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

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.