Never Say Never Measuring Parity Non Conserving Energy Differences

More than 30 years ago, immediately after its theoretical prediction, it was assumed that the tiny parity non-conserving energy difference AEPNC might be experimentally measured for chiral molecules verifying the above outlined wonderful theoretical approaches of Yukio Yamagata, Dieter Rein, Stephen Mason, and others. Besides measurements of specific heat and magnetic susceptibility, spectroscopic methods such as ultraviolet, infrared, generally optical, microwave, and nuclear magnetic resonance (NMR), but also Moßbauer, and Raman spectroscopy (see Bolik et al. 2007) were applied. Often, isolated molecules in the gas phase under well defined conditions but also physico-chemical nucleation processes were studied intensively for individual chiral compounds and their corresponding mirror image enantiomers with the aim to detect a small differential shift (see e.g. Quack 1989, 1993; Wang et al. 2000, 2003). Particular attention was drawn to reactions where very small differences of energy can become manifest by amplification or cascading. Polymerization and precipitation were considered as such reactions, but also the possibility of cascading differential properties of enantiomers by irradiation-induced chain reactions.

In a remarkable experiment, Otto Merwitz studied differences in the radiochemical behaviour of enantiomers at the Jülich Nuclear Research Center in Germany. He irradiated d-, d,l-, and L-ß-phenylalanine with unpolarized y-radiation from a 60Co source. The absorption of y-quanta by the substrate provokes the dissociation of phenylalanine under release of carbon dioxide CO2. The decarboxylation reaction is illustrated in Fig. 5.7.

As depicted, the amino acid ß-phenylalanine was 100% labelled with 14C-isotopes at its carboxylate-carbon atom. By utilizing this kind of 14C-labelled enantiomers it was possible to very precisely quantify the radiochemically-released

Fig. 5.7 y-Quanta induced radiolysis of 14C-labeled P-phenylalanine enantiomers producing 14CO2

14CO2 by measuring its counting rate of 14C in the gas phase. In Table 5.1, the counting rate is given for the individual enantiomers D- and L-P-phenylalanine and the racemic mixture D,L-P-phenylalanine.

The results indicated that the counting rate of 14CO2 and therewith the cleavage rate of the D-P-phenylalanine enantiomer was higher by a maximum factor of 2.7 compared to the L-enantiomer (Merwitz 1976). The D-enantiomer decarboxy-lated faster. Values for the racemic mixture were recorded in almost exactly halfway between the values for the individual enantiomers. The differential behaviour of the enantiomers was valid for doses of the electromagnetic radiation between 5 • 102 rad and 5 • 104 rad. Below 50 rad the counting rates could not be measured with sufficient accuracy. Above 104 rad the differences became smaller.

For further clarification of this crucial experiment, subsequent differential radiolysis experiments with 14C-labelled (Norden et al. 1985) and 13C-labelled (Merwitz et al. 1998) leucine enantiomers in the solid-state point in a very similar way to the preferential decarboxylation of the D-enantiomers. The decarboxylation of D-leucine was proven to be more effective than that of L-enantiomer. One should note that the above asymmetric results were not assumed to be caused by circular polarization of the photons. Norden et al. (1985), however, could not completely exclude the possibility that (a) asymmetric crystal defects or (b) impurities from the original preparations of the amino acids survived the recrystallization and sublimation caused the asymmetric behaviour.

Are these results the ultimate experimental proof for parity non-conserving energy differences as claimed by the author? Discussions continued (Schleser et al. 1991; Merwitz et al. 1991) and are ongoing until today; the final answer to this question cannot be given yet.

Table 5.1 Dose dependent counting rates of 14CO2-cleavage after irradiation of 14C-labeled P-phenylalanine with unpolarized electromagnetic radiation







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