Three-dimensional complex architectures observed in shock processed amino acid mixtures

Abstract

Asteroid and cometary impacts have been considered one of the possible routes for exogenous delivery of organics to the early Earth. It is well established that amino acids can be synthesized due to impact-driven shock processesing of simple molecules and that amino acids can survive the extreme conditions of impact events. In the present study, we simulate impact-induced shock conditions utilizing a shock tube that can maintain a reflected shock temperature of about 5,500 K for 2 ms time scale. We have performed shock processing of various combinations of amino acids with subsequent morphological analysis carried out using Scanning Electron Microscope (SEM), revealing that the shock processed amino acids demonstrate an extensive range of complex structures. These results provide evidence for the further evolution of amino acids in impact-induced shock environments leading to the formation of complex structures and thus providing a pathway for the origin of life.

1. Introduction

Impact delivery of organic compounds and shock synthesis on planetary bodies, such as the Earth, is considered as one of the possible routes for the supply of prebiotic organics on the early Earth and thus might have played significant in the origin of life (Chyba & Sagan, 1992). Impacts release huge amounts of energy, generating shock waves that provide a sudden increase in pressure and temperature due to shock compression and subsequent cooling due to expansion within a very short time scale. Such extreme conditions have the potential to drive complex chemistry. Gilvarry and Hochstim (1963) first proposed the idea of shock synthesis of organic compounds from meteoroids. Hochstim (1963) further developed this theory based on physical and chemical parameters associated with shock around a meteorite and suggested that complex chemical compounds could be synthesized due to meteorite impact. Thus shock generated from meteors, airbursts and giant impact plumes could have produced large quantities of organics (Chyba & Hand, 2006). Bar-Nun et al. (1970) demonstrated the shock synthesis of amino acids in simulated Earth’s primitive atmosphere using the experimental setup of a shock tube. Since then, many experiments have been performed to understand impact-shock chemistry using various experimental designs to simulate various impact environments. As a result of these investigations, many biomolecules, such as amino acids and nucleobases were known to be synthesized by impact-generated shock and related processes (Bar-Nun et al., 1970; Ferus et al., 2019; Ferus et al., 2020; Martins et al., 2013). Molecular dynamics simulations have also shown that complex chemistry can occur as a result of impact induced shock processes (Goldman et al., 2010; Goldman & Tamblyn, 2013). However, limited studies are performed to understand their further fate in impact-induced shock condition.

Natural impact processes are very complicated in terms of their physical and chemical characteristics. It is impossible to conduct a comprehensive investigation of the full-scaled effect of the impact processes. So the laboratory investigations are focused only on a limited aspect of the impact process at a time. Most common impact experiments are performed using accelerating devices capable of firing millimetre-sized projectiles with a velocity of a few km s⁻¹. A wide variety of accelerating devices are available in the laboratory, such as single- and two-stage light gas guns, Titov guns, and Van de Graaf accelerator, and so forth (Burchell et al., 1999; Gerasimov et al., 1998). Such small-scale impact experiments are beneficial for the investigation of physical aspect of impact events, such as the formation of craters, dynamics of ejecta, and others. High-temperature heating effect of impact process can be simulated in the laboratory to investigate the chemical modification induced by impact. Such methods do not reproduce the actual impact process, rather, they simulate characteristic temperatures (thousands of Kelvin) to understand impact-induced melting, vaporization, and chemical reactions at high temperatures.

Among various sources, a powerful pulsed laser is a valuable tool for laboratory simulation of such a process (Berthe et al., 2011; Křivková et al., 2021). Although laser-induced irradiation experiments do not reproduce the actual high-pressure shock effect of impact, they are capable of simulationing the high-temperature thermal effect of impacts, when decompression occurs after impact and pressure value returns to normal (Ebert et al., 2017). Thus they are a useful tool to investigate chemical processes that are induced as a result of the impact. Many laser experiments have been performed to investigate the melting and mixing process of projectile and target during meteorite impact and shown results as observed in natural impact events (Ebert et al., 2017; Gerasimov et al., 2005). The shock tube also offers exciting characteristics, such as a sharp increase in temperature (thousands of kelvin) within a very short time scale and subsequent cooling, with the rate of millions of degrees per second. Such transient events are expected as a result of impact events. Previously, shock tubes have been used for the shock synthesis of amino acids in the laboratory (Bar-Nun et al., 1970). Our previous investigations have reported complex macroscale structure formation due to the shock processing of amino acids and nucleobases, simulating the high-temperature heating effect of shock using a shock tube (Singh et al., 2020; Surendra et al., 2021). In the present investigation, we extend the previous results to by performing the shock studies on other combinations of amino acids at a temperature of ~5,500 K for an approximate time scale of 2 ms.

2. Materials and methods

Shock experiments were performed at the Material Shock Tube (MST1) facility at IISc Bangalore and the High-Intensity Shock Tube for Astrochemistry (HISTA) facility at PRL Ahmedabad for laboratory simulation of impact-induced shock. The detailed experimental method for shock processing of amino acids, measurement of shock speed, reflected shock temperature, and pressure can be found in Surendra et al. (2021). The mixture of amino acids (amino acids were procured from Sigma Aldrich, purity >99%) studied in these experiments are shown in Table 1 as are the shock parameters for each of the experiment.

Table 1. Experimental shock parameters for different experiments

3. Results and discussion

Sample 1, a mixture of four amino acids, containing leucine, serine, lysine, and glutamic acid, was chosen based on a combination of positively charged, negatively charged, and neutral amino acids, as suggested by Miller and Orgel, such combinations of partially complementary components might have stereochemical basis and potentially worth for investigating experimentally (Miller & Orgel, 1974). The amino acids were mixed in equal weight ratios and shock processed at an approximate temperature of 5,490 K, resulting in large aggregates with a size of hundreds of microns. Imaging the postshock sample, using SEM, showed a solid sphere joint to a cone-like structure with large clumps embedded on the surface (Figure 1a). The magnified images show an irregular texture on the surface with a few holes of diameters of ~10 μm (Figure 1b). The clump architectures seem to be bunched together (Figure 1c), with some of them having blunt corners and others having sharp edges (Figure 1d). These smaller clumped structures were observed to be arranged together in various shapes and many having sharp flakes (Figure 1e–h).

Figure 1. SEM micrograph of shocked sample 1 (containing four amino acids) showing various structures with complex morphology.

In Sample 2, a mixture of eight amino acids was used, mixed in equal weight ratios and shock processed at an approximate temperature of 5,560 K. These amino acids have all been shown to be synthesized together in impact processing of a simple ice mixture (Martins et al., 2013). Given the possibility that all these amino acids can be synthesized together from simple molecules in impact process, we intended to investigate their fate under further impact-shock conditions. SEM observations of the shocked material showed entirely different structures to those seen in sample 1. Long thick threads spanning hundreds of microns with fine textures on the threads were observed as shown in Figure 2a. These threads showed a twisted (Figure 2b) and folded (Figure 2c) appearance.

Figure 2. SEM micrograph of shocked sample 2 (containing eight amino acids) showing (a) long-thread morphology with fine surface texture, (b) twisted-thread morphology, and (c) folded threads.

Sample 3 is a mixture of seventeen amino acids mixed in equal weight ratios. Among the twenty proteinogenic amino acids, only seventeen amino acids were chosen, omitting the three amino acids containing a benzene ring. Thses amino acids with benzene ring (e.g. phenylalanine and tryptophan) were omitted because when they were subjected to shock, they do not show any such structure formation, only fine agglomerated particles were seen in these samples as shown in Supplementary Figure S2. When shock processed at a temperature of ~5,700 K, this sample with a mixture of seventeen amino acids produced needle-type nanorod structures (Figure 3a), many of them locked together or branched from a common point (Figure 3b). Similar structures are shown in Figure 3c. In addition, many zigzag structures of varying sizes seem to be hanging with nanorods (Figure 3c).

Figure 3. SEM micrograph of shocked sample 3 (containing 17 amino acids) showing (a) rod-like morphological structure, (b) branching structure, and (c) zigzag structures at various length scales.

Sample 4 was a mixture of all twenty amino acids being mixed in equal weight ratios and shock processed at a temperature of ~5,490 K. The shocked sample showed the formation of flat ribbon structures (Figure 4a), hollow tubular structures (Figure 4b), and folded sheet structures (Figure 4c).

Figure 4. SEM micrograph of shocked sample 4 (containing 20 amino acids) showing (a) micro-ribbon structure, (b) tubular structure, and (c) folded sheet.

Experiments were repeated for all four samples and similar results were obtained in each case, demonstrating that a wide variety of complex structures are formed due to the impact of a strong shock on the mixture of amino acids. The unshocked mixtures of amino acids were also analyzed using SEM and did not show any such complex structures as shown in Supplementary Figure S1. Further, infrared spectroscopy of shock processed residue shows that amino acid signatures are retained in postshock residues, as shown in Supplementary Figures S3–S6.

The formation of such complex structures under simulated conditions of impact bombardment has the potential to offer new pathways for the emergence of life on prebiotic Earth as well as other planetary bodies. The fundamental principle of organization in a cellular system requires organized self-assembly of building blocks of life such as amino acids, peptides, nucleobases, nucleic acids, and lipids in the prebiotic scenario. The demonstration of the formation of such architectures in simulated prebiotic conditions in the laboratory provides an essential step towards understanding prebiotic cellularity. However, it remains a great challenge to design and engineer such well-defined architectures analogous to a biological system, and advanced understanding in this field requires bridging the gap between molecular chemistry and life (Cardoso et al., 2020). Previous attempts have been made to synthesized such well-defined nanostructures such as nanotubes, nanorods, vesicles, and nanofibers by self-assembly process from molecular building blocks relevant to prebiotic chemistry (Folsome, 1976; Fox, 1973; Lakshmanan & Hauser, 2011; Simionescu et al., 1985; Valladas-Dubois & Prudhomme, 1983; Yanagawa et al., 1988; Zhang, 2012). However, the complex organized architectures achieved by shock processing of amino acids reported in this work have not been revealed before.

4. Conclusions

In this study, laboratory simulations of the high-temperature heating effect induced by shock generated in impact events have been performed. We have shown the spontaneous formation of complex structures organized at different scales in various architecture resulting from shock processing of various mixture of amino acids‥ These results provide elucidation of our understanding of the role played by complex molecules and impact events in the origins of life.