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Influence of parity violation on biochemistry measured

Parity violation could matter in biology, after all

Up to the 1950s, people would believe that the laws of physics were invariant under the simple left-right mirror reflection. However, neutrinos were found to be left-handed and other processes linked to the weak nuclear force that violate the left-right symmetry were found in the 1950s. You know, the direction of the electron that leaves a nucleus after it beta-radiates is correlated with the nucleon's spin even though the velocity is a polar vector and the spin is an axial vector – such a correlation couldn't exist in a left-right-symmetric world.

A prejudice has been falsified. Ten years later, the CP symmetry was shown to be invalid as well although its violation in Nature is even tinier. The combined CPT symmetry has to hold and does hold (at least so far), as implied by Pauli's CPT theorem.



Bromocamphor, to become a player below

With the help of chiral spinors (and perhaps self-dual or anti-self-dual middle, (\(2k+1\))-forms i.e. antisymmetric tensors if the spacetime dimension is \(4k+2\)) particle physicists learned to build left-right-asymmetric theories and it became a mundane business. The electroweak theory included in the Standard Model is the most tangible real-world example of a left-right-asymmetric theory.

We also observe some left-right asymmetry in the world around us. Most of us have a heart on the left side – which could be an accident. But such left-right asymmetries exist at a more elementary level. Amino acids and other molecules look different than their images in the mirror – and all the life we know seems to use only one of the two images, typically a "left-handed-screwed" version of such molecules.

Is there a relationship to the violation of the parity at the fundamental level?




Neutrinos don't seem to be directly relevant for biochemistry because they're almost entirely invisible. And the effect of the parity violation on the electrons' energy levels and the probabilities of chemical reactions seems to be too tiny. So I think it's fair to say that most of the competent biochemists and biophysicists would say that it's a coincidence that the life molecules are "skewed", "screwed", "spun", and "twisted" in a particular direction. Sometime at the beginning when life began to expand, it was only the life based on one chirality and it just ate the oppositely chiral "seeds of life", if there were any, as if it were food. Incidentally, I think if you eat the mirror proteins, you won't die but you won't get energy from them, either!




However, there was also a school of thought that those asymmetries could matter, after all. One unusually well-defined version of this paradigm was the so-called Vester-Ulbrict hypothesis which claimed that the cosmic beta-radiation (a beam of electrons from space) had a helicity (an asymmetry in helicities, to say the least), and it selectively killed the now-unobserved copy of the "two mirror images of life".

Now, some experimental evidence in favor of this paradigm was published in PRL:

Chirally Sensitive Electron-Induced Molecular Breakup and the Vester-Ulbricht Hypothesis (J. M. Dreiling and T. J. Gay, Sep 12th)

Weak Nuclear Force Shown to Give Asymmetry to Biochemistry of Life (Elizabeth Gibney, Nature and SciAm, popular, Sep 25th)
They were colliding longitudinally polarized sub-eV electrons (they are spinning around the axis in the direction of motion) with the molecules of \({\rm C}_{10}{\rm H}_{15}{\rm BrO}\), some organic compound called "bromocamphor" with bromine (see the diagram at the top), with the goal of breaking the molecule apart and get the bromine atom out of it.

This may be measured for the two mirror images of the molecule, left-handed and right-handed one, if you wish, and the probability of the breakup (dissociative bromine anion production) depends on the relative sign between the electron's helicity and the molecule's chirality. They found the ratio of the two reaction rates to be\[

\frac{N_+}{N_-} \sim 1.0006.

\] They differ by 0.06 percent. It's arguably measurable – although the guys could have fooled themselves, too – but it's still very small (although arguably larger than what you may expect the "tiny and esoteric" physics of the weak nuclear force to bring to low-energy fields such as biochemistry). I am not sure whether a difference that is this small may play a significant role in the selection of the "right" chiral edition of the organic molecules.

But there are contexts in which one may imagine that this difference gets brutally magnified. For example, imagine that a reservoir of such molecules is needed as "food reserves" at some point and these food reserves drop to a tiny percentage, say \(\exp(-10)\sim 0.0045\%\), of the original size. It's ten (decreasing) \(e\)-foldings. During the same time, if one reaction undergoes ten \(e\)-foldings, the mirror image undergoes \(10.006\) \(e\)-foldings, and because that number ends with \(0.006\), this reaction will leave a smaller amount of the food reserves by \(0.6\) percent. What I want to say is that if you allow the asymmetry to influence exponential processes for a sufficiently long time, the original \(0.06\) percent may increase to a larger relative difference, one multiplied by the number of \(e\)-foldings. The same magnification of the asymmetry may affect exponentially growing, "constructive" processes. But is that enough?

The asymmetry may get magnified in exponential processes. Another paradigm that allows the small asymmetry to be "seen" is some precision cancellation. Imagine that the two mirror images of the bromocamphor molecule are "soldiers in two opposing armies" and they destroy each other, one-against-one, in a very exact way (much like in annihilation of matter and antimatter). Then the survivors will come from the "slightly prevailing" version even if the difference was very tiny.

Well, there could also exist reactions where the difference is much higher than in this – arguably random – dissociative anion production. I remain unconvinced. It's plausible that the asymmetries are inevitably linked but I still find it rather unlikely. Just to be sure, if there are no key reactions that are much more sensitive and no important mechanisms where the asymmetry is magnified or accumulated, as mentioned above, I would say that the odds 49.997% and 50.003% may be considered indistinguishable from 50% and 50% - the chances for both lives would be "basically the same".

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