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Anisotropy (g) values are given at room temperature and indicated wavelength X

Anisotropy factors for selected amino acids are given in Table 6.1 (see Rau 2004 and references therein). Among the proteinaceous amino acids, it is leucine that shows a relatively high anisotropy value of g = 0.0244 (Flores et al. 1977) and has therefore often been selected for enantioselective photolysis experiments.

For the last 25 years, the highest enantiomeric excess achieved for enantioselective photolysis of an amino acid has remained 2.5% in the case of leucine. Very recently, the challenging purely photochemical induction of higher enantiomeric enrichment of amino acids was intensively revisited with the help of innovative ideas, concepts, and instrumentations using four main approaches.

1. pH-dependence. The Yoshihisa Inoue ERATO research team at Osaka University studied the pH-dependence of the chiroptical properties, particularly the anisotropy g (and therefore the ability to induce a high e.e.), of the amino acid leucine. Solutions with different acidity were subjected to right- and left-circularly polarized photons and the resulting enantiomeric excess was determined subsequently. It was shown that enantioselective photodecomposition depends strongly on the degree of protonation of the amino acid's carboxylic group, such that protonated carboxylic groups at pH values close to 1 generate a higher anisotropy. Based on this effect, which was promised also to be studied for other amino acids, a Norrish Type II mechanism was proposed, including y-H abstraction and Norrish type II cleavage of the leucine molecule. The obtained enantiomeric excess was, however, only close to 0.2% (Nishino et al. 2001).

2. Two-photon excitation. In order to increase the anisotropy g, two-photon excitation processes were studied. Femtosecond circularly polarized laser pulses were applied in a specific norbornadiene-quadricyclane system to make use of an additional anisotropy g* derived from the excitation of an electronically excited state. For the chosen system, however, the induction of enantiomeric enrichments during irradiation involving two-photon excitations turned out to be very similar to that obtained by one-photon excitation. Nevertheless, the principal approach is innovative and might provide advanced capabilities.

3. Elliptically polarized light. The potential of right- and left-elliptically polarized light in asymmetric photolytic reactions was systematically studied by William A. Bonner at Stanford University subjecting acidic solutions of leucine to irradiation. It was shown that a decomposition up to 93% justified asymmetries close to 3% enantiomeric excess. As theory predicts, irradiation with "clean"

right-circularly polarized light or left-circularly polarized light in parallel experiments ended with 4% enantiomeric excess and thus a higher degree (Bonner and Bean 2000).

4. Irradiation in the solid state. Studying the interaction of racemic organic molecules with left- and right-circularly polarized light in aqueous solution represents the conventional attempt at enantioselective photolysis. Solitary (n*, n)-electronic transitions of the amino acids' carboxylic groups with values close to 212 nm (5.85 eV) were obtained here, because water absorbs wavelengths below 200 nm making higher energetic electronic transitions (at lower wavelengths) inaccessible. Taking this into account, leucine molecules in their solid state were exposed by our research team to circularly polarized synchrotron radiation (Nahon et al. 1997) of variable polarization (Alcaraz et al. 1999; Nahon and Alcaraz 2004) and energy obtained from a newly developed electromagnetic planar/helical crossed undulator. Using this concept, additional high-energy electronic transitions such as (n*, n1)-, (n*, n2)-, and even (a*, o)-transitions of amino acids (Fig. 6.3) could be excited below 200 nm providing more effective optical anisotropies. Following this approach, we took the energy-dependence of the anisotropy g into consideration, which will now be outlined in more detail.

To establish the most suitable conditions for enantioselective photodecomposi-tion, the absorption spectrum of leucine with its electronic transitions was studied between 120 and 250 nm in the vacuum ultraviolet photon range.

In order to determine the direction of a possible photon-induced enantiomeric excess, chiroptical investigations of amino acids were taken into account, also in the

Fig. 6.3 Vacuum ultraviolet absorption spectrum and electronic transitions of the amino acid leucine recorded in a solid sample film with synchrotron radiation at beamline SA-61 in the Laboratoire pour l'Utilisation du Rayonnement Electromagnetique LURE, Paris. The (n*,n)-transition of leucine was observed at 211 nm, its (n*,^1)-transition at 183 nm, the tentative (n*,^2)-transition at 142 nm, and its potential (o*,o)-transition at 121 nm. Enantioselective photodecomposition of leucine can be considered to be sufficiently active precisely at each of the determined electronic transition energies

Fig. 6.3 Vacuum ultraviolet absorption spectrum and electronic transitions of the amino acid leucine recorded in a solid sample film with synchrotron radiation at beamline SA-61 in the Laboratoire pour l'Utilisation du Rayonnement Electromagnetique LURE, Paris. The (n*,n)-transition of leucine was observed at 211 nm, its (n*,^1)-transition at 183 nm, the tentative (n*,^2)-transition at 142 nm, and its potential (o*,o)-transition at 121 nm. Enantioselective photodecomposition of leucine can be considered to be sufficiently active precisely at each of the determined electronic transition energies vacuum ultraviolet range. Extreme care was required to obtain artefact-free, solidstate circular dichroism spectra. This is because circular dichroism spectra in the solid state can inevitably be accompanied by parasitic signals that originate from macroscopic anisotropies of a sample, such as linear dichroism (LD) and linear bifringence (LB) (Kuroda 2004).

Circular dichroism spectra of D-leucine were recorded in the vacuum ultraviolet region of the spectrum by the team of S0ren Hoffmann at the University of Ârhus, using the specifically dedicated synchrotron beamline UV1-ASTRID. The obtained circular dichroism spectrum is given in Fig. 6.4. It shows a negative band at 207 nm for its (n*,n)-electronic transition and, surprisingly, an intense positive band at 191 nm corresponding to the (n*,n1)-transition. The circular dichroism spectrum in the solid, here microcrystalline, state is noticeably different from previously recorded circular dichroism spectra in solution. The intense band at 191 nm, allowing for relatively high enantiomeric excesses by irradiation at this wavelength with circularly polarized light, attracted our particular interest.

Based on the signs of the obtained CD data one can deduce that the (n*,n)-electronic transition of D-leucine is induced preferentially by right-circularly po-

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125 135 145 155 165 175 185 195 205 215 225 235 245 Wavelength [nm]

Fig. 6.4 Vacuum ultraviolet circular dichroism spectrum of the amino acid d-leucine, recorded in a solid sample film with synchrotron radiation at the Synchrotron Facility ISA, University of Arhus (Denmark). Film thickness 1 |m (bold line), film thickness 0.2, 0.4, 0.6, and 0.8 |m (thin lines). d-leucine micro-crystals were immobilized on UV-transparent MgF2 windows (Meierhenrich et al. 2005b)

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125 135 145 155 165 175 185 195 205 215 225 235 245 Wavelength [nm]

Fig. 6.4 Vacuum ultraviolet circular dichroism spectrum of the amino acid d-leucine, recorded in a solid sample film with synchrotron radiation at the Synchrotron Facility ISA, University of Arhus (Denmark). Film thickness 1 |m (bold line), film thickness 0.2, 0.4, 0.6, and 0.8 |m (thin lines). d-leucine micro-crystals were immobilized on UV-transparent MgF2 windows (Meierhenrich et al. 2005b)

larized light and the positive (n*,ni)-electronic transition of D-leucine is preferentially excited by left-circularly polarized light. Therefore, irradiation of D,L-leucine at the 207 nm (n*,n)-electronic transition with left-circular polarized light is assumed to result in an enantiomeric excess of D-leucine. At the 191 nm (n*,n1)-transition, left-circular polarized light should lead to an enantiomeric excess of L-leucine, and so on. Using right-circular polarized light, inverse enantioenrich-ments are foreseen to be obtained (Meierhenrich et al. 2001a).

With these experiments performed in the solid state, we made the asymmetric 191 nm electronic transition accessible for enantioselective photolysis reactions. Moreover, solid-state conditions were chosen, because we assumed that amino acids that are present in interstellar environments (see upcoming Chaps. 7-9) will typically occur there under low-temperature, low-pressure, and low-gravity conditions in the solid state and not primarily in the liquid state.

Following these chiroptical measurements, the assumption was born that the origin of life's molecular asymmetry is due to irradiation of racemic solid state amino acids in space. In certain interstellar environments, an originally symmetric mixture of amino acids might have become asymmetric by irradiation with 'chiral photons'. In order to find further arguments for this hypothesis, solid state racemic leucine was exposed in the laboratory to individually left- or right-circularly polarized synchrotron radiation, representing astrophysical conditions. The breakthrough of these experiments came indeed with the idea of irradiating an amino acid in its solid state. Previous experiments were restricted to the liquid state.

How can one obtain the required circularly polarized light in the vacuum UV at precisely defined energies? We used the polarizing undulator2 called Ophelie (Nahon et al. 1997) installed in the Super-ACO storage ring at LURE as a source of circularly polarized light showing admirable circular polarization rates above 90%. These rates are given in Fig. 6.5.

Irradiation of solid state D,L-leucine was performed within the SU-5 beamline (Fig. 6.6, Nahon et al. 2001) with the energy of the circular polarized synchrotron radiation first set to 182 nm, closely matching the desired 191 nm (n*,n1)-transition, and second to 170 nm, so that information about the energy dependence of the process could be gained.

After 3 h of irradiation resulting in photodecomposition of approx. 70% of the starting material, chemical analysis of the remaining leucine residue was performed by enantioselective gas chromatography. The application of a new analytical technique enabled our group to identify the selective degradation of left-handed leucine exposed to right-circularly polarized radiation. Due to different molecular absorption coefficients of the two leucine enantiomers, the highest obtained enantiomeric excesses in the residues was +2.6% in D-leucine (Meierhenrich et al. 2005b). The sign of the induced e.e. was dependent on the direction of circular polarization as illustrated in Fig. 6.7.

2 An undulator consists of a periodic arrangement of dipole magnets creating an alternating static magnetic field. Electrons that traverse this alternating field are forced to undergo oscillations and emit electromagnetic radiation called synchrotron radiation.

Fig. 6.5 Circular polarization rates of the applied synchrotron radiation at 182 nm. The absolute circular polarization for left-circularly polarized synchrotron radiation (l-CPSR) is 94% (top) and 91% for right-circularly synchrotron radiation (r-CPSR), respectively (bottom). Thin lines illustrate the perfect circular polarization case (see Nahon and Alcaraz 2004)

Fig. 6.5 Circular polarization rates of the applied synchrotron radiation at 182 nm. The absolute circular polarization for left-circularly polarized synchrotron radiation (l-CPSR) is 94% (top) and 91% for right-circularly synchrotron radiation (r-CPSR), respectively (bottom). Thin lines illustrate the perfect circular polarization case (see Nahon and Alcaraz 2004)

Fig. 6.6 Synchrotron beamline SU-5, part of the Laboratoire pour l'Utilisation du Rayonnement Electromagnetique LURE, Centre Universitaire Paris-Sud, France, was constructed to produce and analyse circularly polarized radiation (Nahon and Alcaraz 2004). Here, samples of amino acids were irradiated allowing the induction of a significant enantiomeric excess in the solid state. Top: High vacuum chamber for irradiation of amino acids in the solid state. Top left: Towards photodiode detector. Top center: Sample holder for 7 MgF2 windows on the surface of which amino acids are deposited which can be moved individually into the synchrotron light. Top right: Towards synchrotron source. Bottom left: Location of amino acid samples on top of turbo molecular pump. Bottom center: Polarimeter, capable of determining 4 Stokes parameters. Bottom right: Photodiodes. Today, this polarimeter is installed on the new VUV beamline DESIRS at SOLEIL (Gif sur Yvette, France) for which a new variable polarization undulator has been designed

Fig. 6.6 Synchrotron beamline SU-5, part of the Laboratoire pour l'Utilisation du Rayonnement Electromagnetique LURE, Centre Universitaire Paris-Sud, France, was constructed to produce and analyse circularly polarized radiation (Nahon and Alcaraz 2004). Here, samples of amino acids were irradiated allowing the induction of a significant enantiomeric excess in the solid state. Top: High vacuum chamber for irradiation of amino acids in the solid state. Top left: Towards photodiode detector. Top center: Sample holder for 7 MgF2 windows on the surface of which amino acids are deposited which can be moved individually into the synchrotron light. Top right: Towards synchrotron source. Bottom left: Location of amino acid samples on top of turbo molecular pump. Bottom center: Polarimeter, capable of determining 4 Stokes parameters. Bottom right: Photodiodes. Today, this polarimeter is installed on the new VUV beamline DESIRS at SOLEIL (Gif sur Yvette, France) for which a new variable polarization undulator has been designed

Fig. 6.7 Tipping the biomolecular balance: d,l-Leucine subjected to irradiation with left-circularly polarized light at 182 nm resulted in an enantiomeric excess of l-leucine, while irradiation with right-circularly polarized light gave an excess of d-leucine

Fig. 6.7 Tipping the biomolecular balance: d,l-Leucine subjected to irradiation with left-circularly polarized light at 182 nm resulted in an enantiomeric excess of l-leucine, while irradiation with right-circularly polarized light gave an excess of d-leucine

In a sample with equal amounts of D-and L-leucine, right-circularly polarized light destroyed slightly more of the L-leucine, while left- circularly polarized light destroyed slightly more of the D-form. Irradiation of solid-state D,L-leucine with circularly polarized light at a higher energy, X =170 nm, did induce a minor value for the enantiomeric excess, which does not change the sign by switching photon helicity. This observation is coherent with the fact that the circular dichroism signal of leucine (see Fig. 6.4) at 170 nm is vanishing.

The detection of enantiomeric excesses in samples of leucine, induced by vacuum ultraviolet asymmetric photochemical reactions such as enantioselective photolysis under realistic interstellar/circumstellar conditions, was demonstrated to be feasible. These experimental data support the assumption that ice mantles of tiny interstel-lar/circumstellar dust grains can experience asymmetric reactions when irradiated by circularly polarized ultraviolet light. In space, this type of irradiation may have led to small but significant local enrichment of L-amino acids, as measured in meteorites (see Chap. 8). This excess could have been enough to lead to autocatalytic processes for the formation of life based on L-amino acids. The results have far-reaching consequences on the understanding of the origin of life on Earth and its evolution, suggesting that the biomolecular asymmetry of amino acids was induced in interstellar space, long before the origin and biological evolution of life on Earth took place. Afterwards, these asymmetric amino acids might have been delivered via (micro-) meteorites, interplanetary dust particles, and/or comets to Earth, where they triggered the appearance of life.

Nevertheless, a more systematic study of asymmetric vacuum ultraviolet photolysis reactions in the solid state for different amino acids would help us to better understand the processes leading to their asymmetric formation and their role in prebiotic chemistry. Using this concept, the variation of the photon's energy enables the determination of the anisotropy factor (g) as a function of the wavelength, and from this value the conditions required to obtain enantioenrichments can in turn be determined. Worldwide, there are only two polarimeter-connected light-sources for circularly polarized synchrotron radiation available. One is based in Tsukuba, Japan, the other in the synchrotron in the South of Paris. We will continue to use this unique technique in the new generation synchrotron SOLEIL, which just finished construction in Gif sur Yvette, France.

Enantioselective photolysis reactions, however, suffer in principle from the logical but inconvenient condition that in order to produce a significant enantiomeric excess which can then be amplified by appropriate mechanisms, high amounts of the amino acids have to be photodecomposed, i.e., destroyed, as determined by the anisotropy factor g(g = Ae/e). The higher the anisotropy factor g of the chiral molecule (g is function of the wavelength and the state of aggregation) and the higher the extent of reaction the higher the enantiomeric excess that can be induced in the system. This relationship is illustrated in Fig. 6.8 (see Balavoine et al. 1974).

As g-values for amino acids are generally small (cf. Table 6.1), high enantiomeric enrichments are only reached at a large where most of the starting material has disappeared. So Fig. 6.8, in some ways, is "deceptive" (Rau 2004).

Enantioselective sensitized photochemical reactions, in which an optically active sensitizer transfers energy to a prochiral or racemic substrate through non-covalent interactions in the excited state (Inoue 2004), might justify higher anisotropies and circumvent this problem. Similar to catalytic and enzymatic asymmetric syntheses, the photosensitization necessitates only a tiny amount in catalytic quantities of the optically active sensitizer. This is what distinguishes this particular strategy from other asymmetric photochemical reactions, which employ circularly polarized light, chiral complexing reagents, chiral auxiliaries, chiral supramolecular hosts, and chiral crystal lattices, since the chiral interaction occurs exclusively in the excited state (Inoue 2004). Suitable optically active sensitizers for enantiodifferenti-ating photoreactions involving amino acids of prebiotic relevance, however, have not yet been identified. Topical research activities focus on this kind of challenging photoreaction.

Another common criticism of the above scenario should be mentioned: the relevance of models for the asymmetric photolysis of amino acids in the origin of biomolecular asymmetry were recently questioned because the non-aliphatic amino acids tryptophan and proline show opposite Cotton effects in circular dichroism spectroscopy compared to aliphatic amino acids and thus would be generated as

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