Asymmetric degradation of DL-leucine with longitudinally polarised electrons

Abstract

FOLLOWING Wu's experimental demonstration¹ of Lee and Yang's predicted violation of the parity principle during β decay² and following Goldhaber's demonstration³ of the circular polarisation of the bremsstrahlung produced by β-decay electrons, Vester and coworkers (ref. 4 and F. Vester, unpublished) proposed the following unique mechanism for the abiotic origin of optically active organic molecules Attempts to demonstrate this mechanism experimentally^4,5 were unsuccessful, however, until Garay's report in 1968 that D-tyrosine in dilute aqueous alkali was more decomposed (as evidenced by greater eradication of its ultraviolet absorption bands) than was L-tyrosine after 18 months exposure to 0.36 mCi of ⁹⁰SrCl₂ in solution⁶. One of us has attempted⁷ to repeat Garay's experiment using DL-tyrosine in alkaline solution, looking for optical rotation after exposing the sample to a dose of 4.1 × 10⁸ rad of β-ray bremsstrahlung in a 61,7000-Ci ⁹⁰Sr–⁹⁰Y source at Oak Ridge National Laboratory for 1.34 yr. No optical activity was noted, using optical rotary dispersion (ORD) measurements in the region 250–630 nm. At the same time⁷ Garay's experiments were extended to include other racemic amino acids, both in the solid state and in neutral, acidic or alkaline solution. In no case, after the same 1.34-yr exposure, did we notice the development of optical activity either by ORD measurements or (in the case of DL-leucine) by the gas-chromatographic determination of the enantiomeric composition of the irradiated samples. Our explanation⁷ for these negative results has been that most of the bremsstrahlung causing radiolysis of the samples was of low energy⁸ and therefore of very slight circular polarisation^3,9. In the hope of overcoming this difficulty, we have now undertaken similar experiments using longitudinally polarised electrons (and their bremsstrahlung) from a linear accelerator.