Wang Jia, Zhang Chaojiang, Bergantini Alexandre, Kuznetsov Oleg V, Evseev Mikhail M, Shishova Anastasia S, Antonov Ivan O, Kaiser Ralf I
W. M. Keck Research Laboratory in Astrochemistry, University of Hawaii at Manoa, Honolulu, Hawaii 96822, United States.
Department of Chemistry, University of Hawaii at Manoa, Honolulu, Hawaii 96822, United States.
J Am Chem Soc. 2025 Aug 13;147(32):29088-29097. doi: 10.1021/jacs.5c07637. Epub 2025 Jul 29.
Lactic acid (CHCH(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 (CHĊHOH) radical in interstellar analog ices composed of carbon dioxide (CO) and ethanol (CHCHOH). 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 (CHCHOCOOH) 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.
乳酸（CHCH(OH)COOH）——一种关键的生物相关羟基羧酸——在生物体内普遍存在，并且由于其在代谢途径中的重要作用，与生命的分子起源密切相关。在早期地球的缺氧条件下，诸如乳酸发酵之类的厌氧代谢途径可能是原始细胞代谢的一种基本机制。尽管在碳质小行星龙宫以及默奇森等各种陨石中已检测到高丰度的乳酸，但其在星际介质极端条件（低温、辐射）下的形成途径仍然难以捉摸。在此，我们报告了在外星环境中由二氧化碳（CO）和乙醇（CHCHOH）组成的星际模拟冰中，通过羟基羰基（HOĊO）自由基与1-羟乙基（CHĊHOH）自由基之间的无势垒自由基-自由基重组首次自下而上形成外消旋乳酸。这些结果朝着通过外星环境中普遍存在的前体分子的非平衡反应对生物相关羟基羧酸的非生物形成有基本理解迈出了第一步。利用异构体选择性真空紫外光电离反射式飞行时间质谱和同位素取代实验，在程序升温脱附过程中在气相中鉴定出了乳酸及其异构体碳酸单乙酯（CHCHOCOOH）。这些发现表明，它们可能通过银河宇宙射线介导的非平衡化学过程在冷分子云中含有二氧化碳和乙醇的星际冰中形成。
