Tuesday, September 30, 2014

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.

  1. 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.

  2. 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.

  3. 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.
TGD based view about dark matter suggests a scenario in which external parity breaking is not necessary. Parity breaking would be a feature of chemical reaction itself and involve "new chemistry". This of course makes no sense in the chemistry supported by the standard model. The basic elements are dark matter identified as a hierarchy of dark phases with effective value of Planck constant given by heff=n× h and the notion of magnetic body. Magnetic body would use biological body as sensory receptor and motor instrument and EEG and its possible scaled variants would make the communications with and control of biological body possible.
  1. 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.

  2. 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.

  3. 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!

  4. 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.

Another analogous idea is based on polarisation of photons in atmosphere, which in turn induces faster decay for the second molecular chirality: see this. Now the polarisation would be due to the scattering of originally unpolarised photons with atmosphere. This would not involve parity breaking but just Raileygh scattering. Now the polarization is however linear rather than circular and I must confess that I failed to understand how this could lead to breaking of parity which would correspond dominance of either circular polarization. If some reader understands this, I would be happy for explanation.

1 comment:

JVKohl said...

After you blocked me, I posted a notice about the link from polarization to biodiversity in the context of testing for bioparticles/analytes that link chirality from autophagy to cell type differentiation in all living genera.

See: "...it allows users to analyze different cell types without using fluorescent or magnetic labels that are typically required to track cells. Instead, the chip separates cells based on their intrinsic electrical properties: When an electric potential is applied across the inkjet-printed strip, cells loaded into the microfluidic chamber get pulled in different directions depending on their "polarizability" in a process called dielectrophoresis. This label-free method to analyze cells greatly improves precision and cuts lengthy labeling processes."

Read more at: https://phys.org/news/2017-02-scientists-lab-chip-cent.html#jCp