How Homochirality Arises

Continuing the discussion from Modeling Studies on Panspermia:

@Guy_Coe, this article has some helpful information on different mechanisms that can give rise to homochirality. Once the cell arises, biological systems will usually find it easier to make chiral molecules than racemic molecules, so the process perpetuates.


Chirality is also very interesting to me. I studied photochemistry of chiral molecules in graduate school and you need (I think always, but could be wrong) chirality to make rotary molecular motors function. I printed out the paper and may post more when I get through it.


I’ve read that article before, and after reading it again, I come away with the sense that it’s mostly just an optimistic hypothetical description of a number of interesting lines of research, with a few interesting but inconclusive laboratory efforts. It strikes me that we are not much closer to solving this basic mystery after 30 years of research. “May be able to” is not a very definite description of what the known processes can do. They need a “nudge,” however that’s accomplished…

A coin has gotta flip heads or tails.

No nudge needed. Rather this shows that homo chirality can arise by several mechanisms. This is an example of symmetry breaking. Much like…

Isn’t a few interesting laboratory efforts infinitely more than the laboratory efforts from Reasons to Believe, whom you support?

Why do you find RTB’s rhetorical efforts so much more convincing than evidence?

RTB’s “rhetorical” efforts on the age of the earth have been immensely helpful and educational. They do a great job on astrophysics, but have triangulated off of biology in many ways.


You guys seem unable to sort fact from fiction with regards to some of your explanatory theories, is all I can say.
I’ve no doubt that there are suggested pathways, so that we’re “spoilt for choice,” but also no doubt that the problem has not been solved. The “nudge” word was actually from the article!

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The “nudge” referenced here is just a slight initial imbalance to break the symmetry. This is exactly analogous to a coin sitting unstably on edge, so that the slightest nudge from the wind will topple it entirely to heads or tails. Their point is that , in many scenarios, racemic mixtures are unstable and will “topple” to one enantiomer, just like a coin flip. In a metabolic network, this has a domino effect, causing other molecules to be homochiral too. Once we arrive at biological catalysts, homochiral catalysis becomes self propagating.

This general mechanism solves the conceptual problem entirely but leaves many of the details as open questions. Essentially we know that homochirality perse is not an insurmountable problem. If abiogensis were true and we knew the precursor molecules, we know the experiments we could to to settle precisely how homochirality would arise from them by purely natural means. Of course, not knowing these precursors yet (a separate problem all together) there is a limit to how far we can go.

Nonetheless, from a conceptual point of view, it does not appear homochirality is difficult to account for in abiogensis.

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Problem is, you need a continuous “nudge,” like two identical homochiral dimers, perhaps aided by a homochiral crystal lattice, despite the presence of an opposite -,handed dimer, to begin to start a process which can result in enantiomeric excess, much less anything approaching L-enantiopurity. And once you get there, nothing exists to “enforce” the drive back to racemity.
Since so much rides on the issue of homochirality, including the transmission of stable and complex information, it still remains a “chicken and egg” question, as regards abiogenesis.
Here’s to hoping for further breakthough discoveries in this area!

@Guy_Coe that’s not how it works. Spontaneous symmetry breaking, such as when a coin falls to one side, does not require a continuous nudge. It is self propagating and self reinforcing.

The point is that there is something enforce this. Once you are homochiral it is impossible to go towards racemic.

It’s impossible NOT to, by the same process of “symmetry breaking” in reverse.
Just how did we get to a penny landing “heads up EVERY time” in the first place, exactly?
Hope this is at least fun when you run into my stubborn streak, in the name of thoroughgoing science!
Remember, I’m not discounting everything about this, just the adequacy of the explanation we have so far.
Peace to 'ya!

Why should one suppose that this “heads up every time” was the case at the start? In the coin toss scenario, propagation of heads vs. tails is the result of a state being frozen in after the initial (or first) flip. Potentially, either R- or S- stereochemistry could form the basis for life. We’re not sure if there is some other bias either way but once the ball begins rolling, the chemistry that supports life (reproduction) is going to head off in either the R- or S- dominant stereochemistry.


There’s nothing even slightly necessarily the case about that view, biochemically. Enantiopurity is highly unlikely. And yet, vitally necessary in order to be able to convey complex information reliably, and get reliable protein folds. Chicken versus egg questions make this little more than a “just so” story, as far as I can tell.
Castigate away, if you must.

But reactions don’t always produce racemic mixtures, there are entioselective reactions that will prefer one handedness over the other, and that handedness can be propagated chemically (not biologically). I don’t think you’d necessarily have to have all the amino acids all randomly pick the same handedness at the same time to get an enantiopure protein, you just need a bias at some point, I think, that keeps getting stronger as the complex structures get optimized. Even if they aren’t enantiopure, so what? Molecules are really good at exploring configuration spaces and we know that the structures of proteins don’t have to be exact in a lot of places, so what if a few of the wrong enantiomers of some amino acids get in there? It may or may not eliminate the protein’s functionality, but so what, you can always try again.

We would expect that the range of biochemical environments and configurations that life would have come from would be much more diverse than what we see today, given millions of years of optimization.


And your experience as a biochemist is?

What is your point? On the age of the earth, they are not claiming to understand the evidence produced by the experts better than the experts themselves do. It’s very different for their articles about chirality.

Chirality is yet another example falsifying @Guy_Coe 's claim that both groups are looking at the same evidence.

Methinks the right hand knows not what the left hand is doing. Enjoyed the bedtime story. Good night!

there is also the question of how many sequences are functional in the huge sequence space of a tipical gene. there are about 4^1000 different possible combinations of a single gene. what make us to think that all functional genes are near each other in such a huge sapce?