Prebiotic Synthesis and Isomerization in Interstellar Analog Ice: Glycinal, Acetamide, and Their Enol Tautomers

C-H Bond Activation by Sulfated Zirconium Oxide is Mediated by a Sulfur-Centered Lewis Superacid

Sulfated zirconium oxide (SZO) catalyzes the hydrogenolysis of isotactic polypropylene (iPP, Mw=13.3 kDa, Đ=2.4, =94 %) or high-density polyethylene (HDPE, Mn=2.5 kDa, Đ=3.6) to branched alkane products. We propose that this reactivity is driven by the pyrosulfate sites SZO, which open under mild conditions to transiently form adsorbed SO3 and sulfate groups. This adsorbed SO3 is a very strong Lewis acid that binds 15N-pyridine or triethylphosphineoxide (TEPO) (ΔEads>-39 kcal mol-1), reacts with Ph3CH to form Ph3C+, and mediates H/D exchange in dihydroanthracene-d4. DFT studies show that pyrosulfate sites open with a modest 26.1 kcal mol-1 barrier to form the adsorbed SO3 and sulfate in the presence of a tetramer of propylene. Hydride abstraction from the tertiary C-H in this model is exothermic and subsequent β-scission forms cleaved products. Analysis of the energetics provided here brackets the hydride ion affinity (HIA) of the adsorbed SO3 between 226.2 to 237.9 kcal mol-1, among largest values reported for a formally neutral Lewis acid. This study explains how SZO, a classic heterogeneous catalyst, can form carbocations by a redox neutral hydride abstraction reaction by very strong Lewis sites.

Interstellar formation of lactaldehyde, a key intermediate in the methylglyoxal pathway

Aldehydes are ubiquitous in star-forming regions and carbonaceous chondrites, serving as essential intermediates in metabolic pathways and molecular mass growth processes to vital biomolecules necessary for the origins of life. However, their interstellar formation mechanisms have remained largely elusive. Here, we unveil the formation of lactaldehyde (CH3CH(OH)CHO) by barrierless recombination of formyl (HĊO) and 1-hydroxyethyl (CH3ĊHOH) radicals in interstellar ice analogs composed of carbon monoxide (CO) and ethanol (CH3CH2OH). Lactaldehyde and its isomers 3-hydroxypropanal (HOCH2CH2CHO), ethyl formate (CH3CH2OCHO), and 1,3-propenediol (HOCH2CHCHOH) are identified in the gas phase utilizing isomer-selective photoionization reflectron time-of-flight mass spectrometry and isotopic substitution studies. These findings reveal fundamental formation pathways for complex, biologically relevant aldehydes through non-equilibrium reactions in interstellar environments. Once synthesized, lactaldehyde can act as a key precursor to critical biomolecules such as sugars, sugar acids, and amino acids in deep space.

Formation of methylglyoxal (CH3C(O)CHO) in interstellar analog ices - a key intermediate in cellular metabolism

Ketoaldehydes are key intermediates in biochemical processes including carbohydrate, lipid, and amino acid metabolism. Despite their crucial role in the interstellar synthesis of essential biomolecules necessary for the Origins of Life, their formation mechanisms have largely remained elusive. Here, we report the first bottom-up formation of methylglyoxal (CH3C(O)CHO)-the simplest ketoaldehyde-through the barrierless recombination of the formyl (HĊO) radical with the acetyl (CH3ĊO) radical in low-temperature interstellar ice analogs upon exposure to energetic irradiation as proxies of galactic cosmic rays. Utilizing vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry and isotopic substitution studies, methylglyoxal and its enol tautomer 2-hydroxypropenone (CH3C(OH)CO) were identified in the gas phase during the temperature-programmed desorption of irradiated carbon monoxide-acetaldehyde (CO-CH3CHO) ices, suggesting their potential as promising candidates for future astronomical searches. Once synthesized in cold molecular clouds, methylglyoxal can serve as a key precursor to sugars, sugar acids, and amino acids. Furthermore, this work provides the first experimental evidence for tautomerization of a ketoaldehyde in interstellar ice analogs, advancing our fundamental knowledge of how ketoaldehydes and their enol tautomers can be synthesized in deep space.

Exploring Chemical Space Using Ab Initio Hyperreactor Dynamics

In recent years, first-principles exploration of chemical reaction space has provided valuable insights into intricate reaction networks. Here, we introduce ab initio hyperreactor dynamics, which enables rapid screening of the accessible chemical space from a given set of initial molecular species, predicting new synthetic routes that can potentially guide subsequent experimental studies. For this purpose, different hyperdynamics derived bias potentials are applied along with pressure-inducing spherical confinement of the molecular system in ab initio molecular dynamics simulations to efficiently enhance reactivity under mild conditions. To showcase the advantages and flexibility of the hyperreactor approach, we present a systematic study of the method's parameters on a HCN toy model and apply it to a recently introduced experimental model for the prebiotic formation of glycinal and acetamide in interstellar ices, which yields results in line with experimental findings. In addition, we show how the developed framework enables the study of complicated transitions like the first step of a nonenzymatic DNA nucleoside synthesis in an aqueous environment, where the molecular fragmentation problem of earlier nanoreactor approaches is avoided.

Enantiodetermining processes in the synthesis of alanine, serine, and isovaline

In this study, quantum chemical calculations were used to explore the synthesis of three chiral α-amino acids, specifically alanine, serine, and isovaline, from reactants found in interstellar space. Our focus is on the crucial step in the synthesis pathway that determines the chirality of the amino acids. The results indicate that in the case of alanine, the determination of enantiomer is primarily influenced by the direction of the collision of molecules or functional groups, which leads to the formation of a chirality center in a crucial intermediate. However, contrary to chemical expectations, the enantiodetermining/enantioselection step for serine and isovaline synthesis occurs prior to the creation of a chirality center.