Stereoselective decarboxylation of amino acids in the solid state, with special reference to chiral discrimination in prebiotic evolution

Summary

The decarboxylations of sublimated solidd- andl-leucine by nonpolarized γ-rays give quite different quantum yields, indicating significant selection. The G(CO₂) value for thed-isomer is higher than that for thel-isomer by a factor of 2 within a dose range of 10³–10⁵ rads. The G value for thedl-racemate is close to that of thed-isomer. The effect vanishes if instead of sublimation, crystallization from aqueous solution is the last preparation step. Our results on sublimated leucine agree well with those reported for γ-induced decarboxylation of solid β-phenylalanine prepared similarly by sublimation. The asymmetry increases with longer cooling periods after irradiation. An intrinsic energy difference due to parity nonconservation between enantiomers is discussed as a possible stereoselective mechanism, with special reference to the prebiotic origin of asymmetry in living matter. Other possible sources of the observed effects are also discussed.