Asymmetric Radiolysis by Polarized Electrons

Our central question will now be whether this asymmetry inherent to the nuclear P-decay can have contributed to the selection of chiral molecules in life's biopolymers. Is it truly reasonable to assume that polarized electrons can interact with racemic or prochiral organic molecules such as amino acids inducing an enantioen-richment? In this case, a flux of spin-polarized P-electrons should react in a different and measurable way with R- and S-enantiomers.

Here, the interested reader will remark the strong analogy between polarized electrons as chiral driving force and 'chiral photons' in the form of circularly polarized light. The influence of 'chiral photons' on racemic organic molecules is indeed measurable which we will illustrate in the next chapter.

As mentioned before, polarized electrons (or positrons) emitted through the radioactive P-decay are highly energetic; their kinetic energy is not adapted to the range of excitation energies within molecules. Best results for the enantioselec-tive interaction of polarized electrons with organic molecules were thus expected in the examination of relatively slow polarized electrons showing kinetic energies of some electron volt (eV) only. If we expect some enantioselective electron-molecule interaction, then it should be in this energy range. Production and handling of spin-polarized electrons in the low energy range is not particularly difficult and experiments of this kind have indeed been performed:

At the University of Edinburgh, Peter Farago studied the absorption of polarized electrons by individual enantiomers. He subjected enantiomers of D-camphor and L-camphor to 5 eV spin-polarized electrons. This experimental conception can be seen in close analogy to chiroptical circular dichroism measurements. Here and there a polarized beam (Farago: electrons; CD: photons) is differentially weakened by passing a left- or right-handed optically active medium. If any effect will be measured here, we may call it an "electronic circular dichroism" or an "electronic Cotton effect" (Rein 1992). And indeed, Farago was able to measure a differential influence: d- and L-camphor did not equally weaken the through-shining spin-polarized electrons! Differences were detected with high precision in the per mille range (Campbell and Farago 1987). Control experiments were performed carefully with racemic d, L-camphor and also with an unpolarized electron beam. The result was surprising: not because the effect was so small, but because the effect was much bigger than one could expect from quantum-theoretical calculations.

In similar experiments at the Biological Research Center in Szeged, Hungary, D-and L-tyrosine in alkali solution were irradiated separately with codissolved 90SrCl2 as ß-ray source. Unexpectedly, after 18 months, the absorption bands of D-tyrosine diminished compared to those of L-tyrosine, suggesting that the former had undergone more degradation than the latter (Garay 1968). At Stanford University, Garay's experiments were repeated in a way that the irradiated D- and L-tyrosine was additionally subjected to ORD and enantioselective gas chromatographic measurements. After 15 months, however, Bonner et al. did not at all notice the development of optical activity (Bonner et al. 1975b). Instead, Bonner et al. irradiated a solid film of the amino acid rac-leucine with 'left-handed' longitudinal polarized electrons but also with unnatural 'right-handed' longitudinal polarized electrons produced in a specifically developed linear accelerator source (Bonner et al. 1976). In agreement with Garay's finding, it was quantitatively observed that natural left-polarized electrons bring about the degradation of D-leucine more extensively than that of L-leucine, whereas the unnatural right-polarized electrons act in strictly the opposite sense. The significant enantiomeric excess generated in the product ranged from - 0.60% to + 1.42%. It is worth mentioning that the natural left-handed electrons produced an asymmetric degradation favouring the formation of an excess of the L-leucine enantiomer occurring in biomolecules, while antinatural right-polarized electrons do precisely the opposite.

Even higher - not to say dramatic - enantiomeric excesses were reported by Wolfram Thiemann and coworkers at the Jülich Nuclear Research Center, Germany (Darge et al. 1976). In the experiment's design, it was assumed that asymmetric processes in aqueous solution are dominated by solution radiochemistry involving symmetrical free radicals that might overwhelm and obscure any asymmetric effect. Because of this, 32P-phosphate in the form of NaH232PO4 was used as buffer and "internal" ß-emitter together with the racemic amino acid D,L-tryptophan in water at -25° C. The low temperature was chosen in order to avoid asymmetric reactions caused by biological (micro-) organisms. The mean energy of the ß-rays was 0.57 MeV with a 93.9% degree of polarization. After 12 weeks of asymmetric interaction, optical rotation was measured at 220 nm. Comparing the data with control experiments it was concluded that 33% of the racemic tryptophan had been decomposed and from the optical rotation of +0.7± 0.4 mdeg as the mean value of sixteen independent runs, the authors concluded that a remarkable 19% enrichment of the D-enantiomer in the residue had been produced. We should not overlook that Bonner and coworkers (1979) tried to repeat this experiment using enantioselective gas chromatography to quantify the tryptophan enantiomers after subjecting them to ß's of KH232PO4 at — 196°C. They were not able to reproduce the effect of asymmetric radiolysis (Blair and Bonner 1980). Darge et al. (1979) immediately replied to this criticism that ORD measurements are capable of detecting the effect of the sum of all potential chiral products while the gas chromatographic method applied by Bonner and coworkers quantified exclusively the enantiomeric composition of the partially destructed residual tryptophan. This gas chromatographic method focused on the destruction of racemates whereas ORD measurements at 220 nm were said to be furthermore sensitive towards the equally probable chance of stereoselective synthesis triggered by left-polarized electrons as a source of optical activity.

The mechanism of the deceleration of polarized electrons in matter is dominated by collision and ionization processes. Details remain hitherto unknown in lack of experimental but also theoretical work. We know that the original polarization of the electrons is at least partly maintained during deceleration. Meiring (1987) was able to calculate asymmetries in radiolysis cross-sections for both p-- and P+-radiation, and to derive the connection of these cross-sections with decomposition rate constants. At Kyoto University, Akaboshi et al. (1979) contributed to elucidate the reaction mechanism by irradiating individual enan-tiomers of the amino acid a-alanine with P-electrons produced by radioactive 90Yttrium-metal. The products were analyzed by electron spin resonance (ESR), a technique capable of characterizing radicals. As assumed, radicals were identified and quantification showed that D- and L-alanine enantiomers remained in distinguishable concentrations. This asymmetry was confirmed by ESR studies of Conte (1985), however, not observed with non-polarized electrons obtained from the Van de Graaff generator (Akaboshi et al. 1982) pointing on a real asymmetric effect. The interaction of Cerenkov radiation, the bluish light emitted as high-energy charged particles including P-particles, with liquid chiral enantiomers of (-)-R- and (+)-S-2-phenylbutyric acid was carefully studied by Garay and Ahlgren-Beckendorf (1990). Also here, a small asymmetry in the interaction was identified.

At this point we can conclude from all of the above experiments that - even if we are confronted with conflicting results - left spin-polarized ('chiral') electrons are emitted by the radioactive p--decay and that such electrons are in principle capable of interacting differently with R- and S-enantiomers. The hitherto measured effects are small and/or difficult to reproduce and we are still waiting for the scientific breakthrough. We will leave it an open question whether or not the direct asymmetric radiolysis of racemic organic molecules by polarized electrons is a viable mechanism for the origin of biomolecular asymmetry. We can expect for the future that (a) more precise beams of polarized electrons showing higher polarization rates and fluxes and (b) better resolution of enantiomers with modern analytical techniques will provide even more convincing information on the possible direct asymmetric radiolysis of organic matter by polarized electrons.

Since the instant kinetic energy of spin-polarized electrons emitted by the P--decay and other natural decay processes is usually some 100 keV and therefore orders of magnitude too high for any enantiomer discriminating absorption by organic molecules, the Vester-Ulbricht Process was proposed to circumvent this problem.

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