In biology, the concept of parity emerges in the context of chiral molecules, where two molecules exist with the same structure but opposite parity. Interestingly, one enantiomer often strongly predominates over the other in natural biological systems (e.g. D-glucose is ubiquitous, L-glucose is rare in nature).

In physics, it has been established that weak interactions violate parity, which (if I understand correctly) implies a physical difference between left- and right-handed systems.

This suggests to me the following questions: Does parity violation in physics affect the physical or chemical properties of chiral molecules? And does this have any implications for our understanding of biological homochirality?

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    $\begingroup$ There are many examples where evolution chooses essentially at random between two geometrically different possibilities. Fish have vertical tail fins, while cetaceans have horizontal ones. The human retina has nerve cells on the front, while molluscs have them on the back. A random choice between two possibilities doesn't require a bias built into the laws of physics, it only requires that one possibility or the other arise by natural variation and then succeed. Differences such as humans having the heart on the left do require parity-asymmetry at the DNA level, but not in the laws of physics. $\endgroup$ – Ben Crowell May 6 '18 at 0:01
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    $\begingroup$ Martin Quack is a chemist at ETH Zurich whose research focuses on this possibility: onlinelibrary.wiley.com/doi/10.1002/9781118959602.ch18 $\endgroup$ – Tyberius May 6 '18 at 3:17
  • $\begingroup$ @BenCrowell Those examples aren't really "random" choices between two equivalent possibilities, there are historical reasons for the differences (e.g. cetaceans evolved from land mammals whose backbones were suited for terrestrial locomotion) and practical implications of the two alternatives (e.g. humans have a blind spot due to backward retina). But your overall point is correct: biological homochirality doesn't necessarily imply a physical cause, it's merely an intriguing possibility. $\endgroup$ – augurar May 29 '18 at 19:18

The idea that there must be some reason that all terrestrial DNA has a right-handed twist (or D- vs. L-glucose, or whatever your favorite chiral biomolecule is), goes all the way back to the discovery of biomolecular chirality by Pasteur.

The connection to the weak nuclear interaction is apparently known as the "Vester–Ulbricht hypothesis," after its first appearance in the literature more or less instantly following the discovery of parity nonconservation in nuclear weak interactions. As Emilio's answer says, it's an interesting idea but it's hard to make the mathematics work out, just because the weak interaction is so feeble. The literature seems to go back and forth between "here's a way that the weak interaction could couple to biological chirality" and "there's no evidence for that coupling."

A review by Bonner (2000) concludes

Consideration of all lines of evidence leads to the conclusion that there is no substantiation for such a causal connection, and that the two levels of parity violation are entirely independent of each other.

But that hasn't closed the issue. Dreiling and Gay (2014) report the breakup of a particular chiral molecule by electrons has a small ($\sim 10^{-4}$) asymmetry in the electron polarization. Since fast electrons in the natural environoment mostly come either from beta decays or from cosmic rays (and the cosmic electrons are secondary or tertiary particles produced in the weak decay of muons), and beta-decay electrons tend to have "left-handed" polarization, this is the sort of asymmetry that could systematically suppress one enantiomer in a pre-biotic environment. So perhaps that puts the pendulum back to "maybe here's a way."

  • $\begingroup$ To be clear, this answer is far better than mine. $\endgroup$ – Emilio Pisanty May 6 '18 at 12:05
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    $\begingroup$ Thanks! I was digging for a dimly-remembered specific reference for the 1950s proposal --- I remembered something written by Gardener or Asimov or Clarke --- and instead of finding it and a debunking, I learned the actual name of the proposal and also about the 2014 paper. $\endgroup$ – rob May 6 '18 at 12:37

The reason that the weak nuclear force is called 'weak' is that it has a minimal role compared to electromagnetism and the strong nuclear force. Generally speaking it only really appears in nuclear decay, and while it can be responsible for the appearance of chiral ground states of nuclei (see e.g. Why are pear-shaped nuclei possible?) its influence on dynamics outside the nucleus is essentially negligible.

It's hard to fully rule out a weak-interaction origin for biological homochirality, simply because we just don't understand the latter, but generally speaking, very few people are seriously considering that possibility (at least, absent some as-yet-undiscovered linking mechanism).


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