
The Chiral Puzzle of Life - bookofjoe
https://iopscience.iop.org/article/10.3847/2041-8213/ab8dc6
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peter303
Perhaps there is slightly slightly different ground energy state of the two
different chiralities and Nature selected the slightly lower one. I have not
seen evidence supporting this hypothesis.

A similar mystery is why DNA chose a base64 nucleic coding system for 21 amino
acids and a few punctuations. Some of the codons are redundant and some
unused. (There appears to be a fossil base16 coding system inside it.) And
synthetic biologists have modified it into a working base216 system with
additional nucleic acids and amino acids. Perhaps again there is some ground
energy argument as to why this universal 3-billion year old coding system is
as complex as it is and no more complex.

~~~
mdturnerphys
One hypothesis around the 20 amino acids relates to optimization of a quantum
search algorithm [0]. Some recent work has added support to this idea [1].

[0] [https://arxiv.org/abs/quant-ph/0002037](https://arxiv.org/abs/quant-
ph/0002037) [1] [https://www.technologyreview.com/2019/09/12/133081/an-
import...](https://www.technologyreview.com/2019/09/12/133081/an-important-
quantum-algorithm-may-actually-be-a-property-of-nature/)

~~~
WhiteSage
That does not seem a good paper. The only references on biology are quite
generic books. Also the stated hypotheses are quite unfounded. As far as I
know, there is barely any evidence of biology using quantum phenomena
directly, except perhaps regarding photosynthesis and enzymatic activity:

[https://en.m.wikipedia.org/wiki/Quantum_biology](https://en.m.wikipedia.org/wiki/Quantum_biology)

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centimeter
Back in uni, I found it deeply unsettling to learn that the weak interaction
violates parity symmetry. It feels like a glaring aesthetic flaw in an
otherwise very elegant physical “design”, so to speak.

~~~
p1mrx
Would you rather live in a perfectly symmetric universe without any matter in
it?

~~~
ramshorns
It's strange that the weak force violates parity symmetry, and it's also
strange that charge–parity symmetry, taken together, is much closer to being
conserved but is still violated a little bit.

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fqrley
Asimov wrote a book on the subject:
[https://en.wikipedia.org/wiki/The_Left_Hand_of_the_Electron](https://en.wikipedia.org/wiki/The_Left_Hand_of_the_Electron)

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aazaa
The origin of homochiral building blocks (amino acids, sugars) is certainly an
old mystery. But it doesn't seem to be a very deep one. By that I mean that
there are many physical processes that could have initially led to a small
preference. Natural selection then would do the rest.

The deeper mystery is how self-replicating chemical systems ever came about in
the first place. There are several fairly steep hurdles to overcome, and so
far nobody has put forward a compelling, testable hypothesis leading from the
"primordial soup" to anything even distantly resembling the simplest
organisms.

~~~
sova
Is it not pure speculation to assume that the world was present before living
beings inhabited it? Who made these observations of a purely physicalist realm
that predates beings with awareness? It may be true, but its unconfirmable, so
we have to ask questions about what we can confirm. Chirality in the present
is confirmable. A world made of physical matter and mind is confirmable. But
strong questions about ontological declarations of life from purely
physicalist material speculative histories are not only hard to answer, they
assume more than they reveal.

    
    
      Edit: must have hit a nerve

~~~
ridgeguy
Nobody's making that assumption. It's a hypothesis.

All evidence we have is consistent with a "world being present before living
beings inhabited it".

Therefore the hypothesis is not disproved, which is all science ultimately
says about facts.

~~~
sova
Chirality is either an underlying property of biological life, or an
epigenetic (so-to-speak) result from having a Spinning Something and then
throwing Life into it. Reality itself could have a spin or orientation and
therefore Biological Life would be subservient to such. In my opinion long-
term adaptations have about as much pertinence to chirality as it does to
magnetism. Either it turns or it doesn't, it doesn't slowly decide to over
millions of years.

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prvc
>a small, but persistent, chiral bias

>If this mechanism dominates, then the handedness of living systems should be
universal.

No matter how large the bias (unless it's "astronomically" close to 1), there
will be, by the law of large numbers, some world populated by life of the
opposite chirality to the favoured one, due to it arriving first there, given
the size of the universe.

~~~
6nf
Yup I don't think the author meant that all life in the entire universe must
be the same chirality, just that the bias would be widespread across the
universe

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djaque
That's a really interesting idea that cosmic rays caused the chiral asymmetry.
Any biologists want to chime in on how this fits in with other ideas on the
problem?

~~~
tanzbaer
That's honestly more a question for physicists and chemists. Biologists in my
experience take a lot for granted because they don't really understand the
fundamentals on small scales.

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Vaslo
The story of Thalidomide and its effects on birth defects is a good story
about the chirality of molecules and their significance. The same molecule on
2d paper suddenly becomes a teratogen when the racemic mixture is given (50/50
mix of the “right hand” and “left hand” versions.) When a simple molecule like
this drug with only a single chiral center at the base of so many issues,
imagine a protein with hundreds or thousands of chital centers and the
potential effects (or lack of effect at all) with simple changes.

~~~
eganp
Your point on complexity is well taken.

I'd just like to recommend against using thalidomide as an example of chiral
effects. Rather unusually, thalidomide has a nitrogen atom as its chiral
center. As such, it rapidly racemizes (interconverts) at physiological pH.
This makes the claim of stereospecific teratogenicity a bit puzzling as it
shouldn't be possible to resolve by administering enantiopure thalidomide.

The mechanistic story of thalidomide is a bit complicated. Ultimately the
mouse and rat models used couldn't possibly predict teratogencitiy as these
models have an amino acid mutation (valine -> isoleucine) that prevents
binding of thalidomide to Cereblon, the protein that mediates thalidomide's
teratogencitiy [0]. Amazingly, although this story is oft-repeated, its
origins are unclear. This is likely the origin of the myth [1], but knowing
what we know now these results should be impossible.

[0]
[https://elifesciences.org/articles/38430](https://elifesciences.org/articles/38430)
[1]
[https://pubmed.ncbi.nlm.nih.gov/583234/](https://pubmed.ncbi.nlm.nih.gov/583234/)

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philsnow
There's a bit of a callout to the handed chirality of life on Earth in Daniel
Suarez' "Change Agent" [https://www.amazon.com/Change-Agent-Daniel-
Suarez/dp/1101984...](https://www.amazon.com/Change-Agent-Daniel-
Suarez/dp/110198466X)

Like all of his books, it's a quick read that feels a bit like a movie
screenplay. Fast and fun, each book of his has a well-enough-developed hook,
but overall I liked Daemon and Freedom best.

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atemerev
Non-chiral protein life (as we know it) simply cannot exist, as enzymes rely
on precise molecular shaping to implement the catalytic pathways for chemical
reactions essential to life. Without chiral selection, it would be impossible
to build these precisely folded polypeptides with well-defined functional
groups.

So, for me, there is no question of “why chirality” — the entire chemical
architecture of life depends on it. The question is, perhaps, “are there other
options possible?”

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slyrus
Actually only 19 of the 20 standard amino acids are chiral. Glycine is
achiral.

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p1mrx
Is anyone working on building a mirror-image version of life in a lab? I want
to know what anti-chiral chicken tastes like.

