Asymmetric Synthesis

Another strategy to circumvent the problem of excessive photodecomposition during induction of an enantiomeric enhancement by asymmetric photolysis reactions is the spontaneous asymmetric photoformation, i.e., asymmetric synthesis, of amino acid structures, which is particularly interesting under interstellar conditions with circularly polarized light.

In contrast to asymmetric photolysis and asymmetric photoisomerization, the pure synthesis of optically active molecules in non-racemic yields induced by circularly polarized light has remained a difficult task to achieve. The first successful attempts were reported 30 years ago. In these experiments, photocyclization of alkenes in solution, performed in the presence of iodine, led to the formation of polyaromatic hydrocarbon molecules. By irradiation with circularly polarized light, the chiral hexahelicene given in Fig. 6.11 was synthesized with optical yields below 2% (Moradpour et al. 1971; Bernstein et al. 1972).

Fig. 6.11 Chemical structure of hexahelicene. M-hexahelicene is depicted on the left, the P-enantiomer is given on the right. This particular chiral molecule possesses no stereogenic center; it had been synthesized with an enantiomeric enhancement by a photochemical reaction using circularly polarized light at X = 313 nm

Fig. 6.11 Chemical structure of hexahelicene. M-hexahelicene is depicted on the left, the P-enantiomer is given on the right. This particular chiral molecule possesses no stereogenic center; it had been synthesized with an enantiomeric enhancement by a photochemical reaction using circularly polarized light at X = 313 nm

The reaction's dependence on wavelength and the substituents' structure has also been examined, and the mechanism was claimed to involve selective excitation of the enantiomeric conformers (Bernstein et al. 1973). Photoproduction of amino acids has been reported to be possible with the help of initial photon acceptors (Sagan and Khare 1971; Khare and Sagan 1971).

Very recently, two groups demonstrated contemporaneously the spontaneous photoformation of a variety of amino acid structures under interstellar conditions (Bernstein et al. 2002; Munoz Caro et al. 2002). Since the two groups used unpo-larized light for the photoreaction the obtained amino acids were chiral but racemic (see comments in Shock 2002). More recently, analogous experiments were performed with circularly polarized light as an asymmetric driving force in order to directly generate enantioenriched amino acids. These experiments show preliminary successes in a way that chiral amino acids and diamino acids were synthesized with circularly polarized light under simulated interstellar conditions. The obtained results will be presented in Chap. 7.

Now, we will continue asking the question if there are natural sources of circularly polarized electromagnetic radiation and, if so, whether they are sufficient in intensity, energy, and polarization to be promising candidates for inducing biomolecular asymmetry.

6.2.5 'Natural' Sources of Circularly Polarized Light

Circularly polarized electromagnetic radiation, which is required for the photochemical introduction of enantiomeric excesses into racemic mixtures of organic molecules or into prochiral educts, was indeed found to be formed by various 'natural' sources. On Earth's surface, circularly polarized light (both in the ultraviolet and visible region of the spectrum) is extremely weak in intensity and handedness.3 Therefore, possible astronomical sources of circularly polarized light were searched intensively and described recently. Circularly polarized light was identified in galaxies, stars, the interstellar medium, and planets by photo-polarimetric measurements with Earth-based telescopes.

6.2.5.1 Double Reflections on Planets' Surfaces

The circular polarization of scattered sunlight from planets and satellites is a general phenomenon. Observations from Earth-based telescopes have been performed in order to determine circular polarizations of scattered sunlight from Venus, Mercury,

3 Electromagnetic radiation emitted by the non-oriented dipoles of the sun is unpolarized. Sunlight scattered by Earth's atmosphere is linearly polarized and non-asymmetric. Only multiple scattering (Mie scattering) in the atmosphere gives circular polarization. However, the circular component is (a) low (0.1%), (b) in the infrared spectral range, ineffective for chemical reactions, and (c) of different sign in the morning and in the evening. Sunlight is therefore considered insufficient for enantioselective photochemistry.

the Moon, Mars, Jupiter, Uranus, and Neptune. In the 30 years since the fundamental work of James Kemp and colleagues at the University of Hawaii, fractional circular polarizations (S) have been measured in electromagnetic radiation from several planets. The obtained global values (So) were compared with the fractional circular polarization of each planet's northern hemisphere (SN) and southern hemisphere (SS). Data from Venus, the Moon, and Jupiter (Kemp et al. 1971a, 1971b) confirmed approximately that

The absolute amount of fractional circular polarization |S| was determined to be a function of the phase angle (9) between the Sun-planet and Earth-planet directions. In the case of Jupiter, SN and SS pass zero at 9 = 0°. Venus which, like Jupiter, has a dense atmosphere shows a polar scattering effect with the same signs of SN and SS as Jupiter, for the same sign of 9. Kemp and Wolstencroft (1971) proposed a scattering, non-magnetic mechanism for the generation of circular polarization at gaseous surfaces (for magnetic mechanisms cf. Lang and Willson, 1983). This model is consistent with the observed signs of the polar effect on Jupiter and Venus. What Kemp et al. called the elliptical scattering power S/9 (or 5S/§9) was determined to be notably smaller for Venus than for Jupiter, at the same wavelength in the deep red.

The model for the generation of circular (or elliptical) polarization of scattered electromagnetic radiation for planets and satellites with little or no atmosphere and condensed, solid, and rough surfaces (e.g. Moon or Mercury) differs from the gas-layer case described above. The relevant mechanism here is based on double reflections of light from crystal grains exposed on a surface and was developed by

Fig. 6.12 Double refection of non-polarized light on a surface can create circular polarization. After the first reflection, the unpolarized light expressed by the electric field vector Eu becomes partially linearly polarized (electric field vector El). After the second reflection, the light becomes circularly polarized (electric field vector EC). Amount and direction of the circular component are functions of the phase angle 9 between the trajectories of incoming and double reflected light

Fig. 6.12 Double refection of non-polarized light on a surface can create circular polarization. After the first reflection, the unpolarized light expressed by the electric field vector Eu becomes partially linearly polarized (electric field vector El). After the second reflection, the light becomes circularly polarized (electric field vector EC). Amount and direction of the circular component are functions of the phase angle 9 between the trajectories of incoming and double reflected light

Bandermann et al. (1972). As depicted in Fig. 6.12, the first reflection linearly polarizes the ray and the second, from an adjacent grain, elliptically polarizes it by virtue of the material being slightly absorbent. The Bandermann model uses geometrical optics and Fresnel's law. It predicts that the hemispheric values SN and Ss have opposite signs for the gas-layer case model in comparison with Venus and Jupiter for all 9 between 0° and a critical phase 9c. As the magnitude of 9 increases toward 90°, S changes sign. The data for the Moon, at 9 = +90°, are consistent with this model since +90° > 9C.

Thus, data obtained for fractional circular polarizations are understood in detail in sign and amount in the cases of Venus, Jupiter, and the Moon. With Mercury, the situation is less certain. The fractional circular polarization data obtained for Mercury showed an asymmetry in So, which is difficult to interpret. The authors took several potential error effects into account but concluded that this asymmetry is real (see also Meierhenrich et al. 2002b).

Nonetheless, energy and zero-sum of the reflected circularly polarized light from planets was almost certainly insufficient for consideration as a chiral field capable of tipping the biomolecular balance towards the selection of L-amino acids.

6.2.5.2 The Bonner-Rubenstein Neutron Star Hypothesis

The subsequently developed Bonner-Rubenstein hypothesis considers another extraterrestrial mechanism for the molecular parity violation observed in the terrestrial biosphere. According to assumptions of Rubenstein et al. (1983), circularly polarized electromagnetic radiation is emitted from the poles of an extremely rapidly rotating neutron star in a way that, at one pole, right-circularly polarized synchrotron radiation is emitted, whereas the other pole emits left-circularly polarized synchrotron radiation. The neutron star itself formed as the result of a supernova explosion. This circularly polarized light, which was proposed to emit mainly in the ultraviolet and visible wavelengths, might have interacted with interstellar organic matter, the precursor of the solid bodies in the universe such as our planet. By this mechanism, the racemic constituents of the organic mantles of the stellar cloud grains could be asymmetrically photolysed, producing an excess of the corresponding more stable enantiomer in the grain mantles. This model has been further developed by Bonner and Rubenstein (1987), Bonner (1991, 1992 and 1995a), and Bonner et al. (1999a), and has resulted in a topical discussion.

Stephen Mason (1997, 2000) from London King's College argued against this model, considering the 'Kuhn-Condon zero-sum rule' (Kuhn 1930, Condon, 1937), that the integral of the circular dichroism over the whole wavelength range is zero. In case, the neutron star would emanate the required radiation over the entire spectrum and no enantiomeric enhancement could be produced by photochemical interactions with organic molecules. Bonner et al. (1999b) replied that only the circular dichroism band within the characteristic absorption spectra of prebiotic molecules is relevant for the incriminated enantioselective photochemistry by synchrotron radiation.

Despite this discussion, circularly polarized synchrotron radiation seems to be difficult to observe and detect in supernova remnants of pulsars (Bailey et al. 1998; Bailey 2000). Such detection has not yet been performed successfully. Consequently, considerable doubt arose against the hypothesis that synchrotron radiation of neutron stars induced enantiomeric enrichments in racemic or prochiral interstellar molecules (see e.g. Keszthelyi 2001).

6.2.5.3 Light Scattering on Aligned Dust Grains

As an alternative to the ideas of double reflections on planets' surfaces or rapidly rotating neutron stars, circular polarization of ultraviolet photons caused by specific interstellar scattering processes was proposed as a plausible original source of biomolecular handedness. This admissible hypothesis came into mind all of a sudden in 1998, when strong infrared circular polarization was observed by Bailey et al. (1998) in Orion OMC-1 using the 3.9-m Anglo-Australian Telescope including a polarimetry system. The team of Jeremy Bailey measured levels of circular polarization as high as 17%. Two years later, Bailey (2000) reported the theoretical background, namely that the OCM-1 region is known to have grains aligned by a magnetic field. Aligned spheroidal grains can cause circular polarization of incoming unpolarized light by Mie scattering. In general, Mie scattering describes the scattering of electromagnetic radiation by spherical particles, named after its developer, the physicist Gustav Mie (1908). In contrast to Rayleigh scattering4, Mie's solutions to scattering embraces all possible ratios of diameter to wavelength. In 2003, a 23% circular polarization was discovered by Bailey (2003) in NGC 6334V.

Bailey's results were confirmed by observations such as the extended search for circularly polarized infrared radiation in additional areas of the Orion OMC-1 molecular cloud reported by Buschermohle et al. (2005). This team used the United Kingdom Infrared Telescope (UKIRT) at Manua Kea Observatory in Hawaii. At 2.2 |m, circularly polarized light was detected as depicted in Fig. 6.13. Distinct regions of the OMC-1 molecular cloud show positive circular polarization depicted in red colour, whereas in other regions negative circular polarization dominates, shown in blue. Most of the areas show very low circular polarization close to the zero-value given in green colour. In order to provide a suitable explanation for the observed interstellar circular polarization of light, the authors discuss that in the first step, partial plane polarization may result from scattering or from dichroism. Circular polarization is then produced in cases of multiple scattering such as scattering of linearly polarized radiation, scattering of unpolarized radiation by aligned aspherical grains or dichroism involving multiple clouds or so-called twisted magnetic field lines.

We have to take into consideration that one cannot directly observe the circular polarization in ultraviolet wavelengths required for asymmetric photolysis of chiral molecules. Observations of circularly polarized light were only possible in the less energetic infrared region of the spectrum, because of the high dust obscuration in

4 As indicated in Chap. 2, Rayleigh scattering describes the scattering of electromagnetic radiation by particles that are much smaller than the wavelength of the electromagnetic radiation.

RA offset (arcsec)

Fig. 6.13 Seeing red: Map of circular polarization in the Orion OMC-1 molecular cloud recorded in the near infrared at 2.2 |m. North is to the top and east is to the left. The square above center (labelled C) designates the area observed by Chrysostomou et al. (2000). The two observed fields are centered to the southwest (SW) and southeast (SE) of BN. Two smaller rectangular areas that lie entirely in area C are also shown. The observation of circularly polarized light is not feasible in the ultraviolet range of the spectrum, due to high dust extinctions. The illustration was first published by Buschermohle et al. (2005). Reproduced with permission of the authors and the American Astronomical Society (AAS)

RA offset (arcsec)

Fig. 6.13 Seeing red: Map of circular polarization in the Orion OMC-1 molecular cloud recorded in the near infrared at 2.2 |m. North is to the top and east is to the left. The square above center (labelled C) designates the area observed by Chrysostomou et al. (2000). The two observed fields are centered to the southwest (SW) and southeast (SE) of BN. Two smaller rectangular areas that lie entirely in area C are also shown. The observation of circularly polarized light is not feasible in the ultraviolet range of the spectrum, due to high dust extinctions. The illustration was first published by Buschermohle et al. (2005). Reproduced with permission of the authors and the American Astronomical Society (AAS)

Orion's massive star formation regions. However, calculations of the scattering of light from aligned grains show that substantial circular polarization could still be produced at ultraviolet wavelengths. When an ultraviolet flux was present, it was calculated to exhibit substantial levels of circular polarization as well. So it is plausible to assume that there are locations in these massive star-forming complexes where light from an ultraviolet emitting star could scatter off the aligned grains.

The circular polarization in OMC-1 shows regions of both signs but the polarization structure is on a larger scale than the expected size of a protostellar disk -or our solar system. Consequently, if this source were responsible for the selection of life's handed molecules, the same direction of circular polarization would have affected all planets and also comets of our solar system.

With respect to the analysis of eventual enantioenrichments in samples of extraterrestrial origin such as meteorites, comets, or Mars (for details see Chaps. 8 and 9), this means, that Mie scattering models on aligned interstellar dust grains can indeed be assumed to induce an asymmetry. Since the circular polarization was observed on an extremely large scale, such processes would generate the same hand-edness of chiral molecules within our Solar System, including comets.

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