Research into the chemistry of the origin of life got a kickstart with the 1953 publication by Stanley Miller reporting on his experiments that produced amino acids, the building blocks of proteins, from simple substances (H2, CH4, NH3, H2O). The energy source used was a spark discharge (emulating lightning) so these are sometimes referred to as spark discharge experiments. They are also known as Urey-Miller or Miller-Urey experiments, attaching the name of Miller’s supervisor, Harold Urey. While Urey had initially tried to discourage Miller from origin-of-life chemistry, thinking it would not yield anything useful, he was very supportive by giving Miller full credit as sole author on the now-famous 1953 paper.
Seventy years later, such experiments continue. We’ve learned a lot since then. One of the important lessons is that to get a wealth of organic compounds in your experiments, you have to embrace the messy! A recent publication by Root-Bernstein and colleagues is the latest iteration. Here’s a picture of the abstract. (The article is open-access so you can read it in full; DOI: 10.3390/life13020265)
As the title suggests, the experiments produce a rich mess: not just amino acids but also sugars, nucleobases, lipids, and even oligomers – the linking of building blocks to make larger molecules that life utilizes. To the solution they add a particular sea salt from the Mediterranean along with hydroxyapatite (a phosphate salt) and magnesium sulfate (Epsom salt), common salts you might expect to be present in prebiotic chemistry. They ran pre-tests to make sure no ATP or other “biological” molecules were present. Other than that, they tried to stick to Miller’s original initial conditions. One crucial way in which the protocol differed from the original, is that instead of a one-pass reaction (Miller’s was seven days), the researchers regularly reintroduced the “atmospheric gases” (H2, CH4, NH3) every seven days and ran the experiment for multiple weeks. This allows fresh “nutrients” to enter the system so that the chemistry can progress much further – and get messier!
At regular intervals, they remove some sample solution to figure out what’s in it. As expected, within the first week, amino acids show up similar to the ones Miller found. Sugars also show up early. This is expected since the Strecker reaction for producing amino acids relies on the building block formaldehyde, which is also the building block for producing sugars in the formose reaction. By the end of the second week, nucleotides are found, i.e., nucleobases were synthesized. Again, not surprising, since precursors such as HCN and formamide are expected to play a role. The next few weeks bring fatty acids and peptides (oligomers of amino acids) into the mix. There are a couple of dicarboxylic acids (malonic and succinic) and even a few sterols. It’s a very impressive mix – you’ve got stuff for membranes, proteins, sugars, nucleic acids, and metabolic diacids.
The main question that arises in this embarrassment of riches is whether there might be contamination, especially when “regassing” or when removing samples for testing. The authors have expected this question and discussing how they have been careful in their protocols, and how they have tested for errant contamination. While contamination cannot be completely ruled out, it seems to me that they have done due diligence and there don’t seem to be any telltale biomarkers present that you’d find in contaminated samples. The other thing that jumped out at me was the presence of the (sulfur-containing) amino acid cysteine. Cysteine is sort of a magic amino acid that catalyzes all manner of reactions. You can produce it by adding H2S to a spark discharge experiment, but in this instance, H2S was probably produced in situ by reduction of sulfate. This means that thioesters could come into play, although no mention is made of them in the paper. I didn’t expect thioesters to be directly observed because they would hydrolyze easily under these experimental conditions.
The authors rightly emphasize the messiness and the advantages of what they call “dirty” experiments. I’m in agreement. The analysis is painful, and teasing apart what’s going on in such reaction mixtures will continue to be challenging, but I think this is the right approach to make progress in the field. Hurrah for the mess!
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