolute configuration determination can be attained by PECD especially with moderate kinetic energy electrons (i.e., KE > 4 eV due to the decreasing sensitivity to the molecular potential). Also, in spite of the complication found in a genuine biological floppy systems such as alanine, a plausible conformer population can be obtained. Figure 11b shows the theoretical CMS-Xα and experimental b1 values associated with the second PES band of L-alanine as a function of the photon energy, which illustrates the orbital dependence of the PECD. We have calculated representative b1 values by incorporating both HOMO-1 and HOMO-2 orbital ionizations. From the calculated ionization energies, it is anticipated5 that although all the three lowest-lying conformers of alanine contribute to the low energy side of the second PES band, only conformers 2 and 3 (energetically separated by 2 meV) contribute to the higher energy side of this experimental band. The theoretical curve plotted in Figure 11b was built by weighting the calculated energy-dependent b1 values for the HOMO-1 and HOMO-2 by the calculated partial cross sections shown in Figure 5 at a given photon energy and for a given conformer. The final theoretical b1 curve is then obtained as the Boltzmann average of all three conformers, at a chosen 2775



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absorption anisotropy factors g,22 and asymmetric photochirogenesis of alanine from achiral precursors in interstellar ice analogues.81 Because of the ion recoil motion associated to photoemission, we showed in ref 47 that PECD could be a photophysical asymmetric process leading to a noticeable ee in the case of alanine photoionized at 10.2 eV. Indeed, if one considers that racemic gas phase alanine is present in a region of space embedded into a partially CPL field with a given and constant helicity, then the fact that b1 is antisymmetric with the swapping of enantiomers means that the PECD-induced electron asymmetry at a given photon energy will be opposite for the two enantiomers. Because of momentum conservation, the corresponding recoiling ions will have opposite asymmetries. This process leads therefore to an asymmetric flux of parent alanine ions in a given line of sight, which may reach a maximum value of 2b1, typically 4%, in the case of alanine photoionized at 10.2 eV (parent-filtered HOMO value).47 In other words, along the photon axis direction, the expanding sphere of recoiling ions present an asymmetry of 4%, which is nothing but a net ee of the same value. This enantio-enriched (of a given handedness) gas-phase-ion cloud, separating from its counterpart, would then be captured, neutralized, and embedded into comets and meteorites, thereby seeding earth with an exogenous organic matter presenting an initial ee. Beyond the already published results obtained at the single Lyman-α photon energy, the present extended experimental and theoretical PECD data over a large VUV range bring some interesting additional clues regarding this asymmetric astrophysical scenario. Note that it is based upon the PECD data of the only HOMO−1 channel, because the electronic ground state is the only state correlating, partly, to the intact parent alanine ion (see Figure 2a,b). First, we confirm the clear temperature insensitivity of the PECD asymmetry around 10.2 eV for which the experimental data obtained via both vaporization methods and therefore with different temperatures are very similar. This is very well reproduced by the theoretical modeling, with the two high- and low-temperature average conformer PECD theoretical curves crossing exactly around 10.2 eV (see Figure 11a). This can be rationalized by the fact that, as we can observe in Figure 9, the three main conformers’ individual PECD curves cross for ∼1.1 eV KE (i.e., around 10.2 eV for the part of HOMO band leading the parent alanine ion). This temperature independence around the Lyman-α radiation energy strengthens the astrophysical scenario by not setting any temperature constraint on the precise type of interstellar/ circumstellar medium (ISM/CSM) environment in which the CPL/amino acids interaction may have taken place and for which the temperature may vary from 10s to 300 K or more in hot cores.81 Besides, an important point for this astrophysical scenario is the fact that the b1 parameter keeps a constant sign over the whole studied VUV range, except for the very limited range between the IP and 9.6 eV, for which b1 has an opposite sign as compared to the rest of the spectrum and for which the photoionization cross section and m/z 89 absolute yield are quite small.11 Overall, by integrating over the whole UV/VUV spectrum encountered either in the ISM/CSM or close to the solar system,15 taking into account the ionization cross sections reaching a plateau around 18 eV and then decreasing,11 and the fact that the PECD effect vanishes in general in the 10s eV above the IP, there is no way, at least in the case of alanine, that the asymmetry would cancel out because of the alternating sign

of b1 or because of a possible blurring of the various conformer contributions.

4. CONCLUSIONS Two complementary vaporization methods (TD and RH) as well as their advantages and drawbacks have been presented and benchmarked in the showcase of the amino acid alanine. The aerosol thermodesorption (TD) method enables us to bring fragile alanine biomolecules to the gas phase in a soft and clean manner, whereas the resistive heating (RH) method, despite the fact that thermal decomposition was noticed, provides in this case a larger signal. A comparison of the two methods using the spectroscopic and dichroism studies as diagnostics, points to a small temperature difference in the neutral internal energy distribution of ∼40K. Because we do not expect any particular cooling of the sample with the TD method, this would mean that the adiabatic expansion that follows in the RH method is not very efficient at cooling the vibrational modes, although the translational temperature has been determined at 72K. TPEPICO spectra of alanine were obtained by both vaporization methods, and the state-selected fragmentation curves were discussed, in particular the ionization potential as well as fragment appearance energies have been reported. The double imaging coincidence scheme allows the extraction of ion 3D translational energies for any given ion and offers the possibility of disentangling fragments from impurities. The PECD has been measured at several photon energies between 9.4 and 18 eV, with the largest asymmetry occurring at the Lyman-α photon energy (b1 = 0.03). The experimental values have been compared to CMS-Xα calculations, and a relatively good agreement is found, especially at high photon energies for which we show that the absolute configuration can be determined by comparison with theory. Such a comparison allowed us to confirm the plausible conformer distribution that we considered, especially in the case of the TD method leading to the “hottest” neutral molecules distribution for which all the three conformers contribute to the signal. In addition, due to its comparatively large magnitude, PECD could have played a role in the origin of life’s homochirality, probably with other asymmetric processes. Such a scenario should be strengthened in the future by extending PECD studies to amino acids other than alanine. Finally, the ability to obtain in a multiplex manner the photoelectron spectroscopy and circular dichroism for a given ion signal and translational energy has an enormous analytical potential because, in principle, one could obtain the molecular structure, including absolute configuration, for any given ion in a complex mixture in a matter of minutes at a fixed photon energy, provided that the different structural details (isomers, conformers) can be either retrieved from the literature or calculated. Such a strategy of using PECD-PICO as an analytical tool has been used here successfully to disentangle spurious thermal decomposition signals from genuine dissociative ionization products of alanine.

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(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) Snyder, P. A.; Vipond, P. M.; Johnson, W. C. Circular Dichroism of the Alkyl Amino Acids in the Vacuum Ultraviolet. Biopolymers 1973, 12, 975−992. (18) Nishino, H.; Kosaka, A.; Hembury, G. A.; Aoki, F.; Miyauchi, K.; Shitomi, H.; Onuki, H.; Inoue, Y. Absolute Asymmetric Photoreactions of Aliphatic Amino Acids by Circularly Polarized Synchrotron Radiation: Critically pH-Dependent Photobehavior. J. Am. Chem. Soc. 2002, 124, 11618−11627. (19) Tanaka, M.; Yagi-Watanabe, K.; Kaneko, F.; Nakagawa, K. First Observation of Natural Circular Dichroism Spectra in the Extreme Ultraviolet Region Using a Polarizing Undulator-based Optical System and its Polarization Characteristics. J. Synchrotron Rad. 2009, 16, 455− 462. (20) Tanaka, M.; Yagi-Watanabe, K.; Kaneko, F.; Nakagawa, K. Chiroptical Study of Alpha-Aliphatic Amino Acid Films in the Vacuum Ultraviolet Region. J. Phys. Chem. A 2010, 114, 11928−11932. (21) Meierhenrich, U.; Filippi, J.-J.; Meinert, C.; Bredehöft, J. H.; Takahashi, J.; Nahon, L.; Jones, N.; Hoffmann, S. V. Amino Acid Vacuum Ultraviolet Circular Dichroism Spectra. Angew. Chem., Int. Ed. 2010, 49, 7799−7802. (22) Meinert, C.; Bredehoeft, J. H.; Filippi, J. J.; Baraud, Y.; Nahon, L.; Wien, F.; Jones, N. C.; Hoffmann, S. V.; Meierhenrich, U. J. Anisotropy Spectra of Amino Acids. Angew. Chem., Int. Ed. 2012, 51, 4484−4487. (23) Miles, A. J.; Wallace, B. A. Synchrotron Radiation Circular Dichroism Spectroscopy of Proteins and Applications in Structural and Functional Genomics. Chem. Soc. Rev. 2006, 35, 39−51. (24) Rau, H. Direct Asymmetric Photochemistry with Circularly Polarized Light. In Chiral Photochemistry; Inoue, Y., Ramamurthy, V., Eds.; Marcel Dekker: New York, 2004; pp 1−44. (25) 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. (26) Berova, N.; Nakanishi, K.; Woody, R. Circular Dichroism Principles and Applications, 2nd ed.; Wiley-VCH: New York, 2000. (27) Muller, T.; Wiberg, K. B.; Vaccaro, P. H. Cavity Ring-Down Polarimetry (CRDP): A New Scheme for Probing Circular Birefringence and Circular Dichroism in the Gas Phase. J. Phys. Chem. A 2000, 104, 5959−5968. (28) Prumper, G.; et al. Upper Limits for Stereoselective Photodissociation of Free Amino Acids in the Vacuum Ultraviolet Region and at the C 1s Edge. Phys. Rev. A 2004, 69, 063717. (29) Sudha, R.; Jarrold, M. F. Left-Handed and Ambidextrous Helices in the Gas Phase. J. Phys. Chem. B 2005, 109, 11777−11780. (30) Zehnacker, A.; Suhm, M. A. Chirality Recognition between Neutral Molecules in the Gas Phase. Angew. Chem., Int. Ed. 2008, 47, 6970−6992. (31) Chiral Recognition in the Gas Phase; Zehnacker, A., Ed.; CRC Press−Taylor & Francis: New York, 2010. (32) Powis, I. Photoelectron Circular Dichroism in Chiral Molecules. Adv. Chem. Phys. 2008, 138, 267−329. (33) Daly, S.; Powis, I.; Garcia, G. A.; Soldi-Lose, H.; Nahon, L. Photoionization of Epichlorohydrin Enantiomers and Clusters Studied with Circularly Polarized Vacuum Ultraviolet Radiation. J. Chem. Phys. 2011, 134, 064306. (34) Stranges, S.; Turchini, S.; Alagia, M.; Alberti, G.; Contini, G.; Decleva, P.; Fronzoni, G.; Stener, M.; Zema, N.; Prosperi, T. Valence Photoionization Dynamics in Circular Dichroism of Chiral Free Molecules: The Methyl-oxirane. J. Chem. Phys. 2005, 122, 244303. (35) Nahon, L.; Garcia, G. A.; Harding, C. J.; Mikajlo, E. A.; Powis, I. Determination of Chiral Asymmetries in the Valence Photoionization of Camphor Enantiomers by Photoelectron Imaging using Tunable Circularly Polarized Light. J. Chem. Phys. 2006, 125, 114309.

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Chemistry and Biology Division, Laboratoire National de Métrologie et d’Essais (LNE, 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. We acknowledge use of the EPSRC U.K. National Service for Computational Chemistry Software (NSCCS) at Imperial College London in carrying out this work under award CHEM535.

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