Degradation of DL-leucine with longitudinally polarised electrons

Abstract

MOLECULAR asymmetry represents a striking characteristic of living organisms and their products. The precise mechanism(s) involved in the origin of this asymmetry in nature is not known, although Vester¹ and Ulbricht² proposed that it may have originated through preferential destruction of one enantiomer of a racemic mixture by spin-polarised electrons arising from β-decay, or circularly polarised bremsstrahlung produced during their deceleration. However, tests of this hypothesis have yielded results which were either negative or, at best, of borderline significance³. Recently, however, Bonner et al.^4,5 reported the asymmetric degradation of the amino acid DL-leucine on irradiation with a beam of longitudinally polarised 120-keV electrons (degree of polarisation 10–27%). Electrons polarised with their spins antiparallel (AP) to their momenta were reported to cause preferential destruction of the D-enantiomer. Moreover, the L-enantiomer of leucine was reported to be preferentially destroyed by electrons polarised with their spins parallel (P) to their momenta. These results were of special interest as electrons emitted in β-decay have AP spins and also it is the L-isomer of leucine which occurs in nature. The precise physical processes responsible for such an asymmetric degradation remain unclear. We report here that using an electron source⁶ which provides a much higher degree of polarisation (typically 43%), we have reinvestigated the effect of irradiation of DL-leucine with longitudinally polarised electrons. In particular, the increased spin polarisation was expected to result in a more pronounced asymmetric degradation. However, no asymmetric decomposition was observed.