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Ketene and Ammonia Forming Acetamide in the Interstellar Medium

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Ketene and Ammonia Forming Acetamide in the Interstellar Medium JULS Research Article Ketene and Ammonia Forming Acetamide in the Interstellar Medium Akash Kothari *,1, Linglan Zhu 1, Jon Babi 1, Natalie J. Galant2, Anita Rágyanszki 1,3 and Imre G. Csizmadia1,4 O O O O O O O O O H3C NH2 H3C NH2 H3C NH2 H3C NH2 H3C NH2 H3C NH2 H3C NH2 H3C NH2 H3C NH2 O O O O O O O O H3C NH2 H3C NH2 H3C NH2 H3C NH2 H3C NH2 H3C NH2 H3C NH2 H3C NH2 O O O O O O O O O H3C NH2 H3C NH2 H3C NH2 H3C NH2 H3C NH2 H3C NH2 H3C NH2 H3C NH2 H3C NH2 O O O O O O O O ABSTRACT H3C NH2 H3C NH2 H3C NH2 H3C NH2 H3C NH2 H3C NH2 H3C NH2 H3C NH2 Background: Peptide bonds are among the fundamental building blocks of life, O polymerizingO aminoO acids to Oform proteins Othat make upO the structuralO componentsO of O living cells and regulate biochemical processes. The detection of glycine by NASA in H C NH H C NH H C NH H C NH H C NH H C NH H C NH H C NH H C NH 3 2 3comet Wild2 2 3in 20092 suggests3 the2 possibility3 of 2the formation3 2 of biomolecules3 2 in3 extra- 2 3 2 terrestrial environments through the interstellar medium. Detected in the dense molecular O O O O O O O O cloud Sagittarius B2, acetamide is the largest molecule containing a peptide bond and is H C NHhypothesizedH C NH to beH theC precursorNH H toC all NHamino acids;H C asNH such,H viabilityC NH of itsH formationC NH is Hof C NH 3 important2 3 biological2 relevance.3 2 3 2 3 2 3 2 3 2 3 2 O Methods:O UnderO a proposedO mechanismO of ammoniaO and ketene Oreactants, whichO O have also been detected in dense molecular clouds in the ISM, the reaction pathway for the formation of acetamide was modelled using quantum chemical calculations in H3C NH2 H3C NH2 H3C NH2 H3C NH2 H3C NH2 H3C NH2 H3C NH2 H3C NH2 H3C NH2 Gaussian16, using Austin-Frisch-Petersson functional with dispersion density functional theory at a 6-31G(d) basis set level of theory to optimize geometries and determine the thermodynamic properties for the reaction. Stability of the reactants, transition states, and products were examined to establish a reasonable mechanism. Conclusion: Product formation of acetamide was found to be highly exergonic and exothermic with a low energy barrier, suggesting a mechanism that is viable in the extreme density and temperature conditions found in ISM. Keywords: Peptide bond, origins of life, interstellar medium, computational chemistry Affiliations *Please direct correspondence to: 1Department of Chemistry, University of Toronto, Canada [email protected] 2University Health Network, Princess Margaret Cancer Centre, University of Toronto, Canada Published online on December 14, 2020 3Department of Chemistry, York University, Canada 4Institute of Chemistry, University of Miskolc, Hungary 1 The Journal of Undergraduate Life Sciences • Volume 14 • Issue 1 • December 2020 | https://doi.org/10.33137/juls.v14i1.35210 Ketene and Ammonia Forming Acetamide in the Interstellar Medium Research Articles INTRODUCTION more difficult to find acetamide, a 9-atom molecule, versus formamide, a 6-atom molecule8,9. The similarity Of all the molecules discovered in the in abundance of acetamide, ketene, and formamide interstellar medium (ISM), acetamide (CH3CONH2) despite their difference in complexity suggests that is of upmost importance because of its unique bond acetamide is a heavily favoured product in ISM. structure that has significant biological implications. The proposed mechanism for the formation of Discovered in a dense molecular cloud of Sagittarius acetamide is based on ketene and ammonia reactants, B2 in 2006, it is the largest molecule discovered to where ketene has been proven to exist at 15 K through date with a peptide bond (-CO-NH-)1. The ISM is often isotopic shift testing using B3LYP level of theory, easily cited when discussing prebiotic chemistry, as this large forming in ice10. Ketene can also be formed from the territory between star systems contains many unique decomposition of acetamide, acetaldehyde, acetone, environments where many organic molecules have and acetic acid while dissociating to make CH2. been discovered2,3. These molecules containing C, H, These are all constituents used in potential acetamide O, and N are essential to all living systems, that consist pathways10. Ammonia, along with acetamide, has been of proteins. The basic subunit of proteins are amino discovered in Sagittarius B2, the eminent source for acids, which are linked by peptide bonds. Currently, the interstellar study of large complex molecules11. all amino acids, except for glycine, are too large to be Ammonia was also the first polyatomic molecule to detected in the ISM. However, the core components of be discovered in the ISM and can be used to form peptide bonds have been discovered through isocyanic formamide, giving high probability of acetamide 4 12 acid (HNCO), formamide (HCONH2), and acetamide . synthesis as well due to their similarity . Due to its The discovery of acetamide in the ISM has important prevalence and use in other reactions, acetamide is biological relevance because it is considered to be probably synthesized frequently. Because of these the precursor to all amino acids. Although discovered, proven formations, molecules with peptide bonds have the possible formation mechanism is still unknown good probability and relative abundance, with acetamide and understanding potential reaction pathways that forming through a promising pathway shown in Figure can be undertaken provides a better understanding of 1. Experimental simulation of radiation indicated, the formation of other molecules as well. Determining through infrared spectroscopy, that acetamide, urea, individual reactions can help establish patterns in and acetone are all present in the ISM, with urea other reactions including transition state structures, hydrolysis forming ammonia and acetone pyrolysis plausibility, energy barriers, mechanisms, and kinetics forming ketene, two probable reactants for acetamide in the ISM. synthesis6. Although molecules larger than 8 atoms, especially ones possessing all four key elements, are rare among the ISM, acetamide has a high relative abundance almost equal to less complex, but related molecules such as ketene (C2H2O), acetaldehyde (CH3CHO), and formamide (HCONH2), a smaller molecule with a peptide bond4. An isomer of acetamide, N-methyl formamide, was found to be 5-22 times less abundant, potentially indicating acetamide being Figure 1 Proposed Acetamide Synthesis Mechanism the most favourable product to be formed of these Through Ammonia and Ketene Reaction structures4. Most organic molecules including formamide are predicted to form on grain surfaces or ice mantles through hydrogenation, yet experimental abundance In the proposed reaction via ketene and levels of formamide in this specific environment ammonia, a nucleophilic attack results in the formation does not correspond with acetamide, despite being of an ionic intermediary step, leading to the reformation structurally similar5. Synthesis of acetamide was of the bonds that form acetamide. After thorough possible in the presence of CO, NH3, and methane at analysis of the reactants to form a viable mechanism, 77 K in irradiated ices, but this environment is not found it is hypothesized that acetamide will form in a highly in the interstellar medium6. Most of the ISM is relatively exothermic spontaneous reaction that can provide a low particle density and low temperatures found in cold precursor to more complex molecules, such as amino cores with dense molecular clouds, not hot cores that acids. This would support the origin of life theories would be ideal for synthesis7. The activation energy proposing that essential biomolecules were initially barrier associated with forming these molecules in low formed in space and later arrived on Earth, due to the density and temperature can be overcome through UV strongly reductive environment hypothesized by Miller’s radiation, proton flux, or shocks associated with star “RNA World” experiment which demonstrated that formation, potentially using comets to spread1. The formaldehyde and hydrogen cyanide were abundant relative amount of energy required illustrates that it is and important intermediates in glycine synthesis3. The Journal of Undergraduate Life Sciences • Volume 14 • Issue 1 • December 2020 | https://doi.org/10.33137/juls.v14i1.35210 2 Research Articles Ketene and Ammonia Forming Acetamide in the Interstellar Medium METHODS To narrow down possible reaction mechanisms, viable constitutional isomers were examined and the Reaction mechanisms and pathways were most stable and probable were selected as reactants calculated at standard ISM conditions of 0 atm pressure for possible acetamide formation mechanisms17. These and 15K temperature. The geometry optimization rearrangement reactions as well as other synthesis and frequencies of the calculated structures were reactants were eliminated because of unviability due computed using density functional theory (DFT) at to the lack of formation of pi bonds, lack of breakdown Austin-Frisch-Petersson functional with dispersion of sigma bonds, or excessive free energy values17,18. (APFD)/6-31G(d) level of theory to produce quantum Based on the sources of energy available in ISM, chemical calculations using the Gaussian16 software any larger molecules would not break down into package13,14,15. This method was chosen as DFT can acetamide as molecules greater than 9 atoms of this give highly accurate results using a moderate basis complexity are less frequently found in ISM19,20. These set size in comparison to ab initio methods, such as molecules, containing oxygen, carbon, hydrogen, and MP2, which require a much larger basis set to get the nitrogen, usually exist only as polymers19,20. The most same result, costly computationally and also in time. likely synthesis came from ketene and ammonia, and The APFD functional is a newer and faster method structures were located and visually confirmed by that is more accurate than more commonly used intramolecular motions created by frequencies made B3LYP, which also lacks the dispersion correction to in GaussView software21.
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