In the 1950s Harold Urey of the University of Chicago and his student Stanley Miller used basic lab glassware containing 200 ml of water and a mix of the gases methane (CH4), ammonia (NH3) and hydrogen sulfide (H2S) to model conditions on the early Earth. Heating this crude analogue for ocean and atmosphere and continuous electrical discharge through it did, in a Frankensteinian manner, generate amino acids. Repeats of the Miller-Urey experiment have yielded 10 of the 20 amino acids from which the vast array of life’s proteins have been built. Experiments along similar lines have also produced the possible precursors of cell walls – amphiphiles. In fact, all kinds of ‘building blocks’ for life’s chemistry turn up in analyses of carbonaceous chondrite meteorites and in light spectra from interstellar gas clouds. The ‘embarrassment of riches’ of life’s precursors from what was until the 20th century regarded as the ‘void’ of outer space lacks one thing that could make it a candidate for life’s origin, or at least for precursors of proteins and the genetic code DNA and RNA. Both kinds of keystone chemicals depend on a single kind of connector in organic chemistry.
Molecules of amino acids have acidic properties (COOH – carboxyl) at one end and their other end is basic (NH2 – amine). Two can react by their acid and basic ‘ends’ neutralising. A hydroxyl (OH–) from carboxyl and a proton (H+) from amine produce water. This gives the chance for an end-to-end linkage between the nitrogen and carbon atoms of two amino acids – the peptide bond. The end-product is a dipeptide molecule, which also has carboxyl at one end and amine at the other. This enables further linkages through peptide bonds to build chains or polymers based on amino acids – proteins. Only 20 amino acids contribute to terrestrial life forms, but linked in chains they can form potentially an unimaginable diversity of proteins. Formation of even a small protein that links together 100 amino acids taken from that small number illustrates the awesome potential of the peptide bond. The number of possible permutations and combinations to build such a protein is 20100 – more than the estimated number of atoms in the observable universe! Protein-based life has almost infinite options: no wonder that ecosystems on Earth are so diverse, despite using a mere 20 building blocks. Simple amino acids can be chemically synthesised from C, H, O and N. About 500 occur naturally, including 92 found in a single carbonaceous chondrite meteorite. They vastly increase the numbers of conceivable proteins and other chain-molecules analogous to RNA and DNA: a point seemingly lost on exobiologists and science fiction writers!
Serge Kranokutski of the Max Planck Institute for Astronomy at the Friedrich Schiller University in Jena, German and colleagues from Germany, the Netherlands and France have assessed the likelihood of peptides forming in interstellar space in two publications (Kranokutski S.A. and 4 others 2022. A pathway to peptides in space through the condensation of atomic carbon. Nature Astronomy, v, 6, p. 381–386; DOI: 10.1038/s41550-021-01577-9. Kranokutski, S.A. et al. 2024. Formation of extraterrestrial peptides and their derivatives. Science Advances, v. 10, article eadj7179; DOI: 10.1126/sciadv.adj7179). In the first paper the authors show experimentally that condensation of carbon atoms on cold cosmic dust particles can combine with carbon monoxide (CO) and ammonia (NH3) form amino acids. In turn, they can polymerise to produce peptides of different lengths. The second demonstrates that water molecules, produced by peptide formation, do not prevent such reactions from happening. In other words, proteins can form inorganically anywhere in the cosmos. Delivery of these products, through comets or meteorites, to planets forming in the habitable ‘Goldilocks’ zone around stars may have been ‘an important element in the origins of life’ – anywhere in the universe. Chances are that, compared with the biochemistry of Earth, such life would be alien in an absolute sense. There are effectively infinite options for the proteins and genetic molecules that may be the basis of life elsewhere, quite possibly on Mars or the moons of Jupiter and Saturn: should it or its chemical fossils be detectable.
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