A team of scientists led by Physical Research Laboratory (PRL) have found that shock-processing of amino acids and nucleobases tend to form complex macroscale structures on a timescale of about 2 milliseconds. This discovery suggests that the building blocks of life could have polymerized not just on Earth but on other planetary bodies through shock-processing of life-forming molecules. The study also provides further experimental evidence for the 'threads' observed in meteorites being due to assemblages of (bio)molecules arising from impact-induced shocks.

The Origin of life remains a well-kept secret with one of the biggest questions and greatest mysteries being “How and where did life start on Earth?”. The molecules that are necessary for life are known to be present in the deeper, darker and the colder regions of space. Simple molecules such as water, ammonia, carbon monoxide and carbon dioxide, methane, nitrogen and oxygen, etc., are processed in the extreme environments of deep space and are converted to larger and complex molecules, such as the amino acids / nucleobases / fatty acids, so called the molecules of life, that are necessary to make the protein, DNA and lipids, respectively, which are basic ingredients of a living cell. Carl Sagan (and his co-workers) had shown that applying shockwaves to such simple molecules can synthesize amino acids in an instant. Instantaneous synthesis of such larger molecules by a shockwave is quite surprising as the conversion of simple to complex molecules by the action of charged particles such as electron, protons and ions takes a long time.

In its early days - billions of years ago, many of the molecules formed in space are thought to have been brought to Earth by cometary or asteroid impacts. During such an impact event the kinetic energy of the incoming bolide is transferred to the target momentarily creating extreme conditions that alter the physico-chemical nature of both the target and the bolide. Shockwaves, of high intensity, generated at the time of such impact events are capable of modifying the chemical nature of not just the simple molecules but even the complex molecules, such as the molecules of life, by providing the energy required to overcome thermochemical barriers and so trigger more complex chemistry. Such high intensity shockwaves can be generated in the laboratory and therefore, conditions experienced in an impact event can be simulated in the laboratory.

In order to understand the fate of the molecules of life in simulated impact induced shock conditions, PRL scientists subjected a range of amino acids and nucleobases, to high intensity shocks using a novel Material Shock Tube (MST)* facility of Indian Institute of Science, Bangalore. A similar shock tube (High-Intensity Shock Tube for Astrochemistry; HISTA) is established at the Physical Research Laboratory, Ahmedabad. Both the shock tubes are similar in their design, and experiments were carried out and repeated at both the shock tube facilities. These shock tubes can generate shockwave speeds of up to Mach number ~5.6, temperature ~8000 K and dwell time ~2 millisecond. Series of experiments were performed with individual and mixture of amino acids (in powder form). We selected amino acids from among the set of 21 amino acids that are found in the living systems. After the shock processing, residue samples were collected at the end of the shock tube. Infrared spectroscopy analysis showed the survivability of amino acids as well as additional band corresponding to peptide bonds to be present in the shock processed sample. We therefore used electron microscopy to image the residue sample.

The results of the imaging analysis were quite remarkable. In the case of shock processed glycine, we could see threads; flat and cylindrical, of length up to 100 micro-meter. In mixtures of amino acids, such as the four mixture; lysine - aspartic acid - glutamic acid - arginine, we could see more complexity in the observed structure (Figure 1). In the case of shock processing a mixture of many of the amino acids (up to 20), we could observe the formation of twisted, threads that split / combine and tube like structures. Whereas when subjecting nucleobases to similar conditions we observed the thread formation, longer and twisted threads forming from randomly oriented solid chunks of nucleobases (Figure 2). More details are provided in two references provided below.

Clearly there is a resemblance in the complex macroscale structures we see in the shocked samples with those found in cellular structures. Similar structures have also been reported to be present in some of the meteorites, although their origin and exact nature, to-date, has been unknown.

These experimental results clearly demonstrate that impact induced shock processes can lead to complex macroscale structures in an instant, a natural selection happening within few milliseconds and brings us a step further in our understanding of the Origin of life from the molecules of life.

Figure 1: Scanning electron micrographs of a mixture of four amino acids (lysine-aspartic acid-arginine-glutamic acid) before and after shock processing. The post shock sample showed the formation of a porous cylindrical structure. Depending on the starting mixture of amino acids the resulting structure differed (see reference 1 for details).