Meteorite-catalyzed synthesis of nucleosides and other prebiotic compounds

The origin of life is a longstanding enigma that fascinates scientists and the general public alike. Scientific efforts have yielded a significant amount of data over the last 100 y following a fundamental cornerstone publication: Proiskhozhozhdenie Zhizny (Origin of Life) by Alexander Ivanovich Oparin (1). Currently, the formamide-based synthesis of biomolecules catalyzed or initiated by extraterrestrial minerals may have become one of the most discussed landmarks in the field of prebiotic chemistry (2). Saladino et al. (2) show that a formamide-rich extraterrestrial body could deliver a significantly large palette of biomolecules to the early Earth. The authors demonstrate that formamide irradiation by protons—used to simulate the solar wind—in the presence of selected extraterrestrial materials (samples of the main meteorite classes) can lead to the synthesis of many basic components of the primordial soup during such a period of wandering through the early solar system. Using mass spectrometry, the authors detected nucleosides, nucleic bases, amino acids, sugars, and carboxylic acids. Although the study does not identify a universal catalyst for the synthesis of all of the biomolecules, considering the RNA world hypothesis, all of the canonical nucleic bases are synthesized together with ribose, in the presence of, for example, the NWA 1465 meteorite. The synthesis of the entire major groups of biomolecules using one family of catalyst represents a significant success.

Importance of Formamide in Prebiotic Synthesis

According to the fundamental formamide-based synthesis theory, the idea that formamide is a universal parent molecule is supported by several arguments: formamide contains all four essential heteroatoms (HCON), its structure represents the simplest model of the peptide bond, and the compound is sufficiently reactive when exposed to heat, UV, or plasma but is stable under ambient conditions.

The foundation of the present study (2), the story of formamide-based synthesis of nucleic bases, began with the work of Bredereck et al. (3) in 1956 and continued in the research of Yamada and Okamoto in the 1970s (4). However, these authors discuss only the chemical synthesis without any relation to the origin of biomolecules or life itself. In 2001, Saladino, Di Mauro, and coworkers published a study proposing that the formamide molecule is the prebiotic precursor of nucleic bases (5). The authors referred to the successful synthesis of adenine, cytosine, purine, and 4(3H)-pyrimidinone when formamide is heated to 160 °C in the presence of alumina or silica

Saladino et al. show that a formamide-rich extraterrestrial body could deliver a significantly large palette of biomolecules to the early Earth.

(5). During the subsequent decade, Saladino, Di Mauro, and other authors developed complex synthetic pathways that not only account for the synthesis of nucleobases (6, 7) but also the formation of amino acids (8, 9), sugars, nucleosides, nucleotides, simple oligonucleotide sequences, amino sugars, and carboxylic acids (10).

Up to now, only a few experiments have simulated the primordial conditions that successfully demonstrate such a complete formation of at least one series of a group of biomolecules in one pot (i.e., all of the nucleic bases, selected amino acids, and sugars, including ribose). In addition to the present study, which is devoted to proton irradiation, Saladino and Di Mauro studied the thermal synthesis of a wide range of biomolecules in the presence of meteorites (10). The one-pot synthesis of canonical nucleic bases and glycine was demonstrated by Ferus et al. (11), which showed the complete radical chemistry-based origin of the canonical nucleic bases and glycine in an asteroid-impact plasma environment that simulated the late heavy bombardment before the origin of the first living structures. Recently, Saitta and Sajia (9) published a computer-simulated Miller-type experiment. The authors calculated that the chemical pathway leading to synthesis of glycine upon an electric field is a complex reaction, which (again) involves formamide.

In the present study, Saladino et al. detect HCN oligomers, suggesting the occurrence of mechanisms based on the generation of radical cyanide species. This finding is in agreement with the previous simulation of the plasma transformation of formamide using a high-powered laser (11). In contrast to the classic aqueous scenario, with either protons or plasma, the chemistry of formamide-based biomolecule formation may be provided by the main dissociation products of all organic molecules containing oxygen and nitrogen atoms: radicals (∙CN, ∙NH) and vibrationally/rotationally highly excited CO. Specifically, the ∙CN radical plays a fundamental role because of its very rigid structure (strong triple bond) and rather complicated spectrum, which allows transitions between a whole series of excited states. The ∙CN radical is thus capable of absorbing a huge amount of energy and populating a large number of energetic levels. Because of this effect, ∙CN can transfer its energy to any molecule within a broad spectral range, from the UV to the microwave region. The role of ∙CN is essential for all of the chemical reactions, leading to the formation of the “origin of life” important molecules.

Concluding Remarks to the Current Study

Research on the formamide-based synthesis of biomolecules initiated upon extraterrestrial conditions is still not complete and must be critically discussed by the scientific community in detail. However, the current theory is very attractive, because it satisfies several criteria of plausibility: simplicity (i), relevance (ii), and viability (iii).

• i)Simplicity: The formamide-based scenario demonstrates that only one molecule might be the parent for the canonical nucleic bases, sugars, nucleotides, nucleosides, and some amino acids. If we consider life to be a product of chemical evolution with strict rules and determined results, this theory eliminates all of the processes that have the characteristics of a coincidence (complicated pathways that involve a wide range of parent molecules and various conditions). Formamide is abundant in extraterrestrial bodies and it could have been formed as a reaction product of cyanide hydrolysis in the early oceans.
• ii)Relevance: The scenario for the basic biomolecular synthesis during a journey of an extraterrestrial body that was exposed to the solar wind is in agreement with conditions of chemical evolution and the origin of life. During the late heavy bombardment period, this organic matter might have been delivered to the Earth (or the molecules might have been also additionally synthetized) (11).
• iii)Viability: High-energy protons are a sufficiently powerful source for biomolecular synthesis. Moreover, bombardment over several hundred million years represents a significant time horizon for the origin of biomolecules.

The three suggested scenarios do not exclude the possibility that similar biomolecules might also be synthesized anywhere in the universe, on the surface or in the atmosphere of any exoplanet outside of our Solar system in the environment of a parent star. The current theory offers an interesting solution to the origin of life enigma: The origin of life might simply be a result of a stellar (solar) system evolution processes; that is, the chemical transformation of molecules in the early stellar (solar) system and their subsequent delivery during a heavy bombardment era. Therefore, we are, in a certain sense, aliens.