Determinate Mechanisms

Determinate mechanisms create predictable non-random enantiomeric enhancements. They involve the interaction of either internal or external physical driving forces - cogencies - with racemic or prochiral substances that are causing prevalence of one enantiomer.

Examples of such deterministic scenarios include the adsorption of organic molecules on the enantiomorph surface of quartz (Karagunis and Coumoulos 1938; Soai et al. 1999), or calcite (Hazen et al. 2001) as we will discuss later. Here, the resulting enantioenrichment in the organic molecules is determined by the crystal's asymmetry in a way that chiral information is transferred from the crystal's asymmetric surface towards initially racemic organic molecules via host-guest interactions. The asymmetric crystal's host surface serves as an external driving force determining the chirality of the organic guest system.

Nowadays, a directed vortex in a liquid is also to be considered an external driving force since this vortex was somehow surprisingly proven to induce an asymmetry into an arrangement of organic molecules. It was indeed the direction of stirring in terms of left or right revolution that determined the left or right helicity of a spiral association of compounds in a predictable manner (Ribo et al. 2001).

Other driving forces that determine the outcome of an enantiomeric excess in a non-random process include directed magnetic fields, the weak force as an internal driving force (Yamagata 1966), spin-polarized electrons (Musigmann et al. 1999), and circularly polarized light (Kuhn and Braun 1929; Kuhn and Knopf 1930) that is capable of inducing asymmetries into racemic organic molecules under particular interstellar conditions. Before describing this "extraterrestrial" scenario, I will briefly introduce the most important alternative physical driving forces.

1.3.2.1 Magneto-Optical Effects

Oriented magnetic fields have been suggested as physical driving forces, capable of inducing enantioenrichments into racemic or prochiral organic molecules. Here, the direction of the magnetic field vector applied to a chemical reaction would determine the stereochemical configuration ((+)- or (-)-enantiomers) of the products. In order to figure out such an intriguing possibility of magneto-optical effects, pho-toreactions were studied in strong directed magnetic fields in which the incoming photons disposed of a fixed orientation relative to the magnetic field. To give an example: hexahelicene, a chiral molecule, was produced photochemically from the cis-phenanthrylnaphthyl-ethene precursor within a homogeneous static magnetic field of 1.1 T, resulting in an enantiomeric excess of 0.07% hexahelicene. Interestingly, the configuration of the photoproduct hexahelicene was determined by the direction of the magnetic field vector. Liquid crystals were often used as hosts for such asymmetric photoreactions (Stobel 1985; Teutsch and Thiemann 1986; Teutsch 1988; Thiemann and Teutsch 1990). For decades, enantiomeric excesses caused only by magneto-optical effects had been small (<1%) and often hard to detect.

In one spectacular case, however, a tremendous enantiomeric excess of 98% was reported. Ph.D. student Guido Zadel performed an enantioselective reaction in a static magnetic field at the University of Bonn. Chiral arylethanols were synthesized starting from prochiral ketones. The resulting enantiomeric excess (20-98%) of arylethanols was published to be a linear function of the applied magnetic field ranging from 0.2 to 1.2 T (Zadel et al. 1994). Other laboratories that immediately investigated this spectacular finding were not able to reproduce Zadel's results (Feringa et al. 1994). Finally, the manuscript had to be withdrawn due to intentional "manipulation" of the data (Golitz 1994).

A significant enantioselective magnetochiral effect was observed by Geert Rikken and Ernst Raupach (2000) that caused the introduction of enantiomeric excess into a prior racemic mixture. A photoreaction was used showing an enantioselective capacity that was dependent on the direction of the magnetic field vector. The handedness of the enantiomeric excess achieved in these experiments was induced by the relative orientation of the non-circularly polarized light towards the magnetic field. May such a magnetochiral anisotropy have been decisive for life's selection of its enantiomerically pure molecular building blocks? The answer will be given in Chap. 6, where we will see that the Earth's magnetic field is, referring to the authors' conclusion, too weak and periodically changing over geological time periods its direction, to have a suitable effect. In order to explain the origin of biomolecu-lar dissymmetry Rikken and Raupach considered extraterrestrial magnetic fields for introducing an enantiomeric excess into organic racemates.

1.3.2.2 Biochirogenesis Based on the Weak Force

Originating in theoretical physics and elementary particle physics, a particularly intriguing explanation for a deterministic origin of biomolecular asymmetry continues to receive international attention: inside atomic nuclei, a tiny intrinsic asymmetry has been identified. This inner-nuclear asymmetry was proposed to influence chiral molecules such as amino acids from their own innermost interior serving as an internal deterministic driving force having favoured today's biomolecular homochirality of living organisms.

This explanation finds its origin in 1956, when Tsung Dao Lee and Chen Ning Yang discovered the violation of the parity law; they found out that the weak force is - among the four fundamental forces - an asymmetric one. The Nobel Prize in Physics honoured this discovery in 1957, as we will see in Chap. 5 referring to the herewith-triggered worldwide discussions among Wolfgang Pauli, Richard P. Feynman, Freeman J. Dyson, and others. The described inherent asymmetry of atomic nuclei was later assumed to cause "parity non-conserving energy differences" between enantiomers (Yamagata 1966), which themselves were proposed by a cluster of researchers mainly originating from Oxford University and London King's College to explain the optical asymmetry of the biosphere (Mason 1984; Tranter 1985b; MacDermott and Tranter 1989).

The first experimental evidence for the above hypothesis was claimed by Otto Merwitz (1976) and Bengt Norden et al. (1985) investigating a radiolysis favouring the selective degradation of D-amino acids compared to their L-enantiomers. These studies used spin-polarized ("chiral") electrons emitted as P-radiation for the interaction with racemic amino acids. We will see later how P-radiation is indeed influenced by the weak nuclear force. In addition to this, carefully measured differences in the chiroptical properties of octahedral cobalt and iridium enantiomers were observed by Andrea Szabo-Nagy and Lajos Keszthelyi (1999). Also, these authors considered the recorded differences between enantiomers as being caused by parity non-conserving energy differences. These and other experimental trials to ultimately evaluate parity non-conserving energy differences will be exposed in Chap. 5.

As a consequence of this attractive and far-reaching theory based on the asymmetry of the weak nuclear force it has consequently been acknowledged that even atoms are principally optically active and that L- and D-molecules are truly diastereoiso-mers instead of enantiomers in the strict sense of the term (MacDermott 1993). This is a consequence that would indeed require considerable rewriting and reediting of the majority of stereochemistry textbooks.

Nevertheless, some authors question this theory because there is little experimental evidence for parity non-conserving energy differences (Bonner 1995a). This is not surprising in view of the extremely small magnitude of these calculated energy differences (in the order of 10"14 eV), that is considered probably too small to break the racemic state of the environment (Keszthelyi 1984; Goldanskii and Kuz'min 1988).

1.3.2.3 Photochirogenesis6

In contrast to the majority of nuclear physicists, photochemists may favour involving "chiral photons" in processes explaining the origin of biomolecular asymmetry. From the experiments of Werner Kuhn and E. Braun (1929) and W. Kuhn and E. Knopf (1930), it is a well known photochemical fact that the interaction of circularly polarized ultraviolet light with chiral but racemic organic molecules is able to introduce an enantiomeric excess into these racemates by disrupting the better absorbing enantiomer, resulting in an excess of the other. Among the above mentioned determinate mechanisms, particular processes involving circularly polarized light as the external chiral force have proved capable of producing significant and reproducible enantiomeric excesses from racemic or prochiral precursors. Nevertheless, these enantioselective photoreactions suffer in principle from two disadvantages in explaining satisfactorily the origin of biomolecular homochirality (Bonner 1995a). On the Earth's surface the circularly polarized light is extremely weak in a) intensity and b) handedness in the ultraviolet and visible region of the spectrum. Due to the discovery of both circularly polarized light in interstellar regions and prochiral

6In this context, the term "chirogenesis" is used to describe the induction of an enantiomeric enrichment into a physico-chemical system composed of prochiral or chiral molecules. An enantiomeric enrichment is generated; the system was composed of chiral (but racemic) molecules before, even if we talk about "chirogenesis".

and chiral organic molecules in meteorites, interstellar ices, and simulated comets, a renewed interest in extraterrestrial scenarios for a circularly polarized light-induced origin of molecular parity violation on Earth has recently arisen (Griesbeck and Meierhenrich 2002).

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