The problem is that no one has any idea how basic that level is. There are probably just about as many candidate primary molecules of life as there are potential means for generating them in specific chiral abundance. In other words, at least for simple molecules, a method for generating an excess of one version or handedness of the molecule over the other.
Although nearly every esoteric force of nature known to man has at some point been conscripted into action on this point by theorists and experimentalists — circularly polarized UV light, the magnetism of the Earth, physical orientation on ordered quartzes, dust grains, or clays, and even the weak nuclear force itself — none of these mechanisms has securely emerged into realm of plausibility.
In the absence of a clear molecular bias generator, the default mode explanation will always circle back to simple auto-amplification: left-handed amino acids or right-handed sugars predominated as a result of a chance excess that seeded and outbred everything else. That’s a highly unsatisfactory resignation. Fortunately, researchers continue to try to recreate conditions in tiny pet universes where handed molecules are eventually overrepresented. The latest idea, just published by a group of Japanese researchers in the journal Chemical Physics Letters, is that chiral amino acids could have been spawned on cold interstellar grains by quantum tunneling hydrogen–deuterium substitution reactions.
At first glance, that looks like a fairly complicated proposition. That’s because it is. However, the researchers were able to show in the lab that for glycine, the simplest amino acid known, H-D substitution can occur through more-or-less spontaneous tunneling at the extremely low temperature of 12 K. That’s good to know, because it happens to be right around the range expected for your generic primordial interstellar molecular cloud.
But what does H-D substitution have to do with chirality? Nothing, directly at least. However some (particularly the chemists out there) may have had a nagging feeling inside that good old glycine is normally an achiral molecule — its mirror looks, and more importantly acts, the same as itself. If you were able to swap one the two hydrogens on the main carbon of glycine, not only would you get a molecule that you can spectrographically detect as unique, it would also now technically be chiral.
When the researchers inoculated their cold glycine incubator with deuterium, this is in fact just what they found. Unfortunately, they could not determine the exact so-called ‘enantiomeric excess’ of the chiral glycine they formed due to the limited amount of their sample (enantiomer is just Greek for ‘opposite part’). A magical thing happens once you have this small excess of ‘d1-glycine’, as it is called. The authors note that several other researchers have found that this homochiral glycine acts as a catalyst for the auto-amplification of much more glycine.
Although the researchers haven’t vetted a definitive mechanism whereby glycine then seeded all life as we now know it, they suggest that it could have effectively sourced molecular chirality to evolving planetary systems. This ‘chirality as commodity’ is an interesting concept. It’s somewhat curious that the authors didn’t reference another recent, and perhaps equally exotic, attempt to explain the chirality of our current lot as a result of nothing less than the weak nuclear force itself.
The authors do acknowledge one particularly glaring, but hopefully not too concerning fact of life: although interstellar gas-phase glycine has been intensively searched for with radio telescopes, it has to date never actually been detected. It can however most definitely be formed by UV or cosmic-ray irradiation in interstellar ice analogues, and that is precisely part of the mechanism for how the other theory of life’s handedness we just mentioned above could potentially work.
Evidence for this weak force theory hinges on a peculiar property known as parity violation, which preferentially leads to the production of left-handed electrons during β-decay. For electrons with a left-handed “helicity,” the directions its of spin and motion are opposite to each other. The authors of the weak force handedness model looked at brominated camphor instead of glycine, and found that polarized electrons could enrich a chiral form of the molecule. There is still some question as to whether the ultimate mechanism involved here would be the electrons or the circularly polarized light they could subsequently generate.
Clearly more experiments on both of these chirality-creating mechanisms is needed before the larger research community might christen one theory or another. In asking the fundamental questions — not only what was the first chiral biomolecule in space, but also what was the first chiral molecule of any kind — we might soon find that we can answer both at once.
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