Life-giving crystals

Another piece of the puzzle that is life has fallen into place, and it is a spiky crystal.

That right there is one powerful crystal: it is the chemical precursor to RNA, the earliest self-replicating molecule that science is aware of, and thus the starting point for life. It grew from a mixture of basic, common organic chemicals in the presence of a few common amino acids, and is stable and will stay put until the next step, converting it to RNA, takes place.

Not only that, but it's the correct form of the two possible enantiomers that the RNA precursor can take, and it grew naturally out of a mixture of chemicals that didn't start out composed of the pure correct enantiomer, but of a racemic mixture plus a nearly racemic mixture of amino acid that had only 1% excess of the enantiomer they wanted.

The amino acids themselves had previously been shown to form from a mixture of methane, ammonia, hydrogen, water, and lightning (the early earth was a pretty stinky place) in experiments dating back to the 1950s, and also amino acids have been found, intact, on meteorites (including quite a few which are not used by earth life) so assuming they were present is also plausible: this RNA crystal could have formed some time after the amino acids either formed or arrived.

But is a 1% excess plausible? Basic chemistry says when you make a chiral molecule such as RNA or an amino acid without various proteins and other biological molecules to mediate the reaction, you'll get a roughly 50/50 split between left-handed and right-handed enantiomers, but how symmetric is that split? Well, amino acids on meteors have been found with over 15% excess of one enantiomer over another, so it looks like 1% excess is probably not out of line for a natural source.

But why would a tiny 1% excess of the desired amino acid enantiomer cause a pure crystal of the desired RNA enantiomer to grow, when it isn't even an ingredient in the RNA precursor? The key, it turns out, is in the side reactions. As a chemical engineer, I normally try to minimize those since they consume my reagents and don't make the product I want, but in this case they allow the purification to happen when the desired reaction would normally make a roughly 50/50 split of the two forms of the crystal. The side reaction uses the RNA precursor ingredients as well as the amino acid, and the amino acids, being chiral molecules, are fairly specific about which form of the other chiral molecules the the solution they react best with. The side reaction is also faster than the RNA precursor main reaction. So, both forms of the amino acid react with both forms of the glyceraldehyde (one of the two ingredients), but since the amino acid has a slight imbalance and one form is present in greater quantity it sequesters more of its corresponding glyceraldehyde, leaving the other available to form the RNA precursor in its enantiopure form in the slower reaction.

Just to make sure they were seeing what they thought they were seeing, the researchers ran the same reaction with a 1% excess the other way—and got an enantiopure RNA precursor with the opposite chirality. That 1% excess really did control the reaction, even though it didn't participate directly.

On top of that, once a self-replicating molecule starts replicating itself, it very strongly favours making more of its own enantiomer to the exclusion of the opposite form.

All of which leads me to think it's no surprise that we can only process one of the two possible enantiomers in biological systems—but which of the two that turned out to be, could have been pure chance.

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