Over the last half-century, interstellar aldehydes (RCHO), with R being organic groups, have garnered significant attention due to their role as crucial intermediates in metabolic pathways and key precursors to biorelevant molecules critically linked to the molecular origins of life. For example, the simple aldehyde acetaldehyde (CH₃CHO) is ubiquitous in the interstellar medium (ISM) and has been tentatively identified in interstellar ices. It can react with carbon dioxide (CO₂) in interstellar environment to form the biorelevant pyruvic acid (CH₃COCOOH), a key building block of metabolites and amino acids¹. Among the more than 300 molecules identified in the ISM, 9 aldehydes were detected². Furthermore, 16 aldehydes, including the simplest one, formaldehyde (H₂CO), have been detected in carbonaceous chondrites³, indicating that aldehydes can form in astrophysical environments and survive the entrance of the parent bodies of the meteorites into the atmosphere of the early Earth⁴. Despite their significance, the interstellar formation mechanisms of complex, biologically relevant aldehydes have remained largely elusive.

Laboratory experiments were conducted to investigate the formation pathways of biorelevant lactaldehyde and its isomers in processed interstellar ice analogs composed of carbon monoxide (CO) and ethanol (CH₃CH₂OH) within an ultrahigh vacuum chamber (a few 10⁻¹¹ Torr).The mixed ices were prepared by depositing a gas mixture of carbon monoxide and ethanol vapor onto a polished silver substrate, cooled to 5 K. The ices were irradiated with energetic electrons to simulate the secondary electrons generated by galactic cosmic rays (GCRs) in cold molecular clouds aged up to seven million years. After processing, the ices were heated to 320 K via a temperature-programmed desorption (TPD) process, mimicking the transition from a cold molecular cloud to a star-forming region. We developed a photoionization reflectron time-of-flight mass spectrometry (PI-ReToF-MS) technique to identify the reaction products during TPD isomer-specifically based on their desorption temperatures and calculated adiabatic ionization energies⁵. Utilizing isotopic substitution studies and tunable vacuum ultraviolet (VUV) PI-ReToF-MS, lactaldehyde (CH₃CH(OH)CHO)and its isomers—3-hydroxypropanal (HOCH₂CH₂CHO), ethyl formate (CH₃CH₂OCHO), and 1,3-propenediol (HOCH₂CHCHOH)—were identified in the gas phase⁶. These results suggest that lactaldehyde (CH₃CH(OH)CHO)forms via barrierless recombination of formyl (HĊO) and 1-hydroxyethyl (CH₃ĊHOH) radicals, revealing fundamental formation pathways for complex, biologically relevant aldehydes through non-equilibrium reactions under astrophysical conditions⁶.

Considering that carbon monoxide is one of the most commonly identified compounds in interstellar ices and ethanol is abundant in the ISM and has been tentatively detected in interstellar ices, lactaldehyde and its isomers could have formed upon exposure to GCRs in interstellar ices composed of carbon monoxide and ethanol within a cold molecular cloud. Once formed, these organics are promising candidates for future astronomical searches via radio telescopes. Notably, 3-hydroxypropanal has already been detected toward two star-forming regions.

Lactaldehyde is a key intermediate in the methylglyoxal pathway in biochemistry, where glucose is converted into pyruvate. Under prebiotic conditions, lactaldehyde acts as a fundamental precursor to essential biomolecules such as amino acids, sugars, and sugar acids. It can be oxidized to form methylglyoxal (CH₃C(O)CHO), which further leads to the formation of pyruvic acid. Additionally, oxidation of lactaldehyde forms lactic acid (CH₃CH(OH)COOH), eventually yielding sugar acids such as glyceric acid (HOCH₂CH(OH)COOH). Through carbon-carbon or carbon-oxygen bond cleavage, lactaldehyde can be converted into glycolaldehyde (HOCH₂CHO) or propionaldehyde (CH₃CH₂CHO). Once formed within cold molecular clouds, these organics could be incorporated into planetesimals and later delivered to planets, such as early Earth, providing an exogenous source for the synthesis of biorelevant molecules and offering valuable insights into the molecular origins of life⁴.

For further details, please refer to our article “Interstellar formation of lactaldehyde, a key intermediate in the methylglyoxal pathway,” published in Nature Communications. 

Written by Dr. Jia Wang and Dr. Ralf I. Kaiser

References

1. Kleimeier, N. F., Eckhardt, A. K., Schreiner, P. R., Kaiser, R. I. Interstellar formation of biorelevant pyruvic acid (CH₃COCOOH). Chem 6, 3385-3395 (2020).

2. McGuire, B. A. 2021 Census of interstellar, circumstellar, extragalactic, protoplanetary disk, and exoplanetary molecules. Astrophys. J., Suppl. Ser. 259, 30 (2022).

3. Aponte, J. C., et al. Analyses of aliphatic aldehydes and ketones in carbonaceous chondrites. ACS Earth Space Chem. 3, 463-472 (2019).

4. Chyba, C., Sagan, C. Endogenous production, exogenous delivery and impact-shock synthesis of organic molecules: an inventory for the origins of life. Nature 355, 125-132 (1992).

5. Turner, A. M., Kaiser, R. I. Exploiting photoionization reflectron time-of-flight mass spectrometry to explore molecular mass growth processes to complex organic molecules in interstellar and solar system ice analogs. Acc. Chem. Res. 53, 2791-2805 (2020).

6. Wang, J., et al. Interstellar formation of lactaldehyde, a key intermediate in the methylglyoxal pathway. Nat. Commun. 15, 10189 (2024).