Peptides are one of the smallest biomolecules and are one of life’s critical building blocks. New research shows that they could form on the surfaces of icy grains in space. This discovery lends credence to the idea that meteoroids, asteroids, or comets could have given life on Earth a kick start by crashing into the planet and delivering biological building blocks.
Peptides are short chains of amino acids, and amino acids are the building blocks of proteins. When peptides join together in a chain, they’re called polypeptides. A chain of polypeptides longer than about 50 is a protein. Sometimes peptides are called the “shorter cousins of proteins.” Proteins are larger biomolecules that play many critical biological roles, so there would be no proteins and no life without peptides. Every cell and all tissue in the body contains peptides.
According to most, Emil Fischer discovered peptides and the peptide bond in the early 20th century. He won the 1902 Nobel Prize in chemistry. Fischer thought the day would come when scientists could use peptide science to synthesize proteins. Now we live in an age of constant peptide discovery and synthesis, leading to more than 80 new therapeutics that treat a wide range of diseases. Peptides are critical, and their use is widespread. Their discovery helped usher in an age marked by a burst in our understanding of biological processes.
Their discovery in space might do the same for the understanding of the origins of life.
Peptides had to originate somewhere. Researchers have discovered other building blocks like amino acids in space in recent years. Astronomers found amino acids in meteorites that fell to Earth, and they’ve discovered glycine in a comet along with ammonium salts and aliphatic compounds. Now it looks like we can add peptides to the list of organic building blocks that occur naturally in space.
If this new research is accurate, natural processes in space can produce basic pre-biological building blocks. This suggests that the possibility of life’s emergence could be widespread and that any fertile planet or moon has likely been seeded with these building blocks.
The research comes from scientists at the University of Jena and the Max Planck Institute for Astronomy. The paper is “A pathway to peptides in space through the condensation of atomic carbon.” The lead author is Serge Krasnokutski, and the paper is published in the journal Nature Astronomy.
“It is an amazing fact that complex organic molecules exist in denser regions between the stars, in protoplanetary disks, primitive meteorites and comets,” said Thomas Henning, co-author of the new study and director at the Max Planck Institute for Astronomy. “They can be formed by a variety of processes from processes in the gas phase, on icy grain surfaces and wet chemistry on the parent bodies of meteorites.”
In their paper, the researchers point out that complex molecules are present in the interstellar medium (ISM). Previous researchers have simulated ISM conditions in labs and produced the same complex molecules. But there’s a limit to that type of research. “Until now, however, only relatively small molecules of biological interest have been demonstrated to form experimentally under typical space conditions,” they explain.
This research focuses on the icy surfaces of dust grains—particularly carbon or silicate atoms—that exist in giant molecular clouds (GMCs.) If we subtract the dominant amounts of hydrogen and helium in GMCs, these atoms make up half of the remaining mass in GMCs. The carbon and silicate atoms are clumped together in conglomerates less than one-millionth of a meter in diameter. Their location inside GMCs is vital because stars, and eventually planets, form from material in GMCs. This is the beginning of the potential link between peptides and life on Earth or elsewhere.
This work is different than previous work that produced small biologically important molecules. Peptides are chains of amino acids, so they’re larger than things like formaldehyde produced previously. This new research focuses on the icy layers of the carbon and silicate atom conglomerates. These layers provide a natural laboratory where materials adhere to the ice and come into close contact with each other. That proximity allows chemical reactions to form more complex molecules.
“Here we prove experimentally that the condensation of carbon atoms on the surface of cold solid particles (cosmic dust) leads to the formation of isomeric polyglycine monomers (aminoketene molecules). Following encounters between aminoketene molecules, they polymerize to produce peptides of different lengths,” the authors write.
This discovery strongly rests on the scientific efforts of lead author Serge Krasnokutski. He’s interested in the chemistry of carbon atoms, particularly cold carbon atoms found in space. Krasnokutski developed and then patented a method to produce cold carbon atoms that allows laboratory experiments to duplicate conditions in space. Labs around the world now use this method.
In 2020 Krasnokutski published results showing that glycine, which is the simplest amino acid, could form on the surface of dust grains with the help of cold carbon atoms. He showed that these chemical reactions didn’t need ultraviolet photons for an energy source.
“Single carbon atoms are surprisingly reactive, even at the lowest temperatures,” Krasnokutski said. “They act as ‘molecular glue’ that joins molecules together, and turns inorganic substances into organic ones.”
Once simple amino acids like glycine form, the next question almost asks itself. Can these acids form into longer chains of peptides or proteins in space?
The only way to find out was to devise and conduct the right experiments. The team of researchers needed to replicate the key conditions of cold carbon atoms in space. They used a method previously developed at MPIA’s Laboratory Astrophysics Group at the University of Jena. The method centers on an Ultra-High Vacuum (UHV) chamber, which creates the vacuum found in molecular clouds in the ISM.
Inside the UHV, the researchers simulated the surface of icy dust grains and deposited atoms and molecules onto their surfaces. They found that aminoketene formed on the cold surface. Aminoketene is the precursor to glycine, the simplest of the amino acids. They also found evidence of peptide bands, a type of chemical bond that bonds amino acids together in short chains of peptides, as well as in longer chains of proteins.
Those peptide bands only showed up when the team warmed their samples up above the temperature inside molecular clouds. So they may occur naturally when a new star forms, or when the dust grains are deposited on a planet’s surface in a star’s habitable zone. “Together, the low-temperature chemistry forming aminoketene and the warming-up letting the aminoketene molecules bond to form peptide could create peptides on interstellar dust grains,” the press release says in summary.
The team has discovered a new pathway to the formation of peptides. And it requires less energy than other pathways, meaning it could happen naturally in the cold of outer space. Also, it requires C atoms, carbon monoxide, and ammonia, which are the most abundant molecule species in the ISM.
Carbon is at the center of this, just as it is in all life. “The single carbon atoms initiate a rich and diverse chemistry. Even under the conditions found in outer space, that chemistry goes much further towards what is needed for the emergence of life than previously thought,” said Krasnokutski.
Scientists are finding that the ingredients for life are more widespread than they thought. With this study, we’re finding that some of these ingredients can combine into biological building blocks in an unlikely spot: the freezing vacuum inside molecular clouds in the ISM. The complexity of those building blocks increases when conditions warm up.
These results strengthen the idea of molecular panspermia. That idea says that while life is rare, the building blocks are widespread. These building blocks have likely spread to every planet and moon, though life is impossible on the majority of worlds. If this is true, then life has likely arisen on multitudes of moons and planets throughout the Universe.
But research shows that many worlds, though they may have experienced a period of habitability, never remained habitable for long. That means Earth is still a rarity, possibly even unique.
It’s the only place that we know of where tiny building blocks forged in the freezing vacuum of outer space eventually evolved into complex life smart enough to study its own origins.