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• W3097950764 abstract "•Pyruvic acid, essential to life, forms in the absence of a biochemical machinery•Interstellar ices are breeding grounds for organic molecules•High-energy electrons initiate chemical reactions in interstellar ices•The results guide the radio-astronomical identification of pyruvic acid in space One of the key questions is how life could have emerged on early Earth and what chemicals and key reactions were involved. Terrestrial biomolecules, such as DNA, RNA, and peptides, formed from building blocks like nucleobases and amino acids. But where do these come from? Simple chemical building blocks could have formed on icy grains in space and may have survived comet impact on the early Earth. Pyruvic acid is widely accepted as a key prebiotic starting material, as it may have served as a fundamental building block for biorelevant molecules. This is underlined by the identification of pyruvic acid in carbonaceous meteorites. This study investigates the formation of pyruvic acid under interstellar conditions to encourage scientists in other fields to consider pyruvic acid as a potential interstellar molecule and include it in their radio-astronomical line searches. For chemists, the study will lead to a better understanding of the fundamental processes of abiotic syntheses of organic molecules. Pyruvic acid represents a key molecule in prebiotic chemistry to form metabolites and amino acids. Without liquid water on the early Earth, endogenous formation of pyruvic acid is unlikely, and an exogenous delivery constitutes an appealing alternative. However, despite the detection of more than 200 molecules in space, pyruvic acid is elusive. Here, we describe its formation by barrierless recombination of hydroxycarbonyl (HOCO⋅) and acetyl (CH3CO⋅) radicals in ices of acetaldehyde (CH3CHO) and carbon dioxide (CO2) modeling interstellar conditions driven by cosmic rays. Exploiting isotopically labeled ices and photoionization reflectron time-of-flight mass spectrometry, the reaction products were selectively photoionized in the temperature-programmed desorption phase and isomers discriminated based on their ionization energies. This reveals a key reaction pathway for pyruvic acid synthesis through non-equilibrium reactions in interstellar cold molecular clouds and star-forming regions, thus offering a unique entry point to abiotic organic synthesis in deep space. Pyruvic acid represents a key molecule in prebiotic chemistry to form metabolites and amino acids. Without liquid water on the early Earth, endogenous formation of pyruvic acid is unlikely, and an exogenous delivery constitutes an appealing alternative. However, despite the detection of more than 200 molecules in space, pyruvic acid is elusive. Here, we describe its formation by barrierless recombination of hydroxycarbonyl (HOCO⋅) and acetyl (CH3CO⋅) radicals in ices of acetaldehyde (CH3CHO) and carbon dioxide (CO2) modeling interstellar conditions driven by cosmic rays. Exploiting isotopically labeled ices and photoionization reflectron time-of-flight mass spectrometry, the reaction products were selectively photoionized in the temperature-programmed desorption phase and isomers discriminated based on their ionization energies. This reveals a key reaction pathway for pyruvic acid synthesis through non-equilibrium reactions in interstellar cold molecular clouds and star-forming regions, thus offering a unique entry point to abiotic organic synthesis in deep space. Pyruvic acid (1, CH3COCOOH) along with its deprotonated pyruvate anion (CH3COCOO−) represent critical molecules in modern biochemistry and plays a key role in contemporary chemical processes that occur within living organisms in order to maintain life (metabolism) (Scheme 1). Serving as the starting material for the Krebs cycle (citric acid or TCA cycle [tricarboxylic acid cycle]) and the pyruvate dehydrogenase complex, 1 links glycolysis with the TCA cycle. After decarboxylation of 1 and nicotinamide adenine dinucleotide (NAD+) reduction, acetyl CoA (2) forms with cofactor A (CoA–SH, Scheme 1). Here, the energy is provided via carbon-carbon bond cleavage and the release of carbon dioxide (CO2) in an oxidative decarboxylation of 1 as the very first step. With thiamine pyrophosphate (TPP, 3)—a thiamine (vitamin B1) derivative with a N-heterocyclic carbene (NHC) as the active entity—the amino enol structure 4 forms, which is referred to as the Breslow intermediate.1Breslow R. Rapid deuterium exchange in thiazolium salts 1.J. Am. Chem. Soc. 1957; 79: 1762-1763Crossref Scopus (239) Google Scholar,2Breslow R. On the mechanism of thiamine action. IV. 1 Evidence from studies on model systems.J. Am. Chem. Soc. 1958; 80: 3719-3726Crossref Scopus (1336) Google Scholar The polarity of the formerly electrophilic carbonyl carbon atom is switched and now reveals nucleophilic character. Therefore, NHC organocatalysis and such Umpolung chemistry has found widespread applications in organic chemistry.3Menon R.S. Biju A.T. Nair V. Recent advances in N-heterocyclic carbene (NHC)-catalysed benzoin reactions. Beilstein J.Org. Chem. 2016; 12: 444-461Google Scholar, 4Flanigan D.M. Romanov-Michailidis F. White N.A. Rovis T. Organocatalytic reactions enabled by N-heterocyclic carbenes.Chem. Rev. 2015; 115: 9307-9387Crossref PubMed Scopus (1116) Google Scholar, 5Hopkinson M.N. Richter C. Schedler M. Glorius F. An overview of N-heterocyclic carbenes.Nature. 2014; 510: 485-496Crossref PubMed Scopus (2379) Google Scholar Only recently, Berkessel et al. characterized several hitherto elusive Breslow intermediates by NMR spectroscopy and X-ray crystallography.6Paul M. Sudkaow P. Wessels A. Schlörer N.E. Neudörfl J.M. Berkessel A. Breslow intermediates from aromatic N-heterocyclic carbenes (benzimidazolin-2-ylidenes, thiazolin-2-ylidenes).Angew. Chem. Int. Ed. Engl. 2018; 57: 8310-8315Crossref PubMed Scopus (35) Google Scholar,7Berkessel A. Elfert S. Yatham V.R. Neudörfl J.M. Schlörer N.E. Teles J.H. Umpolung by N-heterocyclic carbenes: generation and reactivity of the elusive 2,2-diamino enols (Breslow intermediates).Angew. Chem. Int. Ed. Engl. 2012; 51: 12370-12374Crossref PubMed Scopus (131) Google Scholar In prebiotic chemistry, 1 may have served as a fundamental building block for key biological compounds.8Abelson P.H. Chemical events on the primitive earth.Proc. Natl. Acad. Sci. USA. 1966; 55: 1365-1372Crossref PubMed Google Scholar,9Muchowska K.B. Varma S.J. Moran J. Synthesis and breakdown of universal metabolic precursors promoted by iron.Nature. 2019; 569: 104-107Crossref PubMed Scopus (83) Google Scholar Pyruvic acid can be reduced to lactic acid (CH3CH(OH)COOH, 5)10Novikov Y. Copley S.D. Reactivity landscape of pyruvate under simulated hydrothermal vent conditions.Proc. Natl. Acad. Sci. USA. 2013; 110: 13283-13288Crossref PubMed Scopus (40) Google Scholar or acts as a precursor for the amino acid alanine (6) formed in the reductive amination of 1.11Goodfriend G.A. Collins M. Fogel M. Macko S. Wehmiller J.F. Perspectives in Amino Acid and Protein Geochemistry. Oxford University Press, 2000Google Scholar Recently, Muchowska et al. reported a captivating chemical reaction network promoted by Fe(II), in which 1 and glyoxylate (HCOCOO−) build up nine of the 11 intermediates of the TCA cycle.9Muchowska K.B. Varma S.J. Moran J. Synthesis and breakdown of universal metabolic precursors promoted by iron.Nature. 2019; 569: 104-107Crossref PubMed Scopus (83) Google Scholar Adding hydroxylamine (NH2OH) and metallic iron into the system produces four biological amino acids (6, glycine, aspartic acid, and glutamic acid). In this reaction network, both α-ketocarboxylic acids along with hydroxylamine serve as fundamental starting materials. Further, Powner et al. highlighted the critical role of 1 by synthesizing 2-phosphoenolpyruvate (PO42−C(CH2)COO−, PEP)—the highest energy phosphate found in living organisms—from simple prebiotic nucleotide precursors: glycolaldehyde (HOCH2CHO) and glyceraldehyde (HOCH2CH(OH)CHO).12Coggins A.J. Powner M.W. Prebiotic synthesis of phosphoenol pyruvate by α-phosphorylation-controlled triose glycolysis.Nat. Chem. 2017; 9: 310-317Crossref PubMed Scopus (54) Google Scholar Pyruvate synthesis may occur during the abiotic degradation of carbohydrates and their phosphates,13Weber A.L. The sugar model: catalysis by amines and amino acid products.Orig. Life Evol. Biosph. 2001; 31: 71-86Crossref PubMed Scopus (71) Google Scholar,14Keller M.A. Turchyn A.V. Ralser M. Non-enzymatic glycolysis and pentose phosphate pathway-like reactions in a plausible Archean ocean.Mol. Syst. Biol. 2014; 10: 725Crossref PubMed Scopus (109) Google Scholar by the oxidation of lactic acid in aqueous media,15Guzman M.I. Martin S.T. Prebiotic metabolism: production by mineral photoelectrochemistry of α-ketocarboxylic acids in the reductive tricarboxylic acid cycle.Astrobiology. 2009; 9: 833-842Crossref PubMed Scopus (56) Google Scholar and in the presence of iron sulfides in very low yields of only 0.07% under hydrothermal vent conditions, e.g., at elevated temperatures (523 K) and pressures (200 MPa) or in the presence of other transition metals. However, these scenarios rely on very specific reaction conditions to produce labile components, such as 1, which might not have been widespread under prebiotic conditions on the early Earth.16Cody G.D. Boctor N.Z. Filley T.R. Hazen R.M. Scott J.H. Sharma A. Yoder H.S. Primordial carbonylated iron-sulfur compounds and the synthesis of pyruvate.Science. 2000; 289: 1337-1340Crossref PubMed Scopus (314) Google Scholar, 17Roldan A. Hollingsworth N. Roffey A. Islam H.U. Goodall J.B.M. Catlow C.R.A. et al.Bio-inspired CO2 conversion by iron sulfide catalysts under sustainable conditions.Chem. Commun. (Camb). 2015; 51: 7501-7504Crossref PubMed Google Scholar, 18Varma S.J. Muchowska K.B. Chatelain P. Moran J. Native iron reduces CO2 to intermediates and end-products of the acetyl-CoA pathway.Nat. Ecol. Evol. 2018; 2: 1019-1024Crossref PubMed Scopus (72) Google Scholar, 19Preiner M. Igarashi K. Muchowska K.B. Yu M. Varma S.J. Kleinermanns K. Nobu M.K. Kamagata Y. Tüysüz H. Moran J. Martin W.F. A hydrogen-dependent geochemical analogue of primordial carbon and energy metabolism.Nat. Ecol. Evol. 2020; 4: 534-542Crossref PubMed Scopus (50) Google Scholar In the gas phase, 1 thermally decarboxylates (Scheme 1) at elevated temperatures yielding carbon dioxide and methylhydroxycarbene (7, HO–C¨–CH3); the latter undergoes isomerization via [1,2]H-tunneling to acetaldehyde (8, CH3CHO), an observation that leads to the tunneling control reactivity paradigm.20Schreiner P.R. Reisenauer H.P. Ley D. Gerbig D. Wu C.H. Allen W.D. Methylhydroxycarbene: tunneling control of a chemical reaction.Science. 2011; 332: 1300-1303Crossref PubMed Scopus (206) Google Scholar, 21Eckhardt A.K. Gerbig D. Schreiner P.R. Heavy atom secondary kinetic isotope effect on H-tunneling.J. Phys. Chem. A. 2018; 122: 1488-1495Crossref PubMed Scopus (13) Google Scholar, 22Schreiner P.R. Tunneling control of chemical reactions: the third reactivity paradigm.J. Am. Chem. Soc. 2017; 139: 15276-15283Crossref PubMed Scopus (84) Google Scholar In general, hydroxycarbenes undergo barrierless carbonyl ene reactions with carbonyl compounds in the gas phase, yielding biorelevant molecules, including the carbohydrates glycolaldehyde and glyceraldehyde.23Eckhardt A.K. Linden M.M. Wende R.C. Bernhardt B. Schreiner P.R. Gas-phase sugar formation using hydroxymethylene as the reactive formaldehyde isomer.Nat. Chem. 2018; 10: 1141-1147Crossref PubMed Scopus (24) Google Scholar Eckhardt et al.24Eckhardt A.K. Bergantini A. Singh S.K. Schreiner P.R. Kaiser R.I. Formation of glyoxylic acid in interstellar ices: a key entry point for prebiotic chemistry.Angew. Chem. Int. Ed. Engl. 2019; 58: 5663-5667Crossref PubMed Scopus (19) Google Scholar and Turner et al.25Turner A.M. Bergantini A. Abplanalp M.J. Zhu C. Góbi S. Sun B.J. Chao K.H. Chang A.H.H. Meinert C. Kaiser R.I. An interstellar synthesis of phosphorus oxoacids.Nat. Commun. 2018; 9: 3851Crossref PubMed Scopus (20) Google Scholar,26Turner A.M. Abplanalp M.J. Bergantini A. Frigge R. Zhu C. Sun B.J. Hsiao C.T. Chang A.H.H. Meinert C. Kaiser R.I. Origin of alkylphosphonic acids in the interstellar medium.Sci. Adv. 2019; 5: eaaw4307Crossref PubMed Scopus (7) Google Scholar recently demonstrated the formation of glyoxylic acid (HC(O)COOH) and even (di)phosphates (PO43−/P2O74−) in ices modeling interstellar conditions, providing compelling evidence that essential biorelevant molecules, together with their precursors, might form in deep space. At least a fraction of these abiotically synthesized compounds might be incorporated into meteoritic parent bodies and could have survived successive meteorite or comet impact on the Earth, thus reinforcing the theory of an exogenous source of key prebiotic molecules on the Earth.27Engel M.H. Macko S.A. Isotopic evidence for extraterrestrial non- racemic amino acids in the Murchison meteorite.Nature. 1997; 389: 265-268Crossref PubMed Scopus (398) Google Scholar This setting defines an appealing alternative to competing theories like the formation of biorelevant molecules in hydrothermal vents on the prebiotic Earth. Although pyruvic acid is prone to decarboxylation, especially in the presence of transition metals and at elevated temperatures, Cooper et al. identified pyruvic acid in carbonaceous meteorites at 15 nmol g−1, indicating that even such potentially unstable compounds are able to survive the entrance of the meteorite into the atmosphere and participate in further prebiotic processes.28Cooper G. Reed C. Nguyen D. Carter M. Wang Y. Detection and formation scenario of citric acid, pyruvic acid, and other possible metabolism precursors in carbonaceous meteorites.Proc. Natl. Acad. Sci. USA. 2011; 108: 14015-14020Crossref PubMed Scopus (79) Google Scholar Therefore, abiotic formation in the interstellar medium and delivery to the early Earth by meteorites constitutes another plausible source of prebiotic molecules on the Earth. This finding clearly underlines the importance of non-equilibrium processes in the fields of astro- and prebiotic chemistry.29Kaiser R.I. Roessler K. Theoretical and laboratory studies on the interaction of cosmic-ray particles with interstellar ices. III. Suprathermal chemistry-induced formation of hydrocarbon molecules in solid methane (CH4), ethylene (C2H4), and acetylene (C2H2).Astrophys. J. 1998; 503: 959-975Crossref Scopus (109) Google Scholar,30Sutherland J.D. Opinion: Studies on the origin of life — the end of the beginning.Nat. Rev. Chem. 2017; 1Crossref Scopus (101) Google Scholar Here, we demonstrate the first abiotic synthesis of 1 under conditions mimicking extraterrestrial environments via the barrierless radical-radical reaction of the hydroxycarbonyl (HOCO⋅, 9) and acetyl (CH3CO⋅, 10) radicals (Scheme 1) by exposing low-temperature model ices to ionizing radiation with high-energy electrons simulating secondary electrons formed in the path of galactic cosmic ray (GCR) particles penetrating ices on interstellar grains in molecular clouds.31Bennett C.J. Jamieson C.S. Osamura Y. Kaiser R.I. A combined experimental and computational investigation on the synthesis of acetaldehyde [CH3CHO(X1A′)] in interstellar ices.Astrophys. J. 2005; 624: 1097-1115Crossref Scopus (95) Google Scholar,32Bennett C.J. Osamura Y. Lebar M.D. Kaiser R.I. Laboratory studies on the formation of three C2H4O isomers - acetaldehyde (CH3CHO), ethylene oxide (c-C2H4O), and vinyl alcohol (CH2CHOH) - in interstellar and cometary ices.Astrophys. J. 2005; 634: 698-711Crossref Scopus (62) Google Scholar Cold molecular clouds encompass the raw material of stars and planetary systems with nanometer-sized grain particles consisting of amorphous and polyaromatic carbon33Dartois E. Interstellar carbon dust.C. 2019; 5: 80Google Scholar and olivine-type silicates34Rogantini D. Costantini E. Zeegers S. Mehdipour M. Psaradaki I. Raassen A. de Vries C. Waters L. Magnesium and silicon in interstellar dust: an X-ray overview.arXiv. 2020; Google Scholar (“interstellar dust”) accumulating icy layers of mainly water (H2O), methanol (CH3OH), carbon dioxide (CO2), and carbon monoxide (CO) at temperatures as low as 10 K.35Herbst E. van Dishoeck E.F. Complex organic interstellar molecules.Annu. Rev. Astron. 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Laboratory studies on the formation of three C2H4O isomers - acetaldehyde (CH3CHO), ethylene oxide (c-C2H4O), and vinyl alcohol (CH2CHOH) - in interstellar and cometary ices.Astrophys. J. 2005; 634: 698-711Crossref Scopus (62) Google Scholar), ketones (e.g., acetone), and carboxylic acids41Kim Y.S. Kaiser R.I. Abiotic formation of carboxylic acids (RCOOH) in interstellar and solar system model ices.Astrophys. J. 2010; 725: 1002-1010Crossref Scopus (19) Google Scholar,42Zhu C. Turner A.M. Abplanalp M.J. Kaiser R.I. Formation and high-order carboxylic acids (RCOOH) in interstellar analogous ices of carbon dioxide (CO2) and methane(CH4).ApJS. 2018; 234: 15Crossref Scopus (12) Google Scholar (e.g., formic acid43Bennett C.J. Hama T. Kim Y.S. Kawasaki M. Kaiser R.I. Laboratory studies on the formation of formic acid (HCOOH) in interstellar and cometary ices.Astrophys. J. 2011; 727: 27Crossref Scopus (27) Google Scholar and acetic acid44Bergantini A. Zhu C. Kaiser R.I. A photoionization reflectron time-of-flight mass spectrometric study on the formation of acetic acid (CH3COOH) in interstellar analog ices.Astrophys. J. 2018; 862: 140Crossref Scopus (13) Google Scholar).35Herbst E. van Dishoeck E.F. Complex organic interstellar molecules.Annu. Rev. Astron. Astrophys. 2009; 47: 427-480Crossref Scopus (948) Google Scholar When a molecular cloud transits into a star-forming region, matter is incorporated into circumstellar disks, which, in turn, contain the material out of which planets, planetoids, and comets may form. Therefore, 1, initially formed in interstellar ices, can be integrated into matter of solar systems eventually untangling the fundamental chemical reaction(s) of how and where in the universe the molecular precursors to the origins of life can arise. In contrast to the complex mixture of ice constituents found in the interstellar medium, laboratory model ices typically only consist of binary or ternary mixtures of molecules detected in interstellar ices to facilitate the assignment of newly formed molecules based on mass-to-charge ratios, isotopic shifts, and ionization energies. Therefore, only a fraction of the molecules expected to form in interstellar ices are typically found in lab-based experiments. However, molecules formed in different individual simplified mixtures can be expected to form in interstellar ices, based on the availability of the reactants used. The experiments were conceived to unravel the abiotic synthesis of 1 upon exposing polar model ices of 8 and carbon dioxide (CO2) to proxies of galactic cosmic rays in the form of high-energy electrons under conditions mimicking the lifetime of molecular clouds of up to a few million years.45Moore M.H. Hudson R.L. Production of complex molecules in astrophysical ices.Proc. Int. Astron. Union. 2005; 1: 247-260Crossref Scopus (15) Google Scholar These anhydrous model ices were chosen to explore the proof-of-concept that 1 can be synthesized via interaction of interstellar ices with ionizing radiation. Accounting for data from the Spitzer space telescope, ices containing carbon dioxide at levels of up to 37% relative to water were observed toward low- and high-mass stars, field stars, and Galactic center sources46Boogert A.C.A. Pontoppidan K.M. Lahuis F. Jorgensen J.K. Augereau J.C. Blake G.A. Brooke T.Y. Brown J. Dullemond C.P. Evans II, N.J. et al.Spitzer space telescope spectroscopy of ices toward low-mass embedded protostars.Astrophys. J. Suppl. Ser. 2004; 154: 359-362Crossref Scopus (103) Google Scholar, 47Nummelin A. Whittet D.C.B. Gibb E.L. Gerakines P.A. Chiar J.E. Solid carbon dioxide in regions of low-mass star formation.Astrophys. 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Astro Phys. 1999; 343: 966-976Google Scholar but laboratory experiments verified that 8 forms easily on interstellar grains containing carbon monoxide (CO) and methane (CH4).31Bennett C.J. Jamieson C.S. Osamura Y. Kaiser R.I. A combined experimental and computational investigation on the synthesis of acetaldehyde [CH3CHO(X1A′)] in interstellar ices.Astrophys. J. 2005; 624: 1097-1115Crossref Scopus (95) Google Scholar,40Abplanalp M.J. Gozem S. Krylov A.I. Shingledecker C.N. Herbst E. Kaiser R.I. A study of interstellar aldehydes and enols as tracers of a cosmic ray-driven nonequilibrium synthesis of complex organic molecules.Proc. Natl. Acad. Sci. USA. 2016; 113: 7727-7732Crossref PubMed Scopus (72) Google Scholar During the radiation exposure of 8 and carbon dioxide or 8 and carbon dioxide-18O2 (C18O2) ices, multiple novel infrared (IR) absorption features emerged (Figures 1 and S1–S5; Table S1). These absorptions can be linked to four discrete molecules: carbon monoxide (CO, 2,131 cm−1), carbon monoxide-18O (C18O, 2,086 cm−1), the acetyl radical (10, 1,840 cm−1, Figure 1), and methane (CH4, 1304 cm−1).51Jacox M.E. The reaction of F atoms with acetaldehyde and ethylene oxide. Vibrational Spectra CH3CO and CH2CHO free radicals trapped in solid argon.Chem. Phys. 1982; 69: 407-422Crossref Scopus (47) Google Scholar, 52Das P. Lee Y.-P. Bimolecular reaction of CH3 + CO in solid p-H2: infrared absorption of acetyl radical (CH3CO) and CH3-CO complex.J. Chem. Phys. 2014; 140: 244303Crossref PubMed Scopus (10) Google Scholar, 53Kleimeier N.F. Turner A.M. Fortenberry R.C. Kaiser R.I. On the formation of the popcorn flavorant 2,3-butanedione (CH3COCOCH3) in acetaldehyde-containing interstellar ices.ChemPhysChem. 2020; 21: 1531-1540Crossref PubMed Scopus (5) Google Scholar The assignment of 10 is further supported by the isotopic shift in a mixture of acetaldehyde-d4 (CD3CDO) and carbon dioxide-18O2, where the acetyl-d3 radical is detected at 1,851 cm−1, which agrees well with matrix isolation studies of acetyl and previous studies on the irradiation of pure acetaldehyde ices.51Jacox M.E. The reaction of F atoms with acetaldehyde and ethylene oxide. Vibrational Spectra CH3CO and CH2CHO free radicals trapped in solid argon.Chem. Phys. 1982; 69: 407-422Crossref Scopus (47) Google Scholar,53Kleimeier N.F. Turner A.M. Fortenberry R.C. Kaiser R.I. On the formation of the popcorn flavorant 2,3-butanedione (CH3COCOCH3) in acetaldehyde-containing interstellar ices.ChemPhysChem. 2020; 21: 1531-1540Crossref PubMed Scopus (5) Google Scholar However, since electron irradiation can produce a significant array of new species and isomers with close vibrational absorptions, infrared spectroscopy is capable of determining only newly formed functional groups, but often does not allow further identification of individual complex organic molecules. In contrast to inert gas matrix isolation spectroscopy, no inert host gas was used in our experiments, since they aim to replicate interstellar ices; therefore, all observed infrared absorptions were typically very broad and not sharp, which made assignments of specific molecules extremely challenging. The features at 954 cm−1 and 1,052 cm−1 can be linked to CH2 wagging and C–O stretching vibrations, respectively. In addition, a broad absorption feature between 1,200 and 1,300 cm−1 was observed after irradiation that was probably connected to C–O–H deformation vibrations. Due to the low concentration, broad absorptions, and signal overlaps in the critical regions, we could not identify 1 unambiguously. However, since the formation of 10 from 8 released suprathermal hydrogen, which could easily overcome the barrier of addition to carbon dioxide to form 9,54Holtom P.D. Bennett C.J. Osamura Y. Mason N.J. Kaiser R.I. A combined experimental and theoretical study on the formation of the amino acid glycine (NH2CH2COOH) and its isomer (CH3NHCOOH) in extraterrestrial ices.Astrophys. J. 2005; 626: 940-952Crossref Scopus (159) Google Scholar it is very likely that 9 formed in the ices. Unfortunately, in both isotopically labeled and the unlabeled ices, the absorption features of 9 coincided with acetaldehyde absorption features at 1,796 cm−1 and the acetyl feature at 1,833 cm−1, respectively.55Milligan D.E. Jacox M.E. Infrared spectrum and structure of intermediates in the reaction of OH with CO.J. Chem. Phys. 1971; 54: 927-942Crossref Scopus (141) Google Scholar Moreover, no absorptions associated with vinoxy radicals (2-oxoethyl, CH2CHO, 1,542 cm−1) were observed. This is in agreement with a recent study on radical-radical reactions in pure acetaldehyde ices, revealing that reaction products from vinoxy radicals did not form.53Kleimeier N.F. Turner A.M. Fortenberry R.C. Kaiser R.I. On the formation of the popcorn flavorant 2,3-butanedione (CH3COCOCH3) in acetaldehyde-containing interstellar ices.ChemPhysChem. 2020; 21: 1531-1540Crossref PubMed Scopus (5) Google Scholar Comparing the molecular structure of 1 with the radical building blocks 9 and 10 suggests that 1 could have formed in the ices via a barrierless radical-radical recombination (Scheme 1). However, an alternative analytical technique is required for the firm identification of 1. This could be achieved by exploiting photoionization reflectron time-of-flight mass spectrometry (PI-ReToF-MS) during the temperature-programmed desorption (TPD) phase, in which, the irradiated ice was heated to 320 K. This approach allowed us to identify different isomers in the gas phase, based on their distinct ionization energies and desorption profiles, which is outlined below. In contrast to electron impact ionization, which can lead to strong fragmentation even of the parent molecule, photoionization is ideally fragmentation free, thereby facilitating the assignment of mass-to-charge ratios of specific isomers. The PI-ReToF-MS experiments were performed using ices of 8 and 18O isotopically labeled carbon dioxide (C18O2). This combination ensured a unique mass-to-charge (m/z) ratio for the reaction products investigated as the main reaction products from pure acetaldehyde ices subjected to ionizing radiation were diacetyl (m/z = 86), the acetaldehyde dimer (m/z = 88), and the protonated dimer (m/z = 89), all of which had a lower ionization energy than 1.53Kleimeier N.F. Turner A.M. Fortenberry R.C. Kaiser R.I. On the formation of the popcorn flavorant 2,3-butanedione (CH3COCOCH3) in acetaldehyde-containing interstellar ices.ChemPhysChem. 2020; 21: 1531-1540Crossref PubMed Scopus (5) Google Scholar With the isotopic labeling, however, the mass-to-charge ratio of 1-18O2 shifted to m/z = 92, at which no reaction products of 8 or C18O2 were detected at the photon energies used; at the same time, reaction products from pure acetaldehyde ices were not shifted by the isotopic labeling. As adiabatic ionization energies of key complex organic molecules have not yet been determined experimentally, they had to be computed with high confidence. All molecular structures were optimized at B3LYP/cc-pVTZ level of theory in their electronic ground state and in their radical cationic form that resembles the most similar conformation. All electronic energies were improved by extrapolated high level ab initio coupled cluster CCSD(T)/CBS energies and were corrected by zero-point vibrational energies (ZPVEs). The difference of the ZPVE corrected radical cationic electronic energy and the ZPVE corrected energy of the studied molecule equaled the adiabatic ionization energy. As the difference of deuterated (or heavier) and non-deuterated isotopologs in the ZPVEs is generally marginal, we used the ZPVEs of non-deuterated isotopologs for IE calculations and assumed them to be the same for our experiments with heavier isotopologs. For a comparison and error analysis, the adiabatic ionization energies of selected molecules have been computed and compared with experimental values (Table S2). The comparison showed that the computed values were within an error regime of −0.04 eV to +0.19 eV to the experimentally determined ones. This estimated error and a reduction of ionization energies of 0.03 eV due to the electric field of the extractor plate of the time-of-flight spectrometer56Bergantini A. Abplanalp M.J. Pokhilko P. Krylov A.I. Shingledecker C.N. Herbst E. et al.A combined experimental and theoretical study on the formation of interstellar propylene oxide (CH3CHCH2O)—a chiral molecule.Astrophys. J. 2018; 860: 108Crossref Scopus (40) Google Scholar were taken into account for the design of our following experimental studies. In our PI-ReToF-MS studies, 1 and 2-hydroxyacrylic acid (CH2(COH)COOH, 11) could be distinguished based on their experimental (IE = 10.1 eV for 1)57Komorek R. Xu B. Yao J. Ablikim U. Troy T.P. Kostko O. Ahmed M. Yu X.Y. Enabling liquid vapor analysis using synchrotron VUV single photon ionization mass spectrometry with a microfluidic interface.Rev. Sci. Instrum. 2018; 89: 115105Crossref PubMed Scopus (6) Google Scholar and computed adiabatic (CCSD(T)/CBS//B3LYP/cc-pVTZ +ZPVE) ionization energies (Table S3) by tuning the photon energy. Utilizing photons at 10.49 eV, all isomers could be photoionized. By tuning the photon energy down to 9.75 eV, 1 (IE = 9.90–10.02 eV) could not be ionized, leaving only its enol isomer 11 to contribute to the ion signal at m/z = 92. Decreasing the photon energy further to 9.20 eV excluded all conformers of 2-hydroxyacrylic acid (IE = 9.28–9.84 eV). The temperature-dependent mass spectra over the range of m/z = 40–200 at each photon energy are shown in Figure 2. As 8 polymerized quickly upon electron irradiation, it desorbed from its sublimation onset at around 90 K up to a maximum temperature of 320 K and hence dominated the ion counts at 10.49 eV. The spectrum recorded at this energy has therefore been scaled to the height of the highest product peak to provide better visibility of the products formed. To exclude contributions from impurities in the ice, a TPD experiment was also conducted at 10.49 eV on unirradiated ice (Figures 3C and S6), revealing m/z = 117 as the only significant ion signal apart from the reactants. Figure 3A shows the corresponding TPD profiles of m/z = 92 recorded at different ionization energies. At 10.49 eV, a bimodal distribution peaking at 200 K and 235 K was clearly visible, suggesting the formation of both 1 and its enol isomer 11. This was confirmed when comparing the TPD profile recorded at 9.75 eV, which was below the ionization energy of 1. Evidently, the second sublimation event vanished at this photon energy. The early sublimation event only disappeared at 9.20 eV. These findings indicate that the first desorption event (175–225 K) was due to 11 and the second event (200–255 K) was linked to the desorption of 1. The desorption of 1 might have been delayed in comparison with 11 because of one free carbonyl group that might have been involved in an intermolecular hydrogen bonding network within the irradiated ice (see also Figure 3D). In 11, intramolecular hydrogen bonding (cf. relative energies in Table S3, anti-(Z,Z) conformer) was dominating the molecular structure.Figure 3Selected Temperature-Programmed Desorption ProfilesShow full caption(A–D) PI-ReToF-MS data for m/z = 92 collected at 10.49, 9.75, and 9.20 eV, respectively. Collected data (top left, A) and subtraction of signals at 10.49 and 9.75 eV after scaling (top right, B). Blank experiment (bottom left, C) and TPD profiles of pure pyruvic acid (1), pyruvic acid deposited with acetaldehyde-d4 and carbon monoxide-18O2, and the desorption profile of m/z = 92 in the experiments at 10.49 eV after subtracting the contribution from the enol isomer recorded at 9.75 eV (bottom right, D).View Large Image Figure ViewerDownload Hi-res image Download (PPT) (A–D) PI-ReToF-MS data for m/z = 92 collected at 10.49, 9.75, and 9.20 eV, respectively. Collected data (top left, A) and subtraction of signals at 10.49 and 9.75 eV after scaling (top right, B). Blank experiment (bottom left, C) and TPD profiles of pure pyruvic acid (1), pyruvic acid deposited with acetaldehyde-d4 and carbon monoxide-18O2, and the desorption profile of m/z = 92 in the experiments at 10.49 eV after subtracting the contribution from the enol isomer recorded at 9.75 eV (bottom right, D). The individual desorption profiles of 1 and 11 could be extracted from the data by scaling the data collected at 9.75 eV and subtracting them from the TPD profile recorded at 10.49 eV, as visualized in Figure 3B. The assignment of the second desorption event to 1 was further verified when comparing this difference spectrum to that of 1, as shown in Figure 3D. Pure pyruvic acid desorbed at 175–200 K (green curve), however, when co-deposited with acetaldehyde and C18O2 and irradiated using the same parameters as in the experiment (black curve), a second, broad desorption event spanning from 200–235 K became evident. This event originated from 1 trapped inside the polymer matrix of 8, thereby delaying the desorption. This part of the TPD profile matched with that of the desorption event only seen at 10.49 eV (blue curve, Figure 3D) in our experiments, which further confirmed the synthesis of 1 in the ice. The divergence of the profiles at higher temperatures could be explained by 1 trapped in sites of the polymer matrix that were not accessible to the co-deposited pyruvic acid, whereas the absence of the first desorption event was due to the fact that 10 formed from irradiation was trapped in the polymer matrix. These effects were previously also seen for diacetyl forming in pure ices of 8.53Kleimeier N.F. Turner A.M. Fortenberry R.C. Kaiser R.I. On the formation of the popcorn flavorant 2,3-butanedione (CH3COCOCH3) in acetaldehyde-containing interstellar ices.ChemPhysChem. 2020; 21: 1531-1540Crossref PubMed Scopus (5) Google Scholar The results of this study aid in our understanding of how 1 can form abiotically in acetaldehyde-containing interstellar ices upon interaction with ionizing radiation. The molecular structure of 1 suggests a barrierless radical-radical recombination of 9 and 10 as a key formation pathway, since 10 can clearly be identified by infrared spectroscopy and 9 is known to form easily via addition of suprathermal hydrogen to carbon dioxide. Due to overlap of the main absorptions of 1 with other constituents of the ice, its presence in the irradiated ice at 5 K could, however, not be spectroscopically confirmed, and it remains to be seen whether 1 forms upon irradiation at 5 K or during the warm-up phase when radicals become mobile in the ice and can therefore easily recombine. Exploiting TPD with tunable photoionization reflectron time-of-flight mass spectrometry and infrared spectroscopy, this study reveals a non-equilibrium pathway for the formation of 1 in interstellar ices. The non-equilibrium nature of this reaction is evident from the endoergic cleavage of the carbon-hydrogen bond in 8 leading to the formation of the acetyl radical 10 requiring 3.9 eV. Further, the overall reaction leading to the synthesis of 1 from 8 and carbon dioxide is endoergic by 0.8 eV with the energy supplied by the energetic processing of the ice samples." @default.
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• W3097950764 date "2020-12-01" @default.
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• W3097950764 title "Interstellar Formation of Biorelevant Pyruvic Acid (CH3COCOOH)" @default.
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• W3097950764 doi "https://doi.org/10.1016/j.chempr.2020.10.003" @default.
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