New experimental information about chiral selection
Chirality selection is fundamental problem in biology. It means large parity breaking since only second molecular handedness appears in living matter. This is not the case in inanimate matter. This distinction must be somehow fundamental for what it is to be living. The problem with large parity breaking is however that although it is caused by weak interactions it is extremely small effect and it is difficult to imagine how it could emerge. Even more difficult it is to understand why it would emerge only for the in vivo variants of bio-molecules.
One promising idea is that the original parity asymmetry would not be biological but would transferred to biology. In 1967 biohemist Frederic Vester and environmental scientist Tilo Ulbricht proposed that some physical phenomenon could have changed the balance between left and right handed molecule concentrations during earlier stages of evolution. Beta decays are certainly the first candidate to come in mind since in beta decays the breaking of parity symmetry manifests itself as the appearance of only second helicity for electrons. One says that the electrons from beta decays of nuclear neutron to proton are polarized with spin directing to direction opposite from the momentum of electron. For high energies electron is an eigenstate of parity operator and can be said to be left or right-handed. At low energies both chiralities are present and the spin projection to the direction of motion an have both signs. For instance, one could imagine that cosmic rays decaying in the atmosphere producing nuclei and muons suffering beta decay in atmosphere could produce the polarized electrons.
This asymmetry would manifest itself as slightly different decay rates for molecule and its mirror image induced by the absorbtion of the polarized electron. This small difference could be however exponentially amplified by reaction kinetics and lead to chirality selection by say cosmic rays. The process would have taken place long time ago and led to the dominance of second molecular handedness.
The challenge would be to find a chemical process achieving this. A strong constraint is that this effect should be present only for the in vivo variants of biomolecules. It is difficult to imagine why recent physics as we find it in text books could distinguish between in vivo and in vitro.
The attempts to identify the chemical process involved have not been successful hitherto. Now however Nature published an interesting article about a possible chemical mechanism leading to chirality selection.
- Gay and Joan Dreiling working at the University of Nebraska-Lincoln irradiated a organic compound bromocamphor with low energy spin-polarized electrons and achieved a success. The rates of decays induced by absorption of polarized electron differ by about .6 per cent for the two possible polarizations. This is large difference. Note that the chemical reaction is not expected to involve parity breaking. The parity breaking would be inherited from electrons. There are however some problems involved.
- The asymmetry occurs only for electron energies below electronvolt. Electrons from cosmic rays have much higher energies but one could argue that there should exist a slowing down mechanism of electrons or there is some other natural source of very low energy polarized electrons.
- Second problem is that for low energies one cannot anymore have a well-defined electron chirality but one can speak about spin component in the direction of momentum - actually any direction can be chosen for the quantization axis. Does the result mean that it is the sign of spin projection rather than handedness of electron which matters? This would suggest that at the low energy limit the mirror reflection leaves electron invariant and the parity invariance of physical chemistry would imply that there is no distinction between the decay rates! To me this looks a serious problem: maybe I have misunderstood something.
- In dark phase weak bosons would be effectively massless below the scaled up weak scale and weak interactions would be as strong as electromagnetic interaction. The scaling corresponds Lw→ (heff/h × Lw and the resulting dark weak scale can be even of cellular size scale. The otherwise extremely weak parity breaking effects would be large in dark phase. The large parity breaking of weak interaction for the weak decays of dark variants of molecules would directly imply different decay rates for dark magnetic/field bodies of left- and right-handed molecules.
- Parity breaking effects of weak interactions causing chiral selection would large due to the presence of dark effectively massless W bosons and Z boson whose interactions break parity. The description of parity breaking effects in nuclear physics (see this suggests that one parity breaking effect would be induced by the interaction s∇ VZ, where VZ is the scalar potential associated with the classical Z field.
- The additional bonus is that this mechanism would be possible only in vivo since living matter would be made living by large heff phases identifiable as dark matter at magnetic flux tubes!
- Also the observations about bromocamphor could have explanation in terms of dark matter at flux tubes relevant for the observed decays. This would allow to circumvent the problem that the electrons in the experiments involving bromocamphor have so low energy that they do not have well-defined chirality and the difference between the decay rates should be very small.