Stereoselective decarboxylation of amino acids in the solid state, with special reference to chiral discrimination in prebiotic evolution
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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.
Key wordsChiral selection in chemical systems Parity nonconservation in weak interactions Origin of prebiotic asymmetry
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