Interstellar formation of glyceric acid [HOCH₂CH(OH)COOH]—The simplest sugar acid

Abstract

Glyceric acid [HOCH₂CH(OH)COOH]—the simplest sugar acid—represents a key molecule in biochemical processes vital for metabolism in living organisms such as glycolysis. Although critically linked to the origins of life and identified in carbonaceous meteorites with abundances comparable to amino acids, the underlying mechanisms of its formation have remained elusive. Here, we report the very first abiotic synthesis of racemic glyceric acid via the barrierless radical-radical reaction of the hydroxycarbonyl radical (HOĊO) with 1,2-dihydroxyethyl (HOĊHCH₂OH) radical in low-temperature carbon dioxide (CO₂) and ethylene glycol (HOCH₂CH₂OH) ices. Using isomer-selective vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry, glyceric acid was identified in the gas phase based on the adiabatic ionization energies and isotopic substitution studies. This work reveals the key reaction pathways for glyceric acid synthesis through nonequilibrium reactions from ubiquitous precursor molecules, advancing our fundamental knowledge of the formation pathways of key biorelevant organics—sugar acids—in deep space.

Glyceric acid was prepared in interstellar ice analogs through barrierless reaction of hydroxycarbonyl with 1,2-dihydroxyethyl.

INTRODUCTION

Since the first identification of the simplest sugar acid—glyceric acid [HOCH₂CH(OH)COOH, 1]—in the Murchison and Murray meteorites by Cooper et al. (1) more than 20 years ago, 1 has been at the center of attention from the astronomy, astrobiology, and laboratory astrochemistry communities (2–7). 1 plays a key role in contemporary biochemical processes vital to cellular metabolism such as glycolysis (Fig. 1). It serves as the starting point to synthesis of 2-phosphoglyceric acid (C₃H₇O₇P, 2) and 3-phosphoglyceric acid (C₃H₇O₇P, 3) via phosphorylation (8). 1 plays a central role in glycolysis, the tricarboxylic acid (TCA) cycle, and photosynthesis in the Calvin cycle, contributing to carbon metabolism (TCA cycle), carbon fixation, and sugar phosphate production (9). Through nucleophilic substitution, 1 may react with ammonia (NH₃) to yield the proteinogenic amino acid serine [HOCH₂CH(NH₂)COOH, 4], eventually producing complex amino acids and peptides. Through the oxidation with nicotinamide adenine dinucleotide (NAD⁺), 1 can be formed from glyceraldehyde [HOCH₂CH(OH)CHO, 5], which can then be reduced by nicotinamide adenine dinucleotide phosphate (NADP⁺) to glycerol [HOCH₂CH(OH)CH₂OH, 6]—a key molecular precursor to phospholipids—the main component of cell membranes (10, 11).

Fig. 1. Formation of glyceric acid in interstellar ices.

The preparation of glyceric acid (1) in low-temperature ices containing carbon dioxide and ethylene glycol (16) is accomplished through energetic processing by GCR proxies. This process involves carbon-carbon bond coupling via recombination of the hydroxycarbonyl radical (HOĊO, 11) with the 1,2-dihydroxyethyl radical (HOĊHCH₂OH, 17). Glyceric acid (1) serves as a precursor for critical biomolecules including the proteinogenic amino acid serine (4), 2-methyl glyceric acid (7), and lactic acid (15). In contemporary biochemistry, glyceric acid further represents molecular building blocks of 2-phosphoglyceric acid (2) and 3-phosphoglyceric acid (3) via phosphorylation reactions, linking to the TCA cycle (top right) and Calvin cycle (bottom right), respectively.

In prebiotic chemistry, 1 may have served as a fundamental building block for crucial biomolecules (Fig. 1) (8). The methylation of 1 can form deoxy sugar acids such as 2-methyl glyceric acid [HOCH₂C(CH₃)(OH)COOH), 7]. Undergoing carbon-carbon bond cleavage, 1 can be converted into formic acid (HCOOH, 8) and glycolic acid (HOCH₂COOH, 9). Recently, Marks et al. (12) revealed the formation of 9 in irradiated interstellar model ices of carbon dioxide (CO₂) and methanol (CH₃OH, 10); 9 was formed through barrierless radical-radical recombination of the hydroxycarbonyl radical (HOĊO, 11) with the hydroxymethyl (ĊH₂OH) radical. Oxidation of 1 results in the formation of biorelevant hydroxypyruvic acid [HOCH₂C(O)COOH, 12] and tartronic acid [HOOCCH(OH)COOH, 13] (13), which can be decarboxylated into 9. Further, the cleavage of the carbon-oxygen bond in 1 prepares 3-hydroxypropionic acid (HOCH₂CH₂COOH, 14) and the biomolecule lactic acid [CH₃CH(OH)COOH, 15] (14).

Glyceric acid (1) is a simple representative of sugar-related compounds and can form from the simplest sugar molecule glyceraldehyde (5) via oxidation (Fig. 1). Sugar acids and sugar alcohols are vital precursors to the molecular building blocks of biomolecules and could have seeded the evolution of life as we know it (15). Therefore, the elucidation of their formation routes can aid in understanding the molecular mass growth processes of astrobiologically relevant molecules necessary for the origins of life. Among some 300 molecules identified in the interstellar medium (ISM) (16), sugar-related molecules such as glycolaldehyde (HCOCH₂OH) have been identified toward the molecular core Sagittarius B2 (17, 18). Although 1 has not yet been identified in the ISM (16), it has been observed in laboratory simulation experiments. Ices composed of electron-irradiated methanol (10) ice (4) and ultraviolet-irradiated ice mixtures of 10, ammonia (NH₃), and water (H₂O) (19, 20) reveal its formation through the hydrolysis of organic residues remaining after temperature-programmed desorption (TPD) to room temperature. Furthermore, 1 has been identified in the hydrolyzed organic matter of carbonaceous chondrites with an abundance (80 nmol g⁻¹) comparable to amino acids (1), indicating that 1 is able to survive the entrance of the meteorite into the atmosphere of the early Earth when embedded in a matrix (21). However, unraveling the abiotic formation mechanisms of molecular 1 in interstellar analog ices is still elusive. Under interstellar conditions, the formation of biologically relevant molecules is feasible via abiotic (nonbiological) reaction mechanisms, in which these molecules can be formed in interstellar ices composed of simple organic molecules when subjected to ionizing radiation. This understanding is of fundamental importance to the astronomy and astrochemistry communities to rationalize the origin and formation routes of biorelevant molecules linked to the origins of life and the simplest sugar acid 1 in particular.

Here, we demonstrate the very first abiotic synthesis of 1 in low-temperature (5 K) carbon dioxide and ethylene glycol (HOCH₂CH₂OH, 16) ice mixtures. This was accomplished via the barrierless radical-radical reaction of the hydroxycarbonyl (HOĊO, 11) with the 1,2-dihydroxyethyl (HOĊHCH₂OH, 17) radicals (Figs. 1 and 2). These model ices were exposed to energetic electrons mimicking secondary electrons generated in the track of galactic cosmic rays (GCRs) penetrating ices in cold molecular clouds aged a few million years (22). By merging our experiments with calculations, 1 was identified in the gas phase through vacuum ultraviolet (VUV) photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS) during the TPD of irradiated ices based on computed adiabatic ionization energies (IEs) and isotopic substitution studies. This finding notably advances the knowledge of the formation pathways of key biorelevant organics—sugar acids—in deep space. Carbon dioxide is one of the most ubiquitous molecules in interstellar ices and has been detected at fractions of up to 40% with respect to water toward the AFGL 989 source (23). Ethylene glycol (16) generated via radical-radical recombination of the hydroxymethyl (ĊH₂OH) radicals (4, 24) is abundant in the ISM, Murchison meteorites, and comets (1, 18, 25); the abundance of 16 has been found to be 0.25% with respect to water in the Comet C/1995 O1 (Hale-Bopp) (26). Therefore, the formation of 1 in interstellar ices containing carbon dioxide and ethylene glycol represents a plausible mechanistic pathway as demonstrated here. Once formed, 1 can be incorporated into planetesimals and ultimately delivered to planets like early Earth via meteorites, thus participating in a complex chain of chemical reactions leading to the molecular precursors important for the origins of life.

Fig. 2. Reaction scheme leading to three C₃H₆O₄ (m/z = 106) isomers (1, 18, 19) in irradiated carbon dioxide–ethylene glycol ices.

Barrierless radical-radical reactions 11 plus 17 and 11 plus 20 produce 1 and 18, respectively; tautomerization of 1 may lead to the enol 19. The adiabatic IEs are computed at the CCSD(T)-F12b/cc-pVTZ-F12//B3LYP/aug-cc-pVTZ level of theory including zero-point vibrational energies and are corrected by incorporating error (table S4). The insert (bottom right) compiles the computed IEs of isomers (black solid line) and ranges of the conformers (gray area) after error analysis. VUV photon energies (dashed lines) at 11.10 and 8.77 eV were used to photoionize subliming molecules during the TPD process.

RESULTS

Infrared spectroscopy

Fourier transform infrared (FTIR) spectroscopy was used to monitor the chemical evolution of the ices before (black line) and after (red line) the irradiation (figs. S1 to S4). Detailed assignments of the FTIR spectra are summarized in tables S1 to S4. The absorptions of the deposited ices can be attributed to the fundamentals and combination modes of carbon dioxide and ethylene glycol (16). After irradiation at 5 K, several absorption features were detected. The region between 2200 and 1400 cm⁻¹ can be deconvoluted into several Gaussian peaks (fig. S1B). The infrared absorption band at 2140 cm⁻¹ is assigned to the stretch of carbon monoxide (CO); this is confirmed from the observation of C¹⁸O at 2090 cm⁻¹ and ¹³CO at 2092 cm⁻¹ (figs. S2B and S4B) (27). The absorptions at 1767 and 1657 cm⁻¹ are linked to the formation of one or more carbonyl (C═O)–containing species and the bending mode (ν₂) of water, respectively (12). After accounting for vibrational anharmonicity at the B3LYP/aug-cc-pVTZ level of theory, the strongest vibration mode of 1 is predicted at 1803 cm⁻¹ for 1g, i.e., the lowest energy conformer of 1 (table S5). Since vibrational frequencies of matrix-isolated species are often shifted from their gas-phase values (28) and FTIR absorptions can be broadened because of the ice matrix, the anharmonic frequency is close to the observed absorptions at 1767 cm⁻¹, indicating that this absorption may be linked to 1 in a polar environment. The absorptions at 1722 and 1500 cm⁻¹ are associated with the CO stretching mode (ν₂) and CH₂ bending mode (ν₃) of formaldehyde (H₂CO) (27, 29); these features can also be identified in deuterium-labeled (fig. S3) and ¹³C-labeled (fig. S4) ices through absorptions at 1703 cm⁻¹ (D₂CO, ν₂), 1681 cm⁻¹ (H₂¹³CO, ν₂), and 1500 cm⁻¹ (H₂¹³CO, ν₃) (12, 27, 29, 30). The absorptions at 1849 cm⁻¹ in processed CO₂−HOCH₂CH₂OH ice and 1811 cm⁻¹ in irradiated ¹³CO₂−HO¹³CH₂¹³CH₂OH ice (figs. S1 and S4) can be assigned to the trans-hydroxycarbonyl radical (HOĊO, ν₂) and/or formyl radical (HĊO, ν₃). Because of the matrix effect, the absorptions of fundamental (CO stretch) of trans-hydroxycarbonyl radical (ν₂) and formyl radical (ν₃) are close to one another (31, 32) and cannot be clearly distinguished here. Because of the overlapping absorption features resulting from a wide variety of complex organic molecules produced during radiation processing, infrared spectra cannot uniquely identify complex organic molecules such as 1, but only their functional groups, highlighting the need for an alternative analytical technique to detect individual reaction products (33, 34).

Photoionization reflectron time-of-flight mass spectrometry

PI-ReTOF-MS represents an ideal technique to identify individual C₃H₆O₄ isomers in the gas phase based on their distinct adiabatic IEs and mass-to-charge ratios (m/z) (33, 35). Separate experiments exploiting two photon energies of 11.10 and 8.77 eV were selected to distinguish the first-generation products 1 and 2-hydroxyethyl hydrogen carbonate (HOCH₂CH₂OCOOH, 18) formed through radical-radical reactions as well as the second-generation product propene-1,1,2,3-tetrol [HOCH₂CH(OH)C(OH)₂, 19] accessed via tautomerization of 1 (Fig. 2). Photons (11.10 eV) aid in the photoionization of all three isomers if formed, while 8.77 eV photons only ionize 19 (IE = 7.24 to 7.53 eV), but neither 1 (IE = 9.69 to 10.17 eV) nor 18 (IE = 9.79 to 11.09 eV). Figure 3 compiles the PI-ReTOF-MS of the subliming molecules from the exposed carbon dioxide–ethylene glycol ices during the TPD phase. Tentative assignments of C₂H₄O₃ and C₃H₆O₃ isomers are presented in the Supplementary Materials (figs. S7 and S8). Concentrating on the C₃H₆O₄ isomers, the TPD profile of the ion signal at m/z = 106 for the irradiated CO₂–HOCH₂CH₂OH ice at 11.10 eV (black line, Fig. 4A) shows sublimation events at 240 K (peak 1) and 273 K (peak 2). Since the signal at m/z = 106 can be associated with the molecular formulae C₂H₂O₅, C₃H₆O₄, C₄H₁₀O₃, C₆H₂O₂, C₇H₆O, and C₈H₁₀, isotopically labeled reactants were exploited to assign the molecular formula(e). The replacement of the CO₂–HOCH₂CH₂OH ice by ¹³CO₂–HO¹³CH₂¹³CH₂OH ice shifts the m/z by 3 atomic mass unit (amu) from m/z = 106 (C₃H₆O₄⁺) to m/z = 109 (¹³C₃H₆O₄⁺); this finding confirms the presence of three carbon atoms (Fig. 4A). The substitution of CO₂ by C¹⁸O₂ results in products with two ¹⁸O atoms that can be observed at m/z = 110 (C₃H₆O₂¹⁸O₂⁺) in the C¹⁸O₂–HOCH₂CH₂OH ice. Last, deuterated methylene (CD₂) moieties (CO₂–HOCD₂CD₂OH ice) result in products with five deuterium atoms; the observed signals shift to m/z = 111 (C₃HD₅O₄⁺) (Fig. 4B). Hence, the sublimation events (peaks 1 and 2) at m/z = 106 can be clearly assigned to a molecule of the formula C₃H₆O₄.

Fig. 3. PI-ReTOF-MS data as a function of temperature and m/z ratios during the TPD of the carbon dioxide–ethylene glycol ices.

Data were recorded for the unirradiated (blank) CO₂–HOCH₂CH₂OH ice at 11.10 eV (A), the irradiated (30 nA, 60 min) CO₂–HOCH₂CH₂OH ice at 11.10 eV (B) and 8.77 eV (F), the irradiated C¹⁸O₂–HOCH₂CH₂OH ice at 11.10 eV (C), the irradiated CO₂–HOCD₂CD₂OH ice at 11.10 eV (D), and the irradiated ¹³CO₂–HO¹³CH₂¹³CH₂OH ice at 11.10 eV (E).

Fig. 4. Ion signal during TPD of irradiated carbon dioxide–ethylene glycol (16) ices shown as a function of temperature.

(A) TPD profiles measured at 11.10 eV with irradiated (30 nA, 60 min) CO₂–HOCH₂CH₂OH ice (m/z = 106), ¹³CO₂–HO¹³CH₂¹³CH₂OH ice (m/z = 109), and C¹⁸O₂–HOCH₂CH₂OH ice (m/z = 110). (B) TPD profiles measured at 11.10 eV at m/z = 109 and m/z = 111 in irradiated (30 nA, 60 min) CO₂–HOCD₂CD₂OH ice. (C) TPD profiles measured at 8.77 eV at m/z = 106 in CO₂–HOCH₂CH₂OH ice irradiated with a low dose (30 nA, 60 min) and a higher dose (100 nA, 120 min).

The TPD profile of m/z = 106 (C₃H₆O₄⁺) at 11.10 eV reveals sublimation events at 240 K (peak 1) and 273 K (peak 2) (Fig. 4A). A blank experiment conducted without exposing the ices to ionizing radiation under otherwise identical conditions does not reveal any ion signal at m/z = 106 between 210 and 300 K (Fig. 4A). This finding confirms that peaks 1 and 2 are the results of the irradiation of the ices. At 11.10 eV, all three isomers 1 (IE = 9.69 to 10.17 eV), 18 (IE = 9.79 to 11.09 eV), and 19 (IE = 7.24 to 7.53 eV) can be ionized (Fig. 2 and table S5). Thereafter, the photon energy was reduced to 8.77 eV; at this energy, only the enol tautomer 19 can be ionized if present. However, upon reducing the photon energy to 8.77 eV, peaks 1 and 2 are absent (Fig. 4C). An additional experiment was performed with a higher dose of irradiation (100 nA, 120 min) to probe the formation of 19. Even at this increased dose, which corresponds to 12 ± 2 eV molecule⁻¹ of carbon dioxide and 21 ± 3 eV molecule⁻¹ of 16, no ion signal was detectable at m/z = 106, confirming the absence of 19. Therefore, peaks 1 and 2 can only be associated with isomers 1 and/or 18. Because of the overlap of their IEs, it is imperative to confirm their formation using isotopically labeled ices. Here, the identification of isomers 1 and 18 was achieved by exploiting partially deuterated reactants (CO₂–HOCD₂CD₂OH ice) during photoionizing at 11.10 eV. This strategy is highlighted in fig. S5 and reveals that their ion signals can be separated through the detection of m/z = 109 [HOCD₂CD(OH)COOH⁺] for 1 and m/z = 111 (HOCD₂CD₂OCOOD⁺) for 18. In the processed CO₂–HOCD₂CD₂OH ice (Fig. 4B), the TPD profile at m/z = 109 only shows one sublimation event (peak 2), indicating that peak 2 is linked to 1. For ion signals of m/z = 111, the TPD profile shows both peak 1 and peak 2. This is counterintuitive since only one peak connected to m/z = 111 (HOCD₂CD₂OCOOD⁺) for 18 should be present. In the processed CO₂–HOCD₂CD₂OH ice, it is likely that 2-hydroxyethoxy-d₄ (È®CD₂CD₂OH, 20) can isomerize to 17-d₄ (DOĊDCD₂OH), which then reacts with hydroxycarbonyl-d₁(DOĊO, 11) radical to form 1-d₅ [HOCD₂CD(OD)COOD⁺; m/z = 111], leading to the presence of peak 2. Therefore, only peak 1 can be clearly associated with 18. This isomerization was also tackled computationally. Using the AE-CCSD(T)/CBS//AE-MP2/aug-cc-pVTZ level of theory, Wang and Bowie (36) calculated the isomerization reaction from 2-hydroxyethoxy (È®CH₂CH₂OH, 20) radical to 1,2-dihydroxyethyl (HOĊHCH₂OH, 17) radical to be exoergic (44 kJ mol⁻¹) with a reaction barrier of 117 kJ mol⁻¹; this barrier can be overcome by the kinetic energy of the incident electrons. Moreover, the sublimation sequences of 18 at 240 K and 1 at 273 K correlate with an enhanced polarity and increased number of hydroxyl groups (–OH) of the subliming isomers (24, 37–39). In particular, 1 carries an extra hydroxyl group in comparison to 18, leading to an increase of the sublimation temperature by 33 K. The detected counts of 1 and 18 in the CO₂–HOCH₂CH₂OH system were 230 ± 20 and 270 ± 15 counts, respectively. In addition, the TPD profile of the ion signal at m/z = 106 for the irradiated CO₂–HOCH₂CH₂OH ice matches the TPD profile at m/z = 89 (fig. S6), which may be caused by the photodissociation of 1 and 18 by losing a hydroxyl group (–OH) upon ionization at 11.10 eV.

Having provided compelling evidence on the preparation and detection of 1 and 18, we shift our attention now to their formation mechanisms. First, reactions (1) to (3) lead to unimolecular decomposition of 16 to 17 [HOĊHCH₂OH; reaction (1)] and to 20 [È®CH₂CH₂OH; reaction (2)]; the suprathermal hydrogen atoms generated add to the C═O bond in carbon monoxide forming the hydroxycarbonyl radical 11(HOĊO) [reaction (3)]. The decomposition of 16 is endoergic by 398 ± 4 kJ mol⁻¹ for reaction (1) and 443 ± 4 kJ mol⁻¹ for reaction (2) (40, 41); this endoergicity can be compensated for by an energy transfer from the impinging electrons to 16 (42). Reactions (1) and (2) closely resemble the decomposition of methanol (10) that produces hydroxymethyl (ĊH₂OH) and methoxy (CH₃È®) radicals (24, 43). Previous work by Song et al. (3) revealed that reaction is exoergic by 22 kJ mol⁻¹ with an entrance barrier of 106 kJ mol⁻¹ (1.1 eV) calculated at the MRCISD+Q/cc-pVTZ//CASSCF/cc-pVTZ level of theory (44). Note that gas-phase calculations may overestimate the real barrier of the reactions because nearby molecules in the ice phase could be involved in the reaction mechanism (45, 46). Furthermore, the suprathermal hydrogen atoms have excess kinetic energy of a few electron volts (47); this energy can be used to overcome the entrance barrier of reaction (3), leading eventually to 11 (35) as identified through FTIR spectroscopy at 1849 cm⁻¹ for 11 (ν₂; HOĊO) in irradiated CO₂−HOCH₂CH₂OH ice and at 1811 cm⁻¹ for 11-¹³C (ν₂; HO¹³ĊO) in irradiated ¹³CO₂−HO¹³CH₂¹³CH₂OH ice.

$${\text{HOCH}}_2{\text{CH}}_2\text{OH} (\mathbf{16})\to \text{HO}\dot{\mathrm{C}}{\text{HCH}}_2\text{OH} (\mathbf{17})+\dot{\mathrm{H}}$$ (1)

$${\text{HOCH}}_2{\text{CH}}_2\text{OH} (\mathbf{16})\to \dot{\mathrm{O}}{\text{CH}}_2{\text{CH}}_2\text{OH} (\mathbf{20})+\dot{\mathrm{H}}$$ (2)

$${\text{CO}}_2+\dot{\mathrm{H}}\to \text{HO}\dot{\mathrm{C}}\mathrm{O} (\mathbf{11})$$ (3)

Second, the preparation of 1 and 18 relies on barrierless radical-radical recombination of 11 with 17 [reaction (4)] and 20 [reaction (5)] radicals, respectively. Recall that the TPD profile at m/z = 106 (C₃H₆O₄⁺) in CO₂–HOCH₂CH₂OH ice shifts 4 amu to m/z = 110 (C₃H₆O₂¹⁸O₂⁺) in C¹⁸O₂–HOCH₂CH₂OH ice and 5 amu to m/z = 111 (C₃HD₅O₄⁺) in CO₂–HOCD₂CD₂OH ice (Fig. 4), indicating that the formation of 1 or 18 involves one carbon dioxide and one 16 molecule.

$$\text{HO}\dot{\mathrm{C}}\mathrm{O} (\mathbf{11})+\text{HO}\dot{\mathrm{C}}{\text{HCH}}_2\text{OH} (\mathbf{17}) \to {\text{HOCH}}_2\text{CH}(\text{OH})\text{COOH} (\mathbf{1})$$ (4)

$$\text{HO}\dot{\mathrm{C}}\mathrm{O} (\mathbf{11})+\dot{\mathrm{O}}{\text{CH}}_2{\text{CH}}_2\text{OH} (\mathbf{20}) \to {\text{HOCH}}_2{\text{CH}}_2\text{OCOOH} (\mathbf{18})$$ (5)

DISCUSSION

In conclusion, this study presents an abiotic route to a key biomolecule glyceric acid (1)—the simplest sugar acid—via barrierless radical-radical recombination of 11 with 17, providing crucial steps toward a systematic understanding of how sugar acids can be formed in carbon dioxide–containing interstellar ices. 1 and its isomer 2-hydroxyethyl hydrogen carbonate (18) were formed in interstellar model ices of carbon dioxide and ethylene glycol upon exposure to ionizing radiation at low temperatures of 5 K. Using tunable PI-ReTOF-MS along with isotopic labeling experiments, 1 and 18 were detected in the gas phase during the TPD. The overall reaction energy leading to 1 from carbon dioxide and 16 [reactions (1), (3), and (4)] is endoergic by 35 kJ mol⁻¹ as compensated by the kinetic energy of the incident electrons (41, 48), thus highlighting the necessity of nonequilibrium chemistry in its formation in the ISM. Carbon dioxide and 16 are abundant in the ISM and have both been detected in molecular clouds such as Orion-KL—the star-forming region of the Orion Nebula (25, 49), and the presence of 1 and 18 in interstellar environments is therefore plausible. Thus, the hitherto astronomically unobserved 1 and 18 represent promising candidates for future astronomical searches via gas-phase rotational emissions exploiting telescopes such as the Atacama Large Millimeter/submillimeter Array (ALMA). Once formed within interstellar ices in cold molecular clouds, 1 and 18 can be incorporated into accreting planetoids, asteroids, and comets (50). Ultimately, a fraction of these molecules can be delivered to planets like the early Earth and act as an exogenous source of biomolecules and their feedstocks (6). Detailed analyses of meteorites such as Murchison revealed that not only sugar-related organic compounds including 1 but also amino acids and dipeptides can be embedded in meteoritic matter and survive the harsh conditions in space and impact on the Earth (1, 51, 52). Therefore, a firm detection of 1 in the ISM in conjunction with rigorous modeling studies of reaction pathways to form prebiotic biorelevant molecules (Fig. 1) in deep space is of particular importance to our understanding of the role of sugar acids for the molecular complexity and synthesis of key organic molecules linked to the origin of life (35, 53, 54).

MATERIALS AND METHODS

Experimental

Experiments were carried out in a hydrocarbon-free stainless steel chamber under ultrahigh vacuum conditions maintained at pressures of a few 10⁻¹¹ torr by magnetically levitated turbomolecular pumps (Osaka, TG1300MUCWB and TG420MCAB), which are backed by a dry scroll pump (XDS35i, BOC Edwards) (55). A polished silver substrate (12.6 mm × 15.1 mm) for ice deposition was attached to a two-stage closed-cycle helium refrigerator (Sumitomo Heavy Industries, RDK-415E) that can be freely rotated and translated vertically. The chemical samples used in the experiment were carbon dioxide–¹⁸O₂ (C¹⁸O₂, Sigma-Aldrich, 95 atom % ¹⁸O), carbon dioxide–¹³C (¹³CO₂, Sigma-Aldrich, 99 atom % ¹³C), carbon dioxide (CO₂, Airgas, 99.999%), deuterated ethylene glycol-d₄ (HOCD₂CD₂OH, CDN isotopes, 98.9 atom % D), ethylene glycol–¹³C₂ (HO¹³CH₂¹³CH₂OH, Sigma-Aldrich, 99 atom % ¹³C), and ethylene glycol (HOCH₂CH₂OH, Sigma-Aldrich, anhydrous, 99.8%). Ethylene glycol sample was stored in a borosilicate vial that was connected to a high vacuum chamber at pressures of a few 10⁻⁸ torr and subjected to several freeze-pump-thaw cycles to remove residual atmospheric gases. Ices were prepared by passing carbon dioxide gas and ethylene glycol vapor through separate glass capillary arrays directed at the wafer cooled to 5.0 ± 0.3 K. Partial pressures were 2 × 10⁻⁸ torr for carbon dioxide and 4 × 10⁻⁹ torr for ethylene glycol. Ice thickness was measured during the deposition via laser interferometry (56); a photodiode was used to record variations in the intensity of a helium-neon laser (CVI Melles Griot, 25-LHP-230; 632.8 nm) reflected from the ice and silver substrate resulting from thin-film interference. The concentration-weighted average index of 1.32 ± 0.09 was used to derive the thickness of the carbon dioxide−ethylene glycol ices from the refractive indexes of amorphous carbon dioxide ice (n = 1.21) at 10 K (57) and that of ethylene glycol ice (n = 1.43) (58). Ice thicknesses were determined to be 750 ± 80 nm (table S6) by taking into account the densities of carbon dioxide (0.98 g cm⁻³) (57) and ethylene glycol (1.1 g cm⁻³) (59). To monitor the changes of ices in situ before and after the deposition, an FTIR spectrometer (Thermo Electron, Nicolet 6700) was used in the range of 6000 to 500 cm⁻¹ with 4 cm⁻¹ spectral resolution. The ratio of carbon dioxide to ethylene glycol in the ice was determined to be 2.1 ± 0.7:1 using the integrated infrared absorptions of carbon dioxide at 3701 cm⁻¹ (ν₁ + ν₃, 1.8 × 10⁻¹⁸ cm molecule⁻¹) and 2278 cm⁻¹ [ν₃ (¹³CO₂), 6.8 × 10⁻¹⁷ cm molecule⁻¹] (27) and the absorption bands of pure ethylene glycol ice with known thickness (fig. S9).

After the deposition, the ice mixtures were irradiated with 5-keV electrons (SPECS, EQ PU-22) at a 70° angle of incidence for 60 min at a current of 30 nA. On the basis of Monte Carlo simulations performed with the CASINO software suite (60), the irradiation conditions correspond to doses of 1.8 ± 0.3 eV molecule⁻¹ for carbon dioxide and 3.1 ± 0.5 eV molecule⁻¹ for ethylene glycol, respectively, simulating secondary electrons generated in the track of GCRs in cold molecular clouds aged around 5 × 10⁶ years (22). The average penetration depth of electrons in CO₂−HOCH₂CH₂OH ice was calculated to be 300 ± 60 nm based on Monte Carlo simulations using CASINO 2.42 (60). The penetration depth is notably less than the ice thickness (750 ± 80 nm), preventing electron-initiated interactions between the ice and the silver substrate. The changes in the spectrum of the ices were monitored by the FTIR spectrometer before, during, and after irradiation. After irradiation, the ices were subjected to TPD and heated from 5 to 320 K at 0.5 K min⁻¹. Subliming molecules were photoionized 2 mm above the ice surface by pulsed 30-Hz VUV light at 11.10 or 8.77 eV, which was generated through resonant four-wave mixing (ω_VUV = 2ω₁ ± ω₂) schemes using Xenon as a nonlinear medium (table S7). The VUV photons were generated via sum frequency generation (2ω₁ + ω₂; 11.10 eV) and difference frequency generation (2ω₁ − ω₂; 8.77 eV) with 249.628 nm (ω₁; dye laser, Sirah Lasertechnik, Cobra-Stretch) and 1064 nm (ω₂; Nd:YAG laser, Spectra-Physics, Quanta Ray PRO 270-30) (12). A biconvex lithium fluoride lens (Korth Kristalle, R1 = R2 = 131 mm) was used in an off-axis geometry to spatially separate the VUV light from other laser beams. The ions resulting from VUV photoionization were mass-analyzed via ReTOF-MS and detected with a dual microchannel plate (MCP) detector in the chevron configuration (Jordan TOF Products). The MCP signal was amplified with a preamplifier (Ortec, 9305), discriminated (Advanced Research Instruments Corp., F100-TD), and recorded by a multichannel scaler (FAST ComTec, MCS6A, 30 Hz). For each recorded mass spectra, the accumulation time of ion signals was 2 min (3600 sweeps) and the ion arrival time was recorded to 3.2-ns accuracy. Recorded TPD profiles were corrected for variations in the VUV flux throughout each experiment, which was monitored during TPD via a Faraday cup (61). Additional experiment was also performed without irradiation (blank) at 11.10 eV for carbon dioxide−ethylene glycol ice, and no ion signal at m/z = 106 was observed.

Computational

The adiabatic IEs and relative energies are computed as zero-point vibrational energy (ZPVE) corrected adiabatic differences in the energies of the neutral and radical-cation geometries. B3LYP/aug-cc-pVTZ was used to optimize the geometries and compute the harmonic vibrational energies in Gaussian16 (62–66). Explicitly correlated coupled-cluster singles, doubles, and perturbative triples [CCSD(T)-F12b] using the cc-pVTZ-F12 basis set as available within MOLPRO 2020.1 program (67–71) produced single-point energies of C₃H₆O₄ isomers at the optimized B3LYP/aug-cc-pVTZ geometries. These CCSD(T)-F12b energies along with the B3LYP ZPVEs produce the IEs and relative energies (table S5). The CCSD(T)-F12b/cc-pVTZ-F12 single-point energy of PBE0/aug-cc-pVTZ and ωB97XD/aug-cc-pVTZ anharmonic vibrational analysis for both density functionals were calculated for 1 and 18; the shifts in IE and relative energy are shown in table S8. This approach has demonstrated a good correlation with experiments in previous work (72). The computed Cartesian coordinates and harmonic and anharmonic vibrational frequencies are listed in table S9.

Supplementary Materials

This PDF file includes:

Tentative assignments of C₂H₄O₃ and C₃H₆O₃ isomers

Figs. S1 to S9

Tables S1 to S9

References