No Radiation Needed: How Peptide Chains Grow in a Freezing Vacuum

A gold-coated copper plate, chilled to 10 kelvin and sitting in an ultra-high vacuum, makes a serviceable stand-in for a grain of interstellar dust. Freeze the right gases onto it, warm it slowly, and short peptide-like chains assemble on the surface. What remains when the plate comes back to room temperature is a film of stubborn organic residue, and the ordinary thing to do with that film is wash it off. A team led from Jena instead deposited a second helping of the same raw ingredients directly onto the leftovers of the first, and got longer chains than the clean plate had ever produced.

The work was published on 27 August in Communications Chemistry by Serge Krasnokutski of Friedrich Schiller University Jena, Ko-Ju Chuang of Leiden Observatory and colleagues at the University of Poitiers. Its subject is a route to peptides that needs no radiation at all. Atomic carbon, carbon monoxide and ammonia — three of the most abundant reactive species in space — combine on a cold surface into a small, twitchy molecule called aminoketene, with no energy barrier to climb. Warming does the rest. The chains themselves form between 100 and 150 kelvin, the kind of warming a newly lit star supplies to the cloud around it, or a comet's own radioactive decay supplies to its interior.

The plate is not dust. Its careful phrasing is that these processes "mimic the natural condensation of atomic gas onto cold ... dust grains in molecular clouds." What the chamber reproduces is the chemistry of a very cold surface under vacuum, which is the part that carries the argument.

The leftovers were doing the work, not joining in

The catalysis experiment turns on an isotope swap. The first deposition used ordinary carbon; the second used carbon-13, which makes every link built in the second round measurably heavier than its counterpart from the first. Dissolve the whole film afterwards, run it through a high-resolution mass spectrometer, and the two generations of chain are unmistakable. They barely mixed. Chains from the first round did not turn up as building blocks in the second, which is what lets the authors say the old material acted only as a catalyst.

The size of the effect shows up in how far the chain-building got. On a clean gold plate warmed quickly, 0.79 of the chains were dimers, two links and no further. Warmed slowly, which gives the molecules longer in a mobile, liquid-like state, the fraction fell to 0.68. On the residue-coated plate, warmed quickly again, it fell to 0.57, and five-link chains appeared that the fast reference run never produced at all. A coated surface warmed fast beat a bare one warmed far more slowly.

What has not been shown is which component of the residue is responsible. The film is a mixture, and although it is rich in short peptides, the paper has two candidates on the table for this chemistry: the peptides and ammonia, which is a known catalyst for amino-acid polymerization and is present in every run as a reactant. The Methods section is blunt about the position — "Since identity of the catalytically active species is unknown, it is not feasible to test each individual product separately for catalytic activity." The authors' own suspicion is that the peptides are doing it, which would close a loop: peptides promoting the formation of more peptides. They write that if that holds, "the overall process would be autocatalytic," and that "further experiments will be required to reliably confirm this possibility."

Hydrogen builds longer chains out of less material

The second half of the paper adds atomic hydrogen, the most abundant reactive species in the interstellar medium and something any realistic interstellar ice has in quantity. It changes the outcome twice over, in opposite directions. Hydrogen competes for the carbon atoms that the precursor needs, and the amount of aminoketene the system managed to make was about half that of the hydrogen-free run. From that halved supply, the chain-building ran far harder: the dimer fraction dropped below 0.15, and a substantial part of the product would not dissolve at all, which the team reads as long chains too big to go into solution.

Hydrogen also brought side chains. Methyl and hydroxymethyl groups add onto the growing chain to give residues matching alanine and serine, alongside the glycine that dominates the simpler runs. Threonine requires both additions in sequence, and the measurements cannot separate a chain carrying threonine from one carrying alanine and serine at different positions, so the authors call it likely rather than identified.

Two molecules to look for in a meteorite

Most of what forms in these experiments is not a canonical peptide. The chains end in a -COH group where a biological peptide ends in -COOH, a difference that a single step of hydrolysis erases: put them in water and they become ordinary peptides. That detail is what makes the paper's closing proposal specific. Chains written H(NHCH2CO)nH, and a variant capped with NH2 instead, form efficiently here and are unusual enough that finding them in a meteorite or a returned asteroid sample would test whether this pathway runs in space. Nobody has found them there. One related product, the cyclic dimer diketopiperazine, has been reported from carbonaceous meteorites, which is the nearest thing to a precedent.

The analytical capability exists, samples returned from the asteroids Bennu and Ryugu have already yielded amino acids, sugars and nitrogen-rich organics, so the question is what to look for. The authors raise one further possibility worth testing: that some of the amino acids already catalogued from meteorites arrived embedded as insoluble peptide chains, and were freed by the water step of the analysis itself. If that is right, some of those amino acids were peptides until somebody put them in a solvent.