Nonequilibrium Formation of C4 Carbonyls in Interstellar Ices Linked to Amino Acids

[ACS Central Science] Complex organic molecules in interstellar ices and carbonaceous asteroids record key steps in the chemical evolution toward life, yet the origin of branched carbonyl compounds a fundamental class of biorelevant molecules remains unresolved.

Here, we demonstrate the efficient formation of C4 carbonyls, isobutyraldehyde ((CH₃)2CHCHO) and 2-butanone (CH₃CH₂COCH₃) via barrierless radical-radical recombination in carbon monoxide-propane (CO-C₃H₈; 1:1.1 ± 0.2) and ethane-acetaldehyde (C₂H₆-CH₃CHO; 1.5 ± 0.3:1) ice mixtures at 5 K irradiated with 5 keV electrons as proxies for secondary electrons generated by galactic cosmic rays.

Isobutyraldehyde arises from recombination of formyl (HĊO) and isopropyl (CH₃ĊHCH₃) radicals, whereas 2-butanone forms through acetyl (CH₃ĊO) and ethyl (CH₃ĊH₂) radical coupling. Using isomer-selective photoionization mass spectrometry with isotopic labeling, we provided strong evidence for these products together with the enol 2-methylprop-1-en-1-ol ((CH₃)2CCHOH) in the gas phase.

These results establish a plausible low-temperature mechanism for molecular mass growth that generates branched carbon skeletons without activation barriers, bridging a critical gap between simple interstellar species and structurally complex, biorelevant organics.

The demonstrated pathways operate under cosmic-ray-driven, nonequilibrium chemistry in icy grains, providing a plausible route to C4 backbone motifs found in prebiotic molecules, including amino acids and fatty acids.

Our findings show that chemical complexity – including carbon skeleton branching can emerge in deep space prior to planetary accretion, implying a plausible extraterrestrial origin for key molecular precursors delivered to early Earth and exo planetary systems.

• Nonequilibrium Formation of C4 Carbonyls in Interstellar Ices Linked to Amino Acids, ACS Central Science via PubMed (open access)
• Nonequilibrium Formation of C4 Carbonyls in Interstellar Ices Linked to Amino Acids, ACS Central Science (open access)

Astrobiology,