Formation of Lactic Acid (CH3CH(OH)COOH), a Metabolic Keystone for the Molecular Origins of Life, in Interstellar Ice Analogues

Lactic acid (CH₃CH(OH)COOH)–a key biorelevant hydroxycarboxylic acid–is ubiquitous in living organisms and critically linked to the molecular origins of life due to its fundamental role in metabolic pathways. With the anoxic conditions of early Earth, anaerobic metabolic pathways such as lactic acid fermentation may have served as an essential mechanism for primordial cellular metabolisms. Although lactic acid has been detected in high abundances in carbonaceous asteroid Ryugu and various meteorites like Murchison, its formation pathways under extreme conditions of the interstellar medium (low temperature, radiation) have remained elusive. Here, we report the first bottom-up formation of racemic lactic acid via a barrierless radical–radical recombination between the hydroxycarbonyl (HOĊO) radical and the 1-hydroxyethyl (CH₃ĊHOH) radical in interstellar analog ices composed of carbon dioxide (CO₂) and ethanol (CH₃CH₂OH). These results provide a first step toward a fundamental understanding of the abiotic formation of biorelevant hydroxycarboxylic acids via nonequilibrium reactions from ubiquitous precursor molecules in extraterrestrial environments. Utilizing isomer-selective vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry and isotopic substitution experiments, lactic acid and its isomer monoethyl carbonate (CH₃CH₂OCOOH) were identified in the gas phase during temperature-programmed desorption. These findings suggest that they can likely form in interstellar ices containing carbon dioxide and ethanol in cold molecular clouds via galactic cosmic rays-mediated nonequilibrium chemistries.