The complete consecutive Vester-Ulbricht process, however, has not yet been realized successfully in the laboratory. It had been searched for intensively with long-time irradiation experiments at various P-ray sources like 104Rh illuminating different enantiomers such as the amino acids alanine, proline, leucine, valine, and others (Goldanskii and Khrapov 1963). In none of these experiments optical activity was induced, nor could differences in the action of the electrons on individual enantiomers be observed.
At Stanford University, William A. Bonner (1974) subjected aqueous solutions and solid samples of amino acids such as d, L-leucine for 1.34 years to P-radiation. Racemic mixtures but also individual enantiomers were irradiated separately. While slight differences were noted in the percent radiolysis of solid D-leucine (12.7%) and solid L-leucine (16.2%), no significant enantioenrichment was found by enantiose-lective gas chromatographic analysis of irradiated D,L-leucine (Bonner and Flores 1975).
It is interesting to know that 14C-labeled racemic amino acids had been produced with high specific activities long before the discovery of the Fall of Parity in the laboratory at the Lawrence Radiation Laboratory at the University of California. In subsequent years these crystalline samples had undergone self-radiolysis due to the pure P-rays from their 14C-decay and the accompanying y-ray Bremsstrahlung. After having been informed on the parity violation in physics and the resulting asymmetry of P-electrons, Melvin Calvin and coworkers examined the residual amino acid samples (then 12 to 24 years old) for possible asymmetric degradation employing ORD measurements. Despite high total radiation dosages and radiolysis of the extent of 3-80% no optical rotation was observable within the sensitivity of the spectropolarimeters of 0.002° (Bonner et al. 1976). Two years later, these crystalline amino acid samples were analyzed by enantioselective gas chromatography. Also here, the d/l ratios of the radioactive D,L-amino acids were within experimental error 50:50 indicating that these amino acids suffered no asymmetric degradation, despite self-radiolysis of up to 67% (Bonner et al. 1978).4
Today, we have various reasons to assume that these unsuccessful results are not surprising. The reasons are manifold.
1. The energy of ultra-fast electrons dissipates during the electron's movement in matter via collision and ionization processes towards particles and molecules. Only a small part of approximately 10"3 of the initial P's kinetic energy is emitted as Bremsstrahlung.
2. The energy of the Bremsstrahlung is only partly located in the photochemically active range of the spectrum; roughly 10"3 parts of the Bremsstrahlung are in the near ultraviolet.
3. The degree of circular polarization decreases linearly with photon energy and is small at photoenantioselective energies (MacDermott and Tranter 1989). Approximately 10"4 parts of Bremsstrahlung produced by polarized electrons in the low energetic range are circularly polarized. Only this circularly polarized part of the light is active for asymmetric photochemical reactions.
4. The photolytic absorption of the circular polarized component of the Bremsstrahlung by organic molecules is about 10"2 as we will see in the next chapter.
The multiplication of these factors lets us estimate that any asymmetry obtained by the Vester-Ulbricht process should be about 10"12 and thus very small and extremely difficult, if not impossible, to observe (Rein 1992).5 Of course, this experimental difficulty of the involved laboratories requiring high precision does not necessarily exclude the Vester-Ulbricht scenario from having contributed to pre-biotic evolution. Prebiotic amino acids might well have been subjected to natural permanent asymmetric radiation of polarized electrons but also circularly polarized Bremsstrahlung. One requirement would have been that the obtained difference in prebiotic enantiomers is big enough to rise out of the statistical fluctuations of the chiral components. Autocatalytic reactions (we will discuss them in Chap. 10 in more detail) might have been capable of favouring the amplification of a very small excess of one particular enantiomer; even the excess that had been determined by the asymmetry of the weak nuclear interaction might have been sufficient for asymmetric amplification.
4 I would be more than curious to analyze these 14C-labeled amino acid samples from the Laurence Radiation Laboratory or analogous well conserved 14C-labeled amino acids of comparable age by means of enantioselective techniques today.
5 For the specialist reader, I would like to mention that we distinguished for didactic reasons between (a) the direct asymmetric radiolysis of racemic organic molecules by polarized electrons and (b) the Vester-Ulbricht process. This distinction has pedagogic reasons in a way that scientific phenomena are exposed starting from the simple (Sect. 5.2) and moving then to the more complex model (Sect. 5.3). However, this distinction is 'artificial' because an effect of longitudinally polarized electrons on racemic organic molecules might be (a) due to a direct electron-particle interaction or (b) by circularly polarized Bremsstrahlung via photon-particle interaction. Often, the experimentalist is unable to distinguish (see Bonner et al. 1976; Akaboshi et al. 1982).
Discussing the Vester-Ulbricht hypothesis and the hitherto lack of its experimental verification, we should not overlook that ionizing P-radiation could also cause the racemization of amino acids. If the rate of asymmetric radiolysis were lower than the rate of radioracemization, such radioracemization would inevitably diminish the effectiveness of the Vester-Ulbricht mechanism of engendering optical activity (Bonner et al. 1982; Bonner 1984). Effects of radioracemization or secondary symmetrical degradation effects might dominate over asymmetric reactions and thus be responsible for the lack of observing an asymmetric degradation during radioly-sis of racemization-susceptible amino acids such as D,L-leucine with longitudinally polarized electrons.
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