Scientific efforts aimed at reconstructing the beginnings of life
Formamide-based prebiotic chemistry is a reconstruction of the beginnings of life on Earth, assuming that formamide could accumulate in sufficiently high amounts to serve as the building block and reaction medium for the synthesis of the first biogenicmolecules.[1]
The combinatorial power of carbon is manifested in the composition of the molecular populations detected in circum- and interstellar media (see the Astrochemistry.net[16] web site). The number and the complexity of carbon-containing molecules are significantly higher than those of inorganic compounds, presumably all over the universe. One of the most abundant C-containing three-atoms molecule observed in space is hydrogen cyanide (HCN).[17] The chemistry of HCN has thus attracted attention in origin of life studies since the earliest times, and the laboratory synthesis of adenine from HCN under presumptive prebiotic conditions was reported as early as 1961.[18] The intrinsic limit of HCN stems from its high reactivity, which leads in turn, to instability and the difficulty associated with its concentration and accumulation in unreacted form.[19] The “Warm Little Pond” in which life is supposed to have started, as imagined by Charles Darwin[20][21] and re-elaborated by Alexander Oparin,[22] had most likely to reach sufficiently high concentrations to start creating the next levels of complexity. Hence the necessity of a derivative of HCN that is sufficiently stable to survive for time periods extended enough to allow its concentration in the actual physico-chemical settings, but that is sufficiently reactive to originate new compounds in prebiotically plausible environments.[19] Ideally, this derivative should be able to undergo reactions in various directions, without prohibitively high energy barriers, thus allowing the production of different classes of potentially prebiotic compounds. Formamide fulfils all these requirements and, due to its significantly higher boiling point (210 °C), enables chemical synthesis in a much broader temperature range than water.[1][23]
Given the overwhelming variety of the chemically conceivable molecules, the fact that in biological systems we observe only a small subset of organic molecules has raised questions how and which different reaction pathways could have plausibly lead to the synthesis of pre-biological molecules on the primordial Earth. These are the main objectives of prebiotic chemistry research.
Precursor of biogenic molecules
Figure 1 summarizes the basic chemistry of formamide and its chemical connection with HCN and ammonium formate (NH4+HCOO−), considering selected examples of preparative and degradative reactions.[1]
The synthesis of purine from formamide was first reported in 1980.[24] A series of studies building on this observation was started 20 years later: the synthesis of a large panel of prebiotically relevant compounds (including purine, adenine, cytosine, and 4(3H)pyrimidinone) in good yields was reported in 2001.[25] These products were obtained by heating formamide in the presence of simple catalysts such as calcium carbonate (CaCO3), silica (SiO2), or alumina (Al2O3).
Various energy sources, including thermal energy,[25] UV-radiation,[31] irradiation with high-energy (terawatt) laser pulses,[35] or slow protons[26] were tested. Mimics of different formamide-based prebiotic scenarios have been reconstructed and analyzed, including space-wise solar wind irradiation of meteorites,[26] dynamic chemical gardens,[36] and meteorites in aqueous environments.[37] It has been suggested that the stepwise decrease of the temperature of the prebiotic environment could induce a sequence of strongly non-equilibrium chemical events that led to the emergence of more and more complex species from formamide on the early Earth.[23][38]
For each studied combination of catalyst/energy source/environment, formamide condensed into a variety of different prebiotically relevant compounds, each combination giving rise to a specific set of relatively complex molecules, usually encompassing several nucleobases, amino acids, and carboxylic acids.[1] The highest level of complexity was attained for the formamide/meteorite system,[27] using protonirradiation as the energy source, where the one-pot synthesis of four nucleosides (uridine, cytidine, adenosine, thymidine) was observed.[26] So far, no other one-carbon atom compound has shown the versatility of products that can be formed from formamide under plausible prebiotic conditions in a one-pot chemistry (see Figure 2).[39]
In addition to its dual function of substrate and solvent in one-pot syntheses affording prebiotic compounds as complex as nucleosides and long aliphatic chains,[37] it has been observed that formamide plays a role in the generation of molecules which are closer to the biological domain. In the presence of a phosphate source (e.g., phosphate minerals), formamide promotes the phosphorylation of nucleosides, leading to the formation of nucleotides,[40][41] and strongly stimulates the non-enzymaticpolymerization of 3’,5’ cyclic nucleotides, leading to the abiotic synthesis of RNAoligomers.[42] This is the reason why formamide is considered a plausible medium for prebiotic phosphorylation reactions also in the “discontinuous synthesis” scenario of the origin of life.[43][44] As well as phosphorylation, formamide has been shown to be a competent medium for the production of amino acid derivatives from their simple aldehyde and nitrile precursors, demonstrating that water is not the only solvent that this process can occur in. [45] Most notably, formamide provides a medium for the prebiotic synthesis of cysteine derivatives, not considered previously considered plausible in strictly aqueous prebiotic environments.
References
^ abcdefghSaladino, R.; Botta, G.; Pino, S.; Costanzo, G.; Di Mauro, E. (2012). "Genetics first or metabolism first? The formamide clue". Chem. Soc. Rev. 41 (16): 5526–5565. doi:10.1039/c2cs35066a. PMID22684046.
^Schutte, W.A.; Boogert, A.C.A.; Tielens, A.; Whittet, D.C.B.; Gerakines, P.A.; Chiar, J.E.; Ehrenfreund, P.; Greenberg, J.M.; van Dishoeck, E.F.; de Graauw, T. (1999). "Weak ice absorption features at 7.24 and 7.41 MU M in the spectrum of the obscured young stellar object W 33A". Astron. Astrophys. 343 (3): 966–976. Bibcode:1999A&A...343..966S.
^Bockelee-Morvan, D.; Lis, D.C.; Wink, J.E.; Despois, D.; Crovisier, J.; Bachiller, R.; Benford, D.J.; Biver, N.; Colom, P.; Davies, J.K.; Gerard, E.; Germain, B.; Houde, M.; Mehringer, D.; Moreno, R.; Paubert, G.; Phillips, T.G.; Rauer, H. (2000). "New molecules found in comet C/1995 O1 (Hale-Bopp) - Investigating the link between cometary and interstellar material". Astron. Astrophys. 353 (3): 1101–1114. Bibcode:2000A&A...353.1101B.
^Despois, D.; Crovisier, J.; Bockele-Morvan, D.; Biver, N. (2002). Lacoste, H. (ed.). Proceedings of the Second European Workshop on Exo-Astrobiology, ESA-SP Vol. 518. Noordwijk: Esa Publications Division C/O Estec. pp. 123–127. ISBN929092828X.
^Lis, D.C.; Mehringer, D.M.; Benford, D.; Gardner, M.; Phillips, T.G.; Bockelee-Morvan, D.; Biver, N.; Colom, P.; Crovisier, J.; Despois, D.; Rauer, H. (1997). "New molecular species in comet C/1995O1(Hale-Bopp) observed with the Caltech Submillimeter Observatory". Earth Moon Planets. 78 (1–3): 13–20. Bibcode:1997EM&P...78...13L. doi:10.1023/a:1006281802554. S2CID51862359.
^Hudson, R.L.; Moore, M.H. (2004). "Reactions of nitriles in ices relevant to Titan, comets, and the interstellar medium: formation of cyanate ion, ketenimines, and isonitriles". Icarus. 172 (2): 466–478. Bibcode:2004Icar..172..466H. doi:10.1016/j.icarus.2004.06.011.
^Koike, T.; Kaneko, T.; Kobayashi, K.; Miyakawa, S.; Takano, Y. (2003). "Formation of organic compounds from simulated Titan atmosphere: perspectives of the Cassini mission". Biol. Sci. Space. 17 (3): 188–189. PMID14676367.
^Kröcher, O.; Elsener, M.; Jacob, E. (2009). "A model gas study of ammonium formate, methanamide and guanidinium formate as alternative ammonia precursor compounds for the selective catalytic reduction of nitrogen oxides in diesel exhaust gas". Appl. Catal. B: Environ. 88 (1–2): 66–82. doi:10.1016/j.apcatb.2008.09.027.
^Cernicharo, J. (2011). Gargaud, M.; Amils, R.; Cernicharo Quintanilla, J.; Henderson Cleaves, J.; Irvine, W. M.; Pinti, D.; Viso, M. (eds.). Encyclopedia of Astrobiology. Berlin: Springer Verlag. p. 783-783. ISBN978-3-642-11271-3.
^ abSaladino, R.; Crestini, C.; Costanzo, G.; Negri, R.; DiMauro, E. (2001). "A possible prebiotic synthesis of purine, adenine, cytosine, and 4(3H)-pyrimidone from formamide: implications for the origin of life". Bioorg. Med. Chem. 9 (5): 1249–1253. doi:10.1016/s0968-0896(00)00340-0. PMID11377183.
^ abcdeSaladino, R.; Botta, G.; Delfino, M.; Di Mauro, E. (2013). "Meteorites as catalysts for prebiotic chemistry". Chem. Eur. J. 19 (50): 16916–16922. doi:10.1002/chem.201303690. PMID24307356.
^Saladino, R.; Ciambecchini, U.; Crestini, C.; Costanzo, G.; Negri, R.; Di Mauro, E. (2003). "One-pot TiO2-catalyzed synthesis of nucleic bases and acyclonucleosides from formamide: implications for the origin of life". ChemBioChem. 4 (6): 514–521. doi:10.1002/cbic.200300567. PMID12794862. S2CID2349609.
^Saladino, R.; Neri, V.; Crestini, C.; Costanzo, G.; Graciotti, M.; Di Mauro, E. (2008). "Synthesis and degradation of nucleic acid components by formamide and iron sulfur minerals". J. Am. Chem. Soc. 130 (46): 15512–15518. doi:10.1021/ja804782e. PMID18939836.
^Saladino, R.; Barontini, M.; Cossetti, C.; Di Mauro, E.; Crestini, C. (2011). "The effects of borate minerals on the synthesis of nucleic acid bases, amino acids and biogenic carboxylic acids from formamide". Orig. Life Evol. Biosph. 41 (4): 317–330. Bibcode:2011OLEB...41..317S. doi:10.1007/s11084-011-9236-3. PMID21424401. S2CID19132162.
^Šponer, J.E.; Šponer, J.; Di Mauro, E. (2017). "New evolutionary insights into the non-enzymatic origin of RNA oligomers". Wiley Interdiscip Rev RNA. 8 (3): article No. e1400. doi:10.1002/wrna.1400. PMID27785893. S2CID22479877.
^Costanzo, G.; Saladino, R.; Botta, G.; Giorgi, A.; Scipioni, A.; Pino, S.; Di Mauro, E. (2012). "Generation of RNA molecules by a base-catalysed click-like reaction". ChemBioChem. 13 (7): 999–1008. doi:10.1002/cbic.201200068. PMID22474011. S2CID33632225.
^Benner, S.A.; Kim, H.-J.; Carrigan, M.A. (2012). "Asphalt, water, and the prebiotic synthesis of ribose, ribonucleosides, and RNA". Acc. Chem. Res. 45 (12): 2025–2034. doi:10.1021/ar200332w. PMID22455515.