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.