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[cpm/g]a

[cpm/g]a

[cpm/g]a

[cpm/g]a

[cpm/g]a

d-ß-phenyl-alanine

1.3 • 104

2.4 • 104

3.0 • 104

4.6 • 104

1.5 • 105

d,l-ß-phenyl- Alanine

9.0 • 103

1.6 • 104

2.2 • 104

3.5 • 104

1.4 • 105

l-ß-phenyl-alanine

5.7 • 103

9.0 • 103

1.2 • 104

2.5 • 104

1.3 • 105

a Units of cpm for the counting rate are indicated per gram of labeled P-phenylalanine a Units of cpm for the counting rate are indicated per gram of labeled P-phenylalanine

Alternatively, crystallization reactions that form macroscopic crystals via au-tocatalytic growth might offer the potential to amplify tiny energetic differences between enantiomers. They were thought to hold the key to get experimental access to AE PNC. As we have discussed earlier, the atomic number Z of the element in the asymmetry center contributes to AEPNC to the power of 5 Eq. (5.1). Combining these two understandings, Szabo-Nagy and Keszthelyi (1999) studied crystallization experiments with sodium ammonium tartrate, A- and A-tris (1,2-ethanediamine)cobalt(III) and A- and A-tris(1,2-ethanediamine)iridium(III) complexes. The cobalt- and iridium-compounds are chiral octahedral complexes (see Chap. 2 and Fig. 5.8). They were chosen as candidates for the study of crystallization processes since cobalt (Z = 27) and iridium (Z = 77) provide relatively high atomic numbers compared to carbon atoms (Z = 6).

Racemic aqueous solutions of the octahedral complexes were carefully produced and after four weeks of water evaporation at room temperature half of the dissolved molecules were precipitated as polycrystalline material. The crystalline material was dried and again dissolved in water. Circular dichroism spectra of dissolved sodium ammonium tartrate crystals that had been running in parallel were measured to be precisely zero. However, in the case of the cobalt-complex the distribution of the circular dichroism signals for the crystalline material was shifted from zero by a value of -2.3 • 10-4! Even higher records were obtained for the iridium-complexes. The distribution of the crystalline material is shifted in the same direction as for the cobalt-complexes by a value of -39 • 10-4. CD shifts were larger when less material was crystallized, as theoretically expected. Chiral impurities and bacterial contaminations were excluded for the interpretation of these results. After multiple repetitions and a careful statistical evaluation the authors assumed that the recorded shifts in the observed autocatalytically growing crystals, which increased with the atomic number Z, are real and caused by AEPNC. No CD shift was found in the case of sodium ammonium tartrate with carbon in the stereogenic center. Assuming that these differences point to experimental evidence for the existence of AEPNC, Szabo-Nagy and Keszthelyi proposed the AEPNC as the determining agent for the origin of homochirality of biomolecules. To my knowledge this is until today the most hopeful and also most cited experiment for the experimental AEPNC determination.

Nevertheless, we would have to add that even if these reproducible effects in the crystallization behaviour of specific chiral metal complexes were found, how can one guarantee that their origin is intrinsically from parity violation and not from some parasitic effects arising from intermolecular interactions in the macro-

Fig. 5.8 Chemical structure of the chiral octahedral A- and A-tris(1,2-ethanediamine)iridium(III) complexes, crystallized by Szabo-Nagy and Keszthelyi (1999)

scopic experiments? Minor impurities could play major roles. The contemporary environment has been virtually saturated with L-amino acids, and so potential chi-ral contamination is a continuing concern (Deamer et al. 2007). How can one surely guarantee the absence of effects of this type and of other parasitic effects in a macroscopic experiment? The other difficulty is to relate quantitatively the macroscopic observations expressed in a CD value of -2.3 • 10-4 for the octahedral cobalt crystals and a CD value of -39 • 10-4 for the octahedral iridium crystals to the theory of parity violation. This seems to remain extremely difficult, at present (Quack 2002).

In view of the above calculations and experiments, we will have to close this chapter with the conclusion that whether or not the effects of the weak interaction on molecular dynamics are sufficiently large to be able to dominate random conditions or fluctuations has not yet been met with certainty. We do wait for the ultimate experimental evidence of the AEPNC determination in the future.

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