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. 2025 Oct 8;16(44):21111–21120. doi: 10.1039/d5sc05315c

Interstellar formation of 1,2-propanediol (CH3CH(OH)CH2OH) and 1,2-ethenediol (HOCHCHOH)—key precursors to sugars and sugar derivatives

Jia Wang a,b, Chaojiang Zhang a,b, André K Eckhardt c,✉, Ralf I Kaiser a,b,✉
PMCID: PMC12520177  PMID: 41098495

Abstract

Although sugars and sugar derivatives—molecules critical to the origins of life—have been identified in carbonaceous meteorites with total abundances typically higher than that of amino acids, their underlying formation mechanisms in interstellar environments remain poorly understood. This study reports the first formation of 1,2-propanediol (CH3CH(OH)CH2OH) and 1,2-ethenediol (HOCHCHOH) in low-temperature model interstellar ices composed of methane (CH4) and ethylene glycol (HOCH2CH2OH). 1,2-Propanediol forms via the barrierless radical–radical recombination of the methyl (ĊH3) with the 1,2-dihydroxyethyl (HOĊHCH2OH), while 1,2-ethenediol is produced through the decomposition of ethylene glycol. Utilizing vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry and isotopic substitution experiments, 1,2-propanediol and its isomer 2-methoxyethanol (CH3OCH2CH2OH), along with enols 1,2-ethenediol and vinyl alcohol (CH2CHOH) were identified in the gas phase during temperature-programmed desorption based on their adiabatic ionization energies and mass-to-charge ratios. Among these compounds, only 1,2-propanediol has not yet been observed in the interstellar medium; these results suggest that it is a promising target for future astronomical detection. Our findings reveal viable abiotic pathways for the formation of biorelevant 1,2-propanediol and 1,2-ethenediol via non-equilibrium chemistry in ethylene glycol-containing interstellar ices, advancing our understanding of the fundamental formation mechanisms of sugars and sugar derivatives in deep space.

This study reveals viable abiotic routes of the three-carbon deoxysugar alcohol 1,2-propanediol (CH3CH(OH)CH2OH) and enol 1,2-ethenediol (HOCHCHOH)—a critical intermediate in the formose reaction—in interstellar analog ices.graphic file with name d5sc05315c-ga.jpg

Introduction

Since the first identification of methanol (CH3OH, 1) in the interstellar medium (ISM) by Ball et al. more than half a century ago,1 alcohols (ROH), where R represents an organic group, have attracted considerable attention from the astronomy,2–4 astrochemistry,5–7 astrobiology,8,9 and physical organic chemistry10,11 communities mainly due to their central role in the abiotic synthesis of biorelevant molecules essential to the origins of life.12–14 Within cold molecular clouds, complex organics including alcohols can form via non-equilibrium reactions in interstellar ices composed of simple molecules such as water (H2O), carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), and methanol (CH3OH)15 driven by ionizing radiation such as ultraviolet (UV) photons and galactic cosmic rays (GCRs).16 Irradiation of methanol-containing ices with GCR proxies yields glycolaldehyde (HOCH2CHO, 2),17 ethylene glycol (HOCH2CH2OH, 3),18 and glycerol (HOCH2CH(OH)CH2OH, 4)19 as key precursors to sugars and phospholipids. Ethylene glycol serves as a molecular building block of the 3-carbon deoxysugar alcohol 1,2-propanediol (propylene glycol, CH3CH(OH)CH2OH, 5); the latter represents a critical product in the methylglyoxal pathway in biochemistry by which glucose (C6H12O6) is metabolized to pyruvate without generating adenosine triphosphate (ATP).20 Additionally, alcohols can form interstellar enols—alkenes bearing a hydroxyl group connected to a carbon–carbon double bond—such as 1,2-ethenediol (HOCHCHOH, 6) and vinyl alcohol (CH2CHOH, 7),21,22 which are considered key intermediates in the formation of prebiotic sugars and sugar acids.21,23–25 Once synthesized, ices bearing these species may become embedded in circumstellar disks during star formation, providing essential ingredients for the formation of comets and planetesimals,26 which may eventually be delivered to planets like early Earth.12 Analyses of carbonaceous meteorites such as Murchison and Murray have revealed a variety of sugars and deoxysugar derivatives including 3–5 at high concentrations (e.g., 160 nmol g−1 for 4)12,27,28 with their total abundance typically exceeding that of amino acids.29 However, the fundamental formation mechanisms of these species under astrophysical conditions remain largely conjecture, particularly for the biorelevant 1,2-propanediol (5).

In prebiotic chemistry, 5 serves as a fundamental precursor to key biorelevant molecules (Fig. 1). Upon exposure to ionizing radiation, 5 can react with water to form 4, a molecular building block of sugar alcohols and phospholipids.13 Oxidation of 5 produces the simplest sugar, glyceraldehyde (HOCH2CH(OH)CHO, 8), which initiates the synthesis of more complex sugars. Additionally, 5 can be converted into lactaldehyde (CH3CH(OH)CHO, 9) and methylglyoxal (CH3COCHO, 10), the latter of which serves as a direct precursor to pyruvic acid (CH3COCOOH, 11)—a central metabolic intermediate in the synthesis of amino acids and peptides. Further oxidation of 5 leads to lactic acid (CH3CH(OH)COOH, 12), which can access the simplest sugar acid, glyceric acid (HOCH2CH(OH)COOH, 13), thus initiating pathways toward complex sugar acids.30 Through carbon–carbon or carbon–oxygen bond cleavage, 5 can form 1, ethanol (CH3CH2OH, 14), n-propanol (CH3CH2CH2OH, 15), and i-propanol (CH3CH(OH)CH3, 16)—simple alcohols that have been identified in the ISM1,2,31 and are considered potential precursors to essential biomolecules such as glucose and ribose (C5H10O5), which are fundamental building blocks of ribonucleic acid (RNA).9,32 Consequently, 5 represents a fundamental precursor to a suite of sugars and sugar derivatives potentially contributing to the chemical evolution of key biorelevant molecules in extraterrestrial environments. Once formed, these organics may be ultimately delivered to planets like early Earth, as evidenced by their detection in multiple carbonaceous meteorites.12,27–29 Therefore, elucidating the interstellar formation mechanisms of 5 is crucial to understanding the synthesis pathways of astrobiologically relevant molecules and eventually to the emergence of life.

Fig. 1. Proposed formation pathway of 1,2-propanediol in interstellar ices and its potential role as a precursor to biorelevant molecules. 1,2-Propanediol (5) is formed via radical–radical recombination of the methyl (ĊH3, 17) radical with the 1,2-dihydroxyethyl (HOĊHCH2OH, 18) radical in model interstellar ice carrying methane (CH4) and ethylene glycol (HOCH2CH2OH, 3). Upon radiation by galactic cosmic ray proxies in form of energetic electrons, 5 serves as a molecular building block of sugars and sugar-related molecules such as glycolaldehyde (8), lactaldehyde (9), and lactic acid (12). Additionally, 5 can facilitate the abiotic synthesis of glycerol (4), methylglyoxal (10), and pyruvic acid (11), contributing to the synthesis of essential biorelevant compounds such as phospholipids, amino acids, and peptides. In contemporary biochemistry, 5 is a key product in the methylglyoxal pathway (blue arrows), by which glucose is metabolized to pyruvate without the generation of adenosine triphosphate (ATP).

Fig. 1

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Here, we present the first preparation of racemic 5 through the barrierless radical–radical recombination of the methyl (ĊH3, 17) radical with the 1,2-dihydroxyethyl (HOĊHCH2OH, 18) radical (Fig. 1 and 2) in interstellar model ices carrying methane and 3. Low-temperature (5 K) methane–3 ices were exposed to proxies of GCRs in form of energetic electrons to simulate secondary electrons generated along the tracks of low temperature ices condensed on interstellar nanoparticles (grains) in cold molecular clouds aged up to 2 × 107 years.33 During the temperature-programmed desorption (TPD) of the irradiated ices, 5 and its isomer 2-methoxyethanol (CH3OCH2CH2OH, 19), along with enols 6 and 7 were identified in the gas phase using vacuum ultraviolet (VUV) photoionization reflectron time-of-flight mass spectrometry (PI-ReToF-MS) in combination with isotopic substitution experiments (SI). These findings reveal viable formation pathways for 5–7 and 19via GCR-driven non-equilibrium chemistries in interstellar ices (Fig. 2), thereby advancing our fundamental understanding of the formation mechanisms of key biorelevant organics—sugars and sugar derivatives—in deep space. Methane–3 ice can be exploited as a model interstellar ice to comprehensively investigate the formation pathways of 5 and 19, as both methane and 3 are abundant molecules in the ISM. Methane has been identified in interstellar ices at concentrations of a few percent relative to water.35,36 Although 3 has only been observed in the gas phase,37 laboratory simulations have revealed its formation in interstellar ices through surface hydrogenation of carbon monoxide38 and via radical–radical recombination of two hydroxymethyl (ĊH2OH) radicals under exposure to ionizing radiation such as GCRs.18 Additionally, 3 has been detected in the Murchison meteorite and comets,4,12 with an abundance of 0.25% with relative to water in comet C/1995 O1 (Hale–Bopp).4 Therefore, compounds 5–7 and 19 can form in interstellar ices containing methane and 3. Notably, 6, 7, and 19 have been identified in the ISM;39–41 our results suggest that the hitherto astronomically unobserved 5 is a promising target for future astronomical detection. Once synthesized, these organics may lead to the abiotic formation of key biorelevant compounds (Fig. 1), which can be incorporated into planetesimals and ultimately delivered to planets such as early Earth via meteoritic impacts.12 Such exogenous delivery could have contributed to the emergence of essential biomolecules, providing key insights into the molecular origins of life.

Fig. 2. Reaction schemes leading to 1,2-propanediol and 2-methoxyethanol in irradiated methane–ethylene glycol ices. Barrierless radical–radical recombination of methyl (17) with 1,2-dihydroxyethyl (18) and 2-hydroxyethoxy (20) produce 1,2-propanediol (5) and 2-methoxyethanol (19), respectively (a–c). The adiabatic ionization energies (IEs) of 5 and 19 are shown as ranges containing all conformers, computed at the CCSD(T)/CBS//B3LYP/cc-pVTZ level of theory. Relative energies (ΔE) are given with respect to the most stable conformer of each structural isomer.34.

Fig. 2

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Results

Infrared spectroscopy

Fourier-transform infrared (FTIR) spectroscopy was employed to monitor the chemical evolution of the methane–ethylene glycol ices at 5 K before, during, and after the electron irradiation (Fig. 3 and S1–S3). Detailed assignments of the FTIR spectra are provided in Tables S1–S4. The absorption features of the pristine ices correspond to the fundamental and combination modes of methane and ethylene glycol (3).42–45 After the irradiation, several new absorption features emerged and were deconvoluted into multiple Gaussian functions. In the irradiated CH4–HOCH2CH2OH ice, absorptions at 2976 and 822 cm−1 can be attributed to the methyl (CH3) stretching (ν10) and rocking (ν12) modes of ethane (C2H6), respectively (Fig. 3);46 the methyl stretching (ν10) mode shifts to 2967 cm−1 (13C2H6, ν10) in irradiated 13CH4–HO13CH213CH2OH ice (Fig. S1). An absorption at 956 cm−1 is linked to the CH2 wagging (ν7) of ethylene (C2H4).46 The absorptions at 2339 and 2135 cm−1 are assigned to the asymmetric stretching (ν3) of carbon dioxide (CO2) and the C Created by potrace 1.16, written by Peter Selinger 2001-2019 O stretching of carbon monoxide (CO), respectively.42 These assignments are validated by the detection of 13CO2 at 2274 cm−1 and 13CO at 2090 cm−1 in the irradiated 13CH4–HO13CH213CH2OH ice.47 Additional features at 1654, 1499, and 1136 cm−1 are linked to water (H2O, ν2), formaldehyde (H2CO, ν3) and methanol (CH3OH, ν11), respectively.42 The band observed at 1718 cm−1 is assigned to acetaldehyde (CH3CHO, ν4) and/or formaldehyde (ν2); the formation of both species is confirmed from isotopically labeled ices (Fig. S1–S3).5,48,49 The absorption at 2070 cm−1 in irradiated 13CH4–HO13CH213CH2OH ice can be attributed to ketene-13C2 (H213C13CO, ν2).50 The absorption at 1845 cm−1 can be linked to the formyl (HĊO, ν3) radical and/or trans-hydroxycarbonyl (HOĊO, ν2) radical.42,51 The formation of trans-hydroxycarbonyl is confirmed via the detection of HO13ĊO at 1805 cm−1 (ν2) in irradiated 13CH4–HO13CH213CH2OH ice (Fig. S1).52 It is worth noting that the absorption at 1654 cm−1 observed in the irradiated CH4–HOCH2CH2OH ice can also be connected to the C Created by potrace 1.16, written by Peter Selinger 2001-2019 C stretching (ν5) of syn-vinyl alcohol (7, CH2CHOH). This assignment is supported by the shift of this band to 1588 cm−1 for 7-13C2 (13CH213CHOH), 1575 cm−1 for 7-d3 (CD2CDOH), and 1641 cm−1 for 7-d1 (CH2CHOD) in irradiated 13CH4–HO13CH213CH2OH, CH4–HOCD2CD2OH, and CH4–DOCH2CH2OD ices, respectively (Fig. S1–S3).53 However, due to the overlapping absorption features between 3 and the wide suite of irradiation products, FTIR spectroscopy alone is insufficient for a unique identification of complex organics such as 5 and 19. Therefore, an alternative, more sensitive analytical technique is needed to identify individual reaction products.54

Fig. 3. Infrared spectra of CH4–HOCH2CH2OH ice before (black) and after (red) electron irradiation at 5 K. Deconvolution spectral regions are shown for 3900–1900 cm−1 (a) and 1900–800 cm−1 (b). The green lines indicate the total fit of the spectra. Detailed assignments are provided in Table S1.

Fig. 3

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Mass spectrometry

Photoionization reflectron time-of-flight mass spectrometry (PI-ReToF-MS) was utilized to identify individual products including C3H8O2, C2H4O2, and C2H4O isomers in the gas phase during TPD based on their desorption temperatures and adiabatic ionization energies (IEs).54 The PI-ReToF mass spectra of subliming molecules from the irradiated methane–ethylene glycol ices are compiled in Fig. 4.

Fig. 4. PI-ReToF-MS data of methane–ethylene glycol ices during TPD. Data were recorded for the unirradiated (blank) CH4–HOCH2CH2OH ice at 9.60 eV (a), the irradiated CH4–HOCH2CH2OH ice at 9.60 eV (b) and 9.10 eV (c), the irradiated 13CH4–HO13CH213CH2OH ice at 9.60 eV (d), the irradiated CH4–DOCH2CH2OD ice at 9.60 eV (e), and the irradiated CH4–HOCD2CD2OH ice at 9.60 eV (f).

Fig. 4

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1,2-Propanediol and 2-methoxyethanol

Focusing on the C3H8O2 isomers, a photon energy of 9.60 eV was used to photoionize 1,2-propanediol (5, IE = 9.29–9.52 eV) and 2-methoxyethanol (19, IE = 9.24–9.52 eV) formed via radical–radical recombination of methyl (17) with 1,2-dihydroxyethyl (18) and 2-hydroxyethoxy (20) (Fig. 2). The TPD profile of ion signal at mass-to-charge ratios (m/z) of 76 obtained at 9.60 eV exhibits sublimation events peaking at 182 K (peak I) and 218 K (peak II) for the irradiated CH4–HOCH2CH2OH ice (Fig. 5a). To assign the molecular formula for these sublimation events, a fully 13C-labeled ice mixture (13CH4–HO13CH213CH2OH) was used. Replacing CH4–HOCH2CH2OH ice with 13CH4–HO13CH213CH2OH ice shifts the TPD profile by 3 atomic mass units (amu), from m/z = 76 to m/z = 79 (Fig. 5d), verifying the presence of three carbon atoms. Therefore, the sublimation events observed at m/z = 76 in the irradiated CH4–HOCH2CH2OH ice can be assigned to a molecule of the formula C3H8O2. Considering that 9.60 eV photons are capable of ionizing isomers 5 (IE = 9.29–9.52 eV) and 19 (IE = 9.24–9.52 eV), the sublimation events (peaks I and II) of the TPD profile at m/z = 76 (C3H8O2+) can be attributed to isomer 5 and/or 19. It is worth noting that ethylene glycol (3) exhibits a sublimation event peaking at 214 K (Fig. S4) suggesting that peak II may result from cosublimation with 3. A blank experiment was conducted without electron irradiation of CH4–HOCH2CH2OH ice under otherwise identical conditions. No ion signal at m/z = 76 was detected except for a tiny and narrow sublimation event between 211 K and 223 K (Fig. 5a), which is likely due to the cosublimation of impurities with 3. Upon reducing the photon energy to 9.10 eV, at which neither 5 nor 19 can be ionized, peaks I and II are absent, and no sublimation event was detected.

Fig. 5. TPD profiles of C3H8O2 isomers from methane–ethylene glycol ices. TPD profiles of m/z = 76 from irradiated CH4–HOCH2CH2OH ice measured at 9.60 eV and 9.10 eV (a), m/z = 77 and 78 from irradiated CH4–DOCH2CH2OD ice at 9.60 eV (b), m/z = 79 and 80 from irradiated CH4–HOCD2CD2OH ice at 9.60 eV (c), and m/z = 79 from irradiated 13CH4–HO13CH213CH2OH ice at 9.60 eV (d). TPD profiles of m/z = 75, 76, and 77 in blank experiments adding 5 or 19 were recorded at 9.60 eV (e and f). Red shaded regions indicate the sublimation peaks corresponding to 5 and 19.

Fig. 5

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Since the IEs of isomers 5 and 19 overlap, it is imperative to verify their formation through separate experiments at 9.60 eV using isotopically labeled ices including CH4–DOCH2CH2OD ice and CH4–HOCD2CD2OH ice (Fig. 2b and c). From the irradiated CH4–DOCH2CH2OD ice, distinct ion signals at m/z = 78 (CH3CH(OD)CH2OD+) for 5 and m/z = 77 (CH3OCH2CH2OD+) for 19 were detected (Fig. 5b). The TPD profiles at m/z = 77 and 78 show two sublimation event (peaks I and II), indicating the that both peaks are linked to 5 and 19. Similarly, substituting HOCH2CH2OH with HOCD2CD2OH resulted in the ion signals m/z = 79 (CH3CD(OH)CD2OH+) for 5 and m/z = 80 (CH3OCD2CD2OH+) for 19; the TPD profiles of m/z = 79 and 80 show both peaks as well (Fig. 5c), confirming the formation of 5 and 19. Additional blank experiments without irradiation were conducted at 9.60 eV by adding less than 1% of isomer 5 or 19 into the CH4–HOCH2CH2OH ice under otherwise identical conditions. The TPD profile of 5 at m/z = 76 revealed two sublimation events centered at 181 K and 220 K (Fig. 5e), while that of 19 exhibited a minor peak centered at 188 K and a major sublimation event peaking at 218 K (Fig. 5f). The latter peak of both isomers likely result from co-sublimation with ethylene glycol. Their first sublimation events agree with peak I (182 K), supporting the assignment of peak I to 5 and 19.

1,2-Ethenediol

The TPD profile of the ion signal of m/z = 60 in irradiated CH4–HOCH2CH2OH ice at 9.60 eV exhibits a sublimation event peaking at 211 K (Fig. 6a). In the irradiated, fully 13C labeled ice (13CH4–HO13CH213CH2OH), this TPD profile shifts by 2 amu to m/z = 62, suggesting the presence of two carbon atoms and confirming a molecular formula of C2H4O2. Potential C2H4O2 isomers account for this signal can be methyl formate (CH3OCHO), acetic acid (CH3COOH), glycolaldehyde (HOCH2CHO), and their enol tautomer, 1,2-ethenediol (6). Notably, the sublimation event at m/z = 60 remains at a reduced photon energy of 9.10 eV (Fig. 6a). At this energy, only 6 (IE = 8.12–8.43 eV)55 can be ionized, but methyl formate (IE = 10.59–10.85 eV), acetic acid (IE = 10.43–10.67 eV), and glycolaldehyde (IE = 9.75–10.08 eV) cannot be photoionized.56 Therefore, the sublimation event at m/z = 60 in the irradiated CH4–HOCH2CH2OH ice can be attributed to 6. The TPD profile of m/z = 60 from irradiated CH4–HOCH2CH2OH ice matches the previously measured TPD profile of 6 obtained from irradiated 1 ice (Fig. S5),21 confirming the formation of 6. In irradiated CH4–DOCH2CH2OD ice, the TPD profile of m/z = 62 (DOCHCHOD+) agrees with that of m/z = 60 (HOCHCHOH+) in irradiated CH4–HOCH2CH2OH ice (Fig. 6b), indicating that 6 forms via the dehydrogenation of 3 by losing two hydrogen (Ḣ) atoms—one from the central carbon atom and another from the adjacent carbon atom. This mechanism is consistent with the irradiated CH4–HOCD2CD2OH ice experiment, in which the TPD profile of m/z = 62 (HOCDCDOH+) matches that of m/z = 60 (HOCHCHOH+) in the irradiated CH4–HOCH2CH2OH ice (Fig. 6c).

Fig. 6. TPD profiles of C2H4O2 isomers from methane–ethylene glycol ices. TPD profiles of m/z = 60 from irradiated CH4–HOCH2CH2OH ice measured at 9.60 eV and 9.10 eV (a), m/z = 62 from irradiated 13CH4–HO13CH213CH2OH (a), CH4–DOCH2CH2OD ice (b), and CH4–HOCD2CD2OH ices (c) at 9.60 eV.

Fig. 6

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Discussion

Having provided compelling evidence for the formation of 1,2-propanediol (5), 1,2-ethenediol (6), and 2-methoxyethanol (19) in irradiated methane–ethylene glycol ices under astrophysical conditions, we now turn to their potential formation mechanisms. First, upon electron irradiation, methane undergoes C–H bond cleavage to produce a methyl (ĊH3) radical and a hydrogen atom (Ḣ) via reaction (1) with a reaction endoergicity of 439 kJ mol−1.57 Recall that ethane (C2H6) has been identified by the infrared absorptions of the methyl (CH3) stretching (ν10) and rocking (ν12) modes, indicating the formation of methyl radicals. The unimolecular decomposition of ethylene glycol (3) yield the 1,2-dihydroxyethyl radical (18, HOĊHCH2OH) via reaction (2) or the 2-hydroxyethoxy radical (20, ȮCH2CH2OH) via reaction (3)30 with associated endoergicities of 398 and 443 kJ mol−1, respectively.57,58 The isomerization from 18 to 20 is endoergic by 44 kJ mol−1 and proceeds via a reaction barrier of 161 kJ mol−1 calculated at the AE-CCSD(T)/CBS//AE-MP2/aug-cc-pVTZ level of theory, with negligible H-tunneling contributions at low temperatures.59 If a methyl radical has a favorable recombination geometry with nearby 18 or 20 radicals, barrierless radical–radical recombination between methyl radical and 18 or 20 can occur, leading to 5via reaction (4) or 19via reaction (5), with reaction energies of −366 and −359 kJ mol−1, respectively.60,61 X-ray irradiation of matrix-isolated ethanol revealed that 1-hydroxyethyl (CH3ĊHOH) radical is formed preferentially via C–H bond cleavage at the central carbon.62 Recent studies on electron-irradiated CO–CH3CH2OH ice indicate the formation of 1-hydroxyethyl and 2-hydroxyethyl (ĊH2CH2OH) radicals at relatively low irradiation doses.16 Similarly, the formation of 18 may be more favorable than that of 20. This is consistent with the isotopically labeled CH4–HOCD2CD2OH experiment, where the ratio of 5-d3 to 19-d4 was determined as (4.8 ± 0.5): 1 based on their integrated counts. Both 5 and 19 exist numerous conformers (20 for 5 and 12 for 19), making it difficult to identify which specific conformer(s) formed under our experimental conditions. Accurate quantification of their concentrations or branching ratio would further require their photoionization cross sections at 9.60 eV, which have not yet been experimentally determined.

CH4 → ĊH3 + Ḣ 1
HOCH2CH2OH (3) → HOĊHCH2OH (18) + Ḣ 2
HOCH2CH2OH (3) → ȮCH2CH2OH (20) + Ḣ 3
ĊH3 + HOĊHCH2OH (18) → CH3CH(OH)CH2OH (5) 4
ĊH3 + ȮCH2CH2OH (20) → CH3OCH2CH2OH (19) 5

Second, once 18 forms through the decomposition of 3via reaction (2), 6 can be accessed from 18 by the loss of a hydrogen atom (reaction (6)). Recalled that TPD profile of m/z = 60 in irradiated CH4–HOCH2CH2OH ice shifts 2 amu to m/z = 62 in both irradiated CH4–DOCH2CH2OD and CH4–HOCD2CD2OH ices (Fig. 6b and c), indicating that the formation of 6 involves dehydrogenation of 3 through the loss of one hydrogen atom from the central carbon and another from the adjacent carbon. This reaction is endoergic by 142 kJ mol−1.57 Similar reaction mechanisms have been demonstrated in recent study on interstellar analog ices, where 7 can form via the dehydrogenation of ethanol (14) in irradiated carbon monoxide–ethanol ices.22

HOĊHCH2OH (18) → HOCHCHOH (6) + Ḣ 6

Conclusion

This study presents the first abiotic pathways to the biorelevant 1,2-propanediol (5) and 1,2-ethenediol (6) in low-temperature model interstellar ices composed of methane and ethylene glycol (3). These ice mixtures were exposed to energetic electrons as proxies for GCRs, simulating secondary electrons generated along the tracks in interstellar ices in cold molecular clouds aged up to 2 × 107 years.33 Utilizing VUV photoionization reflectron time-of-flight mass spectrometry and isotopic substitution experiments, 5–6 and 2-methoxyethanol (19) were identified in the gas phase during TPD based on their ionization energies and mass-to-charge ratios. These results reveal the formation pathways of 5 and its isomer 19 through radical–radical recombination reactions as well as the enol 6, providing crucial steps toward a systematic understanding of how sugars and sugar derivatives can be formed in 3-containing interstellar ices via non-equilibrium chemistries. Methane has been detected in interstellar ices at a few percent relative to water,35,36 and laboratory simulations revealed that 3 can readily form in interstellar analog ices under ionizing radiation such as GCRs.18 Our results suggest that 5–6 and 19 could be generated in interstellar ices on nanoparticles (interstellar grains) in cold molecular clouds. As dense molecular clouds evolve into star-forming regions, the warmer conditions with rising temperatures (100–300 K) induce the release of complex organics from icy grain mantles into the gas phase.21,23 Among these, 6 and 19 have been detected toward the G+0.693-0.027 molecular cloud40 and the massive protocluster NGC 6334I,41 respectively. Notably, our previous investigation of irradiated low-temperature methanol-bearing ices revealed the formation of 6,21 which was subsequently identified toward the G+0.693-0.027 molecular cloud.40 Given its relatively large dipole moment (2.6 D),34 the yet unobserved 5 represents a promising target for future astronomical searches towards star forming regions.

Once synthesized in interstellar ices, 5 can serve as a precursor to critical biorelevant molecules 8–13 (Fig. 1). Additionally, enol 6 can be produced from 2via keto–enol tautomerism and act as a key intermediate in the formose or Butlerov reaction,63,64 which is fundamental to carbohydrate formation. As a nucleophile, 6 reacts with electrophilic formaldehyde through a favorable six-membered transition state to yield the simplest sugar molecule, 8.21 Therefore, 5 and 6 represent essential prebiotic precursors to sugars and sugar derivatives, providing plausible abiotic pathways for their synthesis in extraterrestrial environments. During star formation, icy grains containing these species can be incorporated into circumstellar disks, contributing to the formation of planetesimals and comets. In fact, isomers of C2H4O2 and C3H8O2 have been detected in dusty coma of comet 67P, with isomers 5 and 19 identified as the most likely contributors to the C3H8O2 signal.3 At least fraction of these complex organics may be delivered to planets like early Earth via meteoritic impacts,12 which has been confirmed by the detection of sugars such as ribose8 and sugar-related compounds in multiple carbonaceous meteorites.27–29 Such exogenous delivery presents a plausible prebiotic scenario for the abiotic synthesis of sugars and their derivatives, potentially initiating key chemical processes that led to the emergence of essential biomolecules central to the origins of life.

Finally, it is important to note that the methane–ethylene glycol ices employed in this study serve as simplified model systems to probe the formation mechanisms of C2H4O2 and C3H8O2 isomers under exposure to GCR proxies. These experiments are designed to investigate fundamental reaction pathways in a controlled environment rather than to replicate the full chemical complexity of interstellar ices. Given that interstellar ices are dominated by water,15 future studies incorporating water into the ice mixtures may reveal additional reaction pathways and products. For instance, the inclusion of water may facilitate the formation of C3H8O3 isomers such as 4 (Fig. 1) in irradiated methane–ethylene glycol–water ices. Additionally, hydroxyl radical formed from water can react with 18 and 20 to produce 1,1,2-ethanetriol (HOCH2CH(OH)2) and 2-hydroperoxyethanol (HOCH2CH2OOH), respectively, thereby competing with the pathways leading to 5 and 19. Future experiments can also investigate the formation of C3H8O2 isomers via the interaction of GCRs with simple ice mixtures such as CH4–H2O and CH4–CH3OH. Through radical–radical recombination, additional C3H8O2 isomers such as ethoxymethanol (CH3CH2OCH2OH), 1-methoxyethanol (CH3OCH(OH)CH3), dimethoxymethane (CH3OCH2OCH3), and ethylmethyl peroxide (CH3CH2OOCH3) may form and co-sublime with water or methanol molecules.

Author contributions

R. I. K. designed the experiments; J. W., and C. Z. conducted the experiments; J. W. analyzed the data; A. K. E. carried out the theoretical analysis; J. W., A. K. E. and R. I. K. wrote the manuscript, which was read, revised, and approved by all authors.

Conflicts of interest

The authors declare no conflict of interest.

Supplementary Material

SC-016-D5SC05315C-s001

Acknowledgments

This work was supported by the U.S. National Science Foundation (NSF), Division of Astronomical Sciences, under grant AST-2403867 awarded to the University of Hawaii at Manoa. The Hawaii group also acknowledges the University of Hawaii at Manoa and the W. M. Keck Foundation for supporting the construction of the experimental setup. The calculations were supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany's Excellence Strategy – EXC 2033 – 390677874 – RESOLV.

Data availability

Additional data are available from the corresponding author upon reasonable request.

Essential data are provided in the main text and the supplementary information (SI). Supplementary information: methods (experimental and computational), identification of vinyl alcohol (CH2CHOH) and its possible formation pathways, FTIR spectra of irradiated methane–ethylene glycol ices (Fig. S1–S3 and Tables S1–S4), TPD profiles of ethylene glycol (Fig. S4) and 1,2-ethenediol (Fig. S5), PI-ReToF-MS data and molecular formula assignments for other ion signals from irradiated methane–ethylene glycol ices (Fig. S6–S12 and Table S5), experimental conditions (Table S6), VUV generation parameters (Table S7), error analysis of IEs (Table S8), and Cartesian coordinates, vibrational frequencies, and IR intensities of computed conformers of 1,2-propanediol and 2-methoxyethanol (Table S9). See DOI: https://doi.org/10.1039/d5sc05315c.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

SC-016-D5SC05315C-s001
SC-016-D5SC05315C-s001.pdf (3.2MB, pdf)

Data Availability Statement

Additional data are available from the corresponding author upon reasonable request.

Essential data are provided in the main text and the supplementary information (SI). Supplementary information: methods (experimental and computational), identification of vinyl alcohol (CH2CHOH) and its possible formation pathways, FTIR spectra of irradiated methane–ethylene glycol ices (Fig. S1–S3 and Tables S1–S4), TPD profiles of ethylene glycol (Fig. S4) and 1,2-ethenediol (Fig. S5), PI-ReToF-MS data and molecular formula assignments for other ion signals from irradiated methane–ethylene glycol ices (Fig. S6–S12 and Table S5), experimental conditions (Table S6), VUV generation parameters (Table S7), error analysis of IEs (Table S8), and Cartesian coordinates, vibrational frequencies, and IR intensities of computed conformers of 1,2-propanediol and 2-methoxyethanol (Table S9). See DOI: https://doi.org/10.1039/d5sc05315c.

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