Polarized electrons see mirrored molecules differently

Amino acids and other biologically important molecules can be distinguished from their mirror images; the two chiral forms, designated as right-handed and left-handed, are called enantiomers. For reasons that remain unknown, only one of the two possible enantiomers is found in living organisms. Longitudinally polarized electrons, too, exist in a pair of mirror-related forms. The right-handed electrons with spin and momentum parallel are distinct from left-handed electrons with spin and momentum antiparallel. The difference is of more than academic interest; in particle physics the weak interactions break parity (inversion) symmetry, and electrons produced in beta decay are predominantly left-handed. Could the preferred handedness chosen by the weak interactions be related to the preferred handedness in living organisms? A necessary (but not sufficient) condition would be that longitudinally polarized electrons can react differently with mirror-related enantiomers. To test that requirement, Joan Dreiling and Timothy Gay (University of Nebraska–Lincoln) studied dissociation reactions in which a polarized electron impinges on a gas of the chiral molecule 3-bromocamphor (C₁₀H₁₅BrO) and knocks out a bromine ion Br⁻. In a series of experiments, the researchers shot left- and right-handed electrons at chirally pure enantiomers of C₁₀H₁₅BrO and determined reaction rates by measuring the current of Br⁻ produced in the dissociation. For the lowest-energy electrons studied (less than 0.1 eV), when the handedness of the electrons and C₁₀H₁₅BrO matched, the current obtained was greater than in the mismatched case by a few parts in 10⁴. No current difference was observed when polarized electrons reacted with a 50-50 mixture of the two molecular enantiomers. (J. M. Dreiling, T. J. Gay, Phys. Rev. Lett. 113, 118103, 2014, doi:10.1103/PhysRevLett.113.118103 .)