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Comment by roxolotl | Hacker News Reader

I don’t know a ton about how chirality works. Couldn’t it just be that half(or some number) the asteroids contain left handed and half contain right? We only have a sample of one. Or is there something fundamental about left handed molecules that gives us reason to believe that if we see right handed ones once we would rarely see left handed ones in similar disconnected systems?

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Chirality means that there is a mirror image of a molecule that cannot be twisted into the original shape, despite being structurally identical. Due to the particular ways molecules tickle each other in living organisms to do interesting things, that means that the mirror image (racemate) of a molecule does something different.

In chemical synthesis, most (but not all) processes tend to preserve chirality of molecules: replacing a bunch of atoms in a molecule with another set will tend to not cause the molecule to flip to a mirror image. If you start from an achiral molecule (one where its mirror image can be rotated to the original), almost all processes tend to end up with a 50-50 mix of the two racemates of the product.

In biochemistry, you can derive all of the amino acids and sugars from a single chiral molecule: glyceral. It turns out that nearly all amino acids end up in the form derived from L-glyceral and nearly all sugars come from D-glyceral. The question of why this is the case is the question of homochirality.

There's as yet no full answer to the question of homochirality. We do know that a slight excess in one racemate tends to amplify into a situation where only that racemate occurs. But we don't know if the breakdown into L-amino acids and D-sugars (as opposed to D-amino acids and L-sugars) happened by pure chance or if there is some specific reason that L-amino acids/D-sugars is preferred.

"I Applied Wavelet Transforms to AI and Found Hidden Structure" https://news.ycombinator.com/item?id=42956262 re: CODES, chirality, and chiral molecules whose chirality results in locomotion

Do any of these affect the fields that would have selected for molecules on Earth? The Sun's rotation, Earth's rotation, the direction of revolution in our by now almost coplanar solar system, Galactic rotation

That’s definitively helpful thank you. I guess I’m still surprised that this finding is being taken to mean left handed molecules couldn’t have come from asteroids. If anything it seems to me to increase the likelihood of the panspermia hypothesis.

The reason this makes panspermia less likely is that the panspermia hypothesis would prefer to put the point at which all amino acids become L-amino acids before arrival on Earth, whereas the evidence is that the amino acids on this asteroid is before that point.

That makes sense. I’m just thinking statistically though. All we’ve proven is that some asteroids have R-amino acids. If it’s a 50/50 shot you could easily find a dozen R asteroids because it starts to look really bad for it being a 50/50 split.

I guess the reason it makes things more likely is because homochirality happens so aggressively that finding right handed molecules at all drastically reduces the odds of left.

This is a good explanation of chirality but I think the parent meant to say enantiomer instead of racemate:

– enantiomer refers to each mirror image of the molecule. https://en.wikipedia.org/wiki/Enantiomer

– racemate refers to a 50-50 mix of the two enantiomers. https://en.wikipedia.org/wiki/Racemic_mixture

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