my favored explanation for homochirality has always been autocatalytic reactions. like, 5 billion years ago some asymmetric reaction by chance produced an L-chiral amino acid before a D-, which catalyzed more L-handedness. and now all biogenic amino acids are L! en.wikipedia.org/wiki/Soai_re... - ThreadSky

jmelville.science • 30 days ago

my favored explanation for homochirality has always been autocatalytic reactions.

like, 5 billion years ago some asymmetric reaction by chance produced an L-chiral amino acid before a D-, which catalyzed more L-handedness. and now all biogenic amino acids are L!

https://en.wikipedia.org/wiki/Soai_reaction?wprov=sfla1

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tscmacdonald.bsky.social•30 days ago

Interestingly, chiral autoamplification is entirely a kinetic phenomenon - not thermodynamic. Left to run for long enough, the unflavoured chirality would "catch up" and you'd get a 50:50 mixture.
Homochirality needs not only amplification, but also a way to start and stop the reaction system!

jmelville.science•30 days ago

this makes sense physically, but my intuition is that this only holds over infinite timescales and given unlimited quantities of starting material. it's not incompatible with homochiral life on Earth! the finite resources available should provide the necessary bounding conditions.

tscmacdonald.bsky.social•30 days ago

Not sure what your chemistry background is, but this is a great and relatively readable paper on these systems and how they do/don't work: https://onlinelibrary.wiley.com/doi/full/10.1002/anie.200804566
(on scihub if you don't have access)

tscmacdonald.bsky.social•30 days ago

The core issue here is microscopic reversibility: if an (auto)catalyst helps make one chirality form faster from an achiral starting system, it will also accelerate the reverse reaction. Autocatalysis can get you a transient spike in one chirality, but it can't sustain homochirality alone.

tscmacdonald.bsky.social•30 days ago

The challenge is that homochirality needs a way to separate chiralities (chemically). That's doing work against entropy - it takes energy to do work in a way that discriminates by chirality. Autocatalysis helps set up a chiral enriched starting point, but homochirality takes a bit more effort :)

jmelville.science•30 days ago

hmmm okay, I buy that this is true thermodynamically, but just because it's a kinetic phenomenon doesn't mean it's not "real", bc life is not a steady-state equilibrium.

if you looked purely at thermodynamics over kinetics, you'd conclude that benzene (∆Hf>0) is unstable relative to elemental C/H!

jmelville.science•30 days ago

my PhD is in physical inorganic chemistry — will definitely give this a read! thank you for sharing

tscmacdonald.bsky.social•30 days ago

Great, you'll be fine - enjoy the read!

manlius.bsky.social•30 days ago

That's surely a possibility.

We are facing the very same question with @ricardsole.bsky.social in an upcoming paper.

My take is that cosmic rays might have played a role, but the difference between 🌎 and Bennu is that natural selection took place on the former.

jmelville.science•30 days ago

hold up, what's this about cosmic rays? you have my full attention

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